Another Bolger lapstrake, Nord Coaster, 16 foot Cat Yawl [Archive]

T. Traddles

03-08-2011, 06:24 AM

brucehallman

03-08-2011, 09:49 AM

Several years ago I wrote Bolger and inquired about the Nord Koster. He had a chance to sail the boat but noted it did not sail as well as he would have liked. He was not sure why that was the case, although he did speculate that the builder's modification of the c/b to case to accommodate a rowing station may have had something to do with it. boat.

Also, judging from the photos, the cut of the mainsail seems to sag towards the aft. A better cut mainsail could probably improve performance. Still, any 16 foot boat is going to be slow. That said, sailing at any speed can still be fun, especially in pretty boat.

My isometric model was created using the sourceforge software Free!Ship version 2.6. It is very quick and handy for calculating bulkhead shapes and expanded strake shapes.

Plans for Nord Coaster are available from:

Phil Bolger and Friends,
P.O. Box 1209
Gloucester, MA 01930
Fax: (978) 282-1349

DGentry

03-08-2011, 10:18 AM

Several years ago I wrote Bolger and inquired about the Nord Koster . . . He recommended another design . . .

Which one did he recommend?

Nord Koster is only a 16' boat, but weighs 850lbs(!), and it's beamy and has a centerboard slot and a salient keel and end plates on the rudder - and only 140 sq ft of sail area. I'm just speculating, but, with all that weight and drag, and so little sail area, there's little wonder that it might not be a spirited sailer (depending on the conditions). She is very pretty, though!

brucehallman

03-08-2011, 11:35 AM

... there's little wonder that it might not be a spirited sailer (depending on the conditions). She is very pretty, though!

I think by design. The design spec for this boat was for "a pretty boat with a large cockpit for four adults, space for sleeping bags for two people, and capable to be sailed in an unstrenuous fashion with a minimum of scrambling". This boat design seems to meet that specification exactly, and being a spirited sailer might require more stress than would be fun for the type of person this boat is meant for.

T. Traddles

03-08-2011, 09:27 PM

Bolger recommended his 18ft camper design. A birdwatcher type sharpie. Ugly, in my mind.

DGentry

03-09-2011, 08:21 AM

Huh. I can't imagine Camper 640 (a lee board sharpie) being a better sailer than Nord Koster! The accommodations would be more amenable to overnighting, and the hull itself would be quicker to build, but I surely don't see any advantages other than that. The NK is certainly in an entirely different universe, looks wise.
Perhaps Mr. Bolger was emphasizing the sleep-over capacity, rather than sailing ability?

As a pointless aside, I am taken with the visual style of the original Birdwatcher, and also with Bolger's Berengaria. But Camper 640, William D Jochems and Birdwatcher 2 leave me cold. Whalewatcher is, well, maybe a whole other topic of discussion.

T. Traddles

03-09-2011, 09:11 AM

DGentry, I agree with you assessment of the designs mentioned in your previous post. I have plans for Spartina and will probably build that boat within the next year or two.

brucehallman

03-09-2011, 10:06 AM

Huh. I can't imagine Camper 640 (a lee board sharpie) being a better sailer than Nord Koster!

Water hydrostatics doesn't care about "ugly".

Camper 18 ft long, 5'3" wide x 6 inch draft (150 sf sail)

Spartina 15'4" long, 7' wide, 6 inch draft (143 sf sail)

Nord Coaster, 16 ft long, 6'6" wide, (141 sf sail)

I dunno, (not paying attention to ugly) I would bet that the longer narrower boat with greater sail area will sail faster than the shorter wider boats with less sail areas.

DGentry

03-09-2011, 11:09 AM

Good point, and I agree - the Camper should be often be faster. But, I should qualify my earlier thought: When I think of sailing ability, I am always subconsciously thinking of windward ability, and I think Spartina and Nord Koster would be superior in that respect. Camper's hard chine sharpie hull form, and those leeboards, are not ideal for windward work. And, too, the Camper might be a little top heavy, and that, combined with the narrow hull, could make her not nearly so able to stand up to a press of sail when going to weather. Spartina, with hopefully less weight, and definitely less underwater drag, than Nord Koster, might be the best at sailing anywhere above a beam reach, despite being shortest.

This is all just theorizing on my part, never having sailed any of these boats. There's more to sailing than just windward ability, too, of course, as we have discussed elsewhere.

brucehallman

03-09-2011, 12:02 PM

I am always subconsciously thinking of windward ability, and I think Spartina and Nord Koster would be superior in that respect. Camper's hard chine sharpie hull form, and those leeboards, are not ideal for windward work.

I am in the process of studying these designs now. Modeling and hopefully isometrics soon.

Camper 640 doesn't have leeboards, it has a swinging centerline daggerboard. So do Spartina and Nord Coaster.

That said, I don't understand the problem with leeboards relative to lateral plane for windward work. The big complaint here against leeboards (I understand) is that they are ugly and a pain to handle, and that they take a special effort to tend. (And maybe the noise?) As far as the wind and water is concerned, I am thinking that lateral plane is lateral plane. The water has no idea whether the lateral plane is located on the centerline or off the centerline of the boat.

And, it is also worth noting that Camper 640 uses a distributed lateral plane shared between the large rudder and the daggerboard being a bit forward of the location considered "normal". This reflects things learned in water tests done with Bolger's Dart Dinghy and later with his St. Vallery design.

Eyeballing it, I would bet that Camper 640 actually has more lateral plan than both Nord Coaster and Spartina. I don't understand any physical reason that the greater lateral plane would not make better to windward that both the Nord Coaster and Spartina boats. Why does the pretty boat seem to go better to windward?

I could hear an argument about the effect of the slot top cabin on Camper, but this is offset by the greater width of the other two boats that also contributes significant top hamper too. I am just trying to tease out the physical reasons from the aesthetic reasons in the 'conventional wisdom' here.

DGentry

03-09-2011, 01:28 PM

Aesthetics were never in my consideration or explanation for sailing ability here.

The width of the low sided Koster and Spartina is not adding to instability - it is substantially adding stability. Camper, being narrower and having a much, much taller hull/deck - ergo much more mass up high - is naturally going to be less resistant to heeling forces than the lower, wider boats. I'd rather be in a Camper than a Spartina if they were sitting on their sides (or if I was camp cruising), but traditional hard chine sharpies, even those with low sides, are typically not renowned for their windward ability, esp in choppy conditions.

Here's the Camper 640, for those who are curious.
http://i926.photobucket.com/albums/ad108/pramsafloat/004.jpg

I hadn't so much as glanced at a pic of Camper in months, and forgot she didn't have leeboards like the William D Jochems.
But lateral plane is definitely not lateral plane. High aspect ratio underwater foils are definitively more efficient for upwind work, for one thing (assuming a hull and sails that can take advantage of that), and leeboards themselves - surface piercing foils - just don't work as efficiently as end-stopped, fully submerged foils like centerboards, daggerboards, or bilgeboards. This is both well established, and something that Bolger said himself (which, offhand, I can not document). Practically speaking, it is evident in the fact that purpose built racing boats of all types that could opt for leeboards never do so.
- Excepting for homebuilt boats, of course.

All that being said, Camper 640 might sail just fine. I still think that Spartina - and likely Nord Koster - would be better going to weather, though.

brucehallman

03-09-2011, 02:13 PM

DG:"The width of the low sided Koster and Spartina is not adding to instability - it is substantially adding stability. "

There are three ways to resist overturning: width (like Spartina), deep ballast (like Marconi sloops), or high reserved buoyancy (like Birdwatcher or Camper 640).

Boats like Birdwatcher and Camper640 are supposed to heel over a lot while sailing. This isn't a failure of "stability", but merely the nature of their design.

DG:"...but traditional hard chine sharpies, even those with low sides, have never been known for their windward ability, esp in choppy conditions."

I agree when compared to fin keeled Marconi rigs. This is attributable to three things: the high tension jib stay (aerodynamics), the effect of engineered high aspect ratio foils (hydrodynamics), and the high mass of their ballast (mass for busting through chop).

We were comparing Nord Coaster, Camper640 and Spartina, and none of these have a jib stays, massive ballast or high aspect foil keel.

DG:"... end-stopped, fully submerged foils...well established, and something that Bolger said himself (which, offhand, I can not document). "

I agree with you about this...if we are talking about engineered high aspect ratio foils which most swinging boards are not.

I am trying to remember the passage of Bolger about leeboards that you are recalling...it would be interesting to collect together his opinions about leeboards...I agree that he stated that they have their drawbacks.

One benefit is that the lateral plane area is measured from the waterline in the case of leeboards, and not measured from the lowest point of the hull as with centerboards.

DG: "All that being said, Camper 640 might sail just fine. I still think that Spartina - and likely Nord Koster - would be better going to weather, though.

I am just trying to tease out the physical reason you believe this. Is it that Camper640 is hard chined? Is there evidence that hard chined boats don't go to windward as well as short wide rounded boats like Spartina. I was tempted to guess that this was a "ugly vs. pretty" subconscious bias, but you say it is not.

James McMullen

03-09-2011, 03:32 PM

A friend and I did an experiment with a couple of super quick and dirty Bolger Teal's many years ago. Sam built his exactly according to spec, clamp-on leeboard and all. Because I had been a little dis-satisfied with the performance of the leeboard on the Chamberlain dory-skiff I had built, I made a daggerboard case on the other one and jammed the stock leeboard in it as a daggerboard. The difference was unequivocal, even for a couple of young, brash, not particularly skilled sailors. Both Sam's Teal and the dory-skiff got daggerboards too, immediately after, and I've never bothered with a leeboard since.

The explanation I've heard is that the surface-piercing foil of a leeboard is much more prone to developing unfavorable spanwise turbulence than an end-stopped fully submerged foil like a CB or DB.

The most interesting thing to me about the Nord Koster design is that I am using essentially the same size rig in a boat that weighs less than half as much in Rowan. Different strokes for different folks, of course. But I will note that the double-ended lapstrake lug yawl Rowan, with her fully submerged CNC machined NACA section foils, is indeed renowned for her windward ability, esp in choppy conditions. :D

brucehallman

03-09-2011, 03:41 PM

surface-piercing foil of a leeboard is much more prone to developing unfavorable spanwise turbulence than an end-stopped fully submerged foil like a CB or DB.

Why might this be? What is the physics making this happen. I cannot imagine that a home made Teal daggerboard/leeboard is achieving much NACA foil laminar flow either "end stopped" or not. Without laminar flow, I cannot imagine this 'lack of end stopping' increasing the turbulence of flow. Excuse me for asking, I am just trying to tease out the physics from the conventional wisdom.

Steve Paskey

03-09-2011, 06:08 PM

Boats like Birdwatcher and Camper640 are supposed to heel over a lot while sailing. This isn't a failure of "stability", but merely the nature of their design.

That's true in principle, but in the case of Camper (no. 640) it appears that the original design does, in fact, lack sufficient stability.

If I remember correctly, the only person to build Camper sold his boat without trying to sail it. The builder had sailing experience, but after raising the mast without a sail he apparently concluded that the boat was too top heavy to stand up to sail in even modest winds. I haven't seen any updates, so I don't know whether the new owner has tried to sail it.

brucehallman

03-09-2011, 06:26 PM

That's true in principle, but in the case of Camper (no. 640) it appears that the original design does, in fact, lack sufficient stability.
If I remember correctly, the only person to build Camper sold his boat without trying to sail it. The builder had sailing experience, but after raising the mast without a sail he apparently concluded that the boat was too top heavy to stand up to sail in even modest winds. I haven't seen any updates, so I don't know whether the new owner has tried to sail it.

True. I am personally of the opinion that the report from the owner was naive. (I think that a lot of people would find it disconcerting to sail at extreme angles of heel.) And, I interpret SA's comment to be that disconcerted owners deserve to feel better about extreme angles of heel, and this could be fixed with 200 pounds of lead fastened to the bottom.

That said, with the windows being watertight in this type of sailboat, they are stable in principle. (Meaning, they retain righting moment at extreme angles of heel.) Both Camper640 and Birdwatcher are designs that rely on the occupants of the boat being seated and serving as live ballast. When I look at the stability curves, these boats appear to be self righting from even a 90 degree knockdown. (Except with this caveat!) The Achilles heel with both of them is the wildcard of the shifting live weight of gear. If there is 300 pound of beer in a cooler that gets shifted to a high C.G. in a knockdown, then it would be less 'self righting' until after you drink, or relocate the beer to a lower center of gravity. I don't see these boats easily turning turtle.

James McMullen

03-09-2011, 09:12 PM

Without laminar flow, I cannot imagine this 'lack of end stopping' increasing the turbulence of flow.

No need to imagine anything, the phenomenon of surface piercing foils has been very extensively studied, both mathematically and with empirical and tank testing. High speed hydrofoil vessels are especially concerned with these surface piercing effects, and there has been an awful lot of published research on it. The primary issue is ventilation from the air/water interface. This sort of turbulence rapidly degrades lift, especially for foils operating at an angle of attack as is seen of a boat sailing to windward. Some other issues can include the wave drag and from interaction turbulence of a "separate hull" moving above its natural hullspeed if the leeboard is far enough from the main hull to behave somewhat independently of the flow from the main hull.

Fully submerged bodies behave differently than those working at the air-water interface. The interface necessarily adds some drag--this is why modern nuke subs can go so very much faster when submerged than when traveling on the surface.

brucehallman

03-09-2011, 11:19 PM

No need to imagine anything, the phenomenon of surface piercing foils has been very extensively studied, both mathematically and with empirical and tank testing. High speed hydrofoil vessels are especially concerned with these surface piercing effects, and there has been an awful lot of published research on it. The primary issue is ventilation from the air/water interface. This sort of turbulence rapidly degrades lift, especially for foils operating at an angle of attack as is seen of a boat sailing to windward. Some other issues can include the wave drag and from interaction turbulence of a "separate hull" moving above its natural hullspeed if the leeboard is far enough from the main hull to behave somewhat independently of the flow from the main hull.

Fully submerged bodies behave differently than those working at the air-water interface. The interface necessarily adds some drag--this is why modern nuke subs can go so very much faster when submerged than when traveling on the surface.

And experiment with a Teal's leeboard/daggerboard is neither high-speed nor a foil. I just glanced at some of this hydrofoil research and see that the essence, I believe, is the problem with air ingress from vortices. How is this applicable at slower speeds where their is not air ingress?

James McMullen

03-10-2011, 10:04 AM

There is an air ingress vortex that travels down the leading edge at even very moderate speed, my friend. The faster you go, the further it can travel, but it is always there at the air-water interface. But I dont ask you to take my word for it, build yourself a couple of Teals and test it out for yourself.

You don't need to do thought experiments and careful theorizing or complex computer modeling to answer this question. Even someone with minimal sailing experience can figure out better pointing and less leeway slippage with a handheld GPS, or better yet by sailing side-by-side with an otherwise identical boat. Do a real-life empirical test rather than all this theorizing and parsing of the sacred revealed texts and scripture of the Mighty Bolger. It won't take you more than a weekend or two and a couple hundred bucks to conclusively answer the question to your own satisfaction without needing to guess . . .and then you'll even have a couple of cute little sailboats to use or even just give away to the local Boy Scouts or something as a bonus.

brucehallman

03-10-2011, 10:11 AM

There is an air ingress vortex that travels down the leading edge at even very moderate speed, my friend.

Amazing, you see an air vortex on your Teals? I don't see it on my Teal. I guess I will have to look again.

DGentry

03-10-2011, 12:07 PM

Two boat testing is well proven - just ask DC. But, regardless of the perceived results of any two boat Teal testing, I reiterate that one need only look at serious racing boat design. Remember - there are rich men's egos on the line! If it isn't constrained by arbitrary rules, racing boats are designed for maximum performance in real world conditions. With the advent of computers, nowadays millions of dollars are invested in scientific research and testing to develop the utmost efficiency in every aspect of boat design - whether for rowing shells, racing kayaks or round-the-world ocean racing sailboats. I've even seen doctoral dissertations on the bio-mechanics of the ideal racing stroke for kayak paddling! Looks are not a factor, and convenience is never trumped by performance. If flat bottomed, hard chine hulls, of sharpie width, were advantageous, they'd sure as heck be using them. And, if leeboards had any performance advantage over - or even parity with - other foil configurations, you'd be seeing them, too.

That doesn't mean that sharpie hulls or leeboards don't have their place, or can't perform - it just means that they aren't the best option when maximum performance (even in light air) is the issue, and money isn't.

brucehallman

03-10-2011, 12:25 PM

...when maximum performance (even in light air) is the issue, and money isn't.

Agreed. And if maximum performance is the criteria, one should stay away from choosing Spartina, Nord Coaster and/or Camper640.

(And in my imaginary triangle course race between these three boats, my money bet would be on Camper640 because it is narrow, longer, has more lateral plane and more sail area.)

If the criteria is non-strenuous pretty boat sailing, I like Spartina best. (This surprises me because I usually prefer double enders.)

brucehallman

03-10-2011, 12:34 PM

I reiterate that one need only look at serious racing boat design. Remember - there are rich men's egos on the line!

OMG serious racing?!? <big smile>

The hidden hand fallacy here is the fact that these rich men write their own racing rules. And they have shown a long clear history that their rule committees selectively choose expensive criteria to advance the elitist nature of their clubs. (Originally pushing "working boat" gaff sails out of the club via measurement rules, and since then, much more.)

I would bet that if the racing rules included shoal draft water, that we would see leeboards, chine runners and more.

My favorite proposed rule is that all boats are handicapped by the price of the boat, with the owner declaring their price before the race. The winning boat is then obligated to put their boat up for sale at the declared price at the end of the race. That would equalize things!

BBSebens

03-10-2011, 01:27 PM

Unless your going for historical accuracy, why wouldn't you try to make your boat the best it can be? Within reason, of course. And everyones reason will vary.

For example, Im making my boat the best it can be by upgrading the rudder, building a new rig, and messing with the ballast a bit. I may re-make the CB as well...

brucehallman

03-10-2011, 02:08 PM

why wouldn't you try to make your boat the best it can be?...And everyones reason will vary.

'Best' is completely subjective. One criteria might be "easiest to build by an all thumbs first carpenter", or another "voluptuously curvy sculpture", or "most ostentatious", or another "best patio platform in the marina". All those are legitimate criteria to wish for in a boat.

BBSebens

03-10-2011, 10:31 PM

Quite right you are.

What is the "Best" that you are going for? (not trying to be snippy, snide, or sarcastic.. Im honestly curious)

brucehallman

03-11-2011, 10:39 AM

Quite right you are.

What is the "Best" that you are going for? (not trying to be snippy, snide, or sarcastic.. Im honestly curious)

Personally, I favor owning many boats, with each one tailored to do the exact thing you want at any moment in time. I personally don't like to have boats that compromise trying to do two opposing things well. For instance, I believe if you design a boat to be both a racer and a cruiser you end up getting a boat mediocre in both.

slidercat

03-11-2011, 11:28 AM

brucehallman

03-11-2011, 12:28 PM

What is an 8-ball?

slidercat

03-11-2011, 05:52 PM

It's an 8 foot pram from Glen-L. Makes a decent tender.

brucehallman

03-12-2011, 09:42 AM

James, a lot of folks have had similar experiences, trading leeboards for daggerboard and seeing an unmistakable increase in windward performance. Thomas Firth Jones is one, and I'm another. We had an 8-Ball as a tender for one of our big boats. Someone gave me a rig of the right size, and I built leeboards to go with it. It was okay, but when I finally got fed up with the leeboards and cut in a board case, it was day and night.

I just looked at the Eightball lines, and if you rested the leeboard against the gunnel and sheerstrake at the balance point, the toe-in angle seems very steep. Excessive toe-in might be the reason for your disappointing experience with a leeboard on an Eightball.

Nicholas Scheuer

03-12-2011, 10:07 AM

Concerning leeboards:

My Shearwater Yawl has a pair of assymetric laminar foil leeboards like illustrated in this sketch.http://farm6.static.flickr.com/5212/5467657031_5a936bfc43_b.jpg

The performance of the bat to windward is had and shoulders above her performance employing the original flat, fan profile boards, both of which fractured right across below their bearing surface against the hull on different occasions under boistrous conditions.

There is one other Shearwater sporting a pair of NACA00 assymetric foils (60%/40% chord proportions) and it is this boat whose performance to windward I was attempting to match. I was more of less successful; a equivacal judgement resulting from the other boat having a more efficient rig, as well as a more able Skipper.

I posted reccently on the WBF in another thread, I cannot understand why those who build leeboard designs still employ flat leeboards when kthe evidence in favor of assymetric foils is so convincing.

There is a wooden sharpie ketch in Australia equipped with a pair of centerboards hung in a common case. The mating surfaces of the boards are flat, while the outboard surfaces are curved. The board appropriate for generating lift is employed on alternate tacks. The boat sails quite well.

As I said, the evidence is convincing, as least to those not totally committed toward engraving ALL the words of Bolger and Peter Duff in stone.

Moby Nick

slidercat

03-12-2011, 10:10 AM

That's really not what "toe-in" means, if I'm understanding you. In any case, I added a rubbing strip partway down the board to support it at the proper angle.

Endstopped foils are just more efficient than surface-piercing foils. The science is really not in dispute. I can give you a real-world example that seems pretty unassailable. The Stiletto 27 catamaran was originally produced with a central swing-up foil. This was a very fast boat, much advanced for its time, built of Nomex, very light and well-designed in almost every respect. However. a few were produced with in-hull daggerboard, and these boats were markedly faster than the ones with a surface-piercing foil.

There are applications where leeboards would be appropriate. The Dutch have made quite an art out of boats with leeboards. They have very shallow inland waters, and for them, leeboards were an excellent solution. In very small monohulls, leeboards get the leeway resistance mechanism out of the cabins. They are easier to maintain, for shallow draft working boats.

But if performance is part of your design brief, leeboards, in my opinion, are less than optimal.

Nicholas Scheuer

03-12-2011, 10:11 AM

My foil leeboards have wooden cores sheathed in a stout schedule of fiberglass/epoxy.

The lower end design is predicated on my opinion of what constitutes the most efficient shape with the boards trailing aft 45-degrees over a shoal bottom.

Moby Nick

Nicholas Scheuer

03-12-2011, 10:13 AM

My boards appear to have no "toe-in" because the designer of the foil told me that the foil itself has a 3-degree angle of attack with the chord centerline parallel to the keel.

Moby Nick

James McMullen

03-12-2011, 02:17 PM

boat fan

03-12-2011, 04:15 PM

I just looked at the Eightball lines, and if you rested the leeboard against the gunnel and sheerstrake at the balance point, the toe-in angle seems very steep. Excessive toe-in might be the reason for your disappointing experience with a leeboard on an Eightball.

I`m really confused now Bruce ....

I was always under the impression that "toe-in " referred to the angle the board makes in relation to the longitudinal center line , in plan view ?.

Have I had this all wrong all this time ?

htom

03-12-2011, 04:22 PM

You could (not going to say it would be easy) build leeboards with both endplates and "surface plates", just below the DWL. How large would the plates have to be to make them equal the performance of an foil stopped in the hull, or midway between plateless and hull-stopped?

boat fan

03-12-2011, 04:30 PM

John Meachen

03-12-2011, 05:17 PM

I just realized that I am shooting myself in the foot by condemning leeboards like that. In fact, I would most definitely enjoy it if every boat but mine were sporting leeboards when rounding the leeward mark. :

They'd probably just handicap my PHRF rating though, but line honors are always glorious no matter what the judges with the calculators say!

Only six days ago I was looking at a C-Class catamaran that had assymetric foil section daggerboards.The boats designers are employed by Airbus and might be expected to know a bit about fluid dynamics.Only a few inches of location between the definitions of leeboards and daggerboards.Good design always succeeds.

Nicholas Scheuer

03-12-2011, 07:00 PM

No, boat fan, you have "toe-in" right, angled in the plan view.

There are advantages both ways in angling leeboards either inward or outward (relave top an end view) but all that has to do more with roll resistance than anything else.

As for end plates, htom, I may be biased, but after years of sailing Dovekie and Shearwater (both advertised by E&D as "Shoal Draft Champions") I would not be at all interested in leeboards having end plates. I've sailed too many leagues with a leeboard trailing aft over a shoal bottom. Wouldn't it be lovely having an end plate dragging through the water at an angle like a drogue.

Moby Nick

htom

03-12-2011, 07:24 PM

Making the end/top plates tilt would complicate things, one scheme would be the foil in two parts that slide on each other, the plates attached to each of the foil parts.

brucehallman

03-12-2011, 08:54 PM

That's really not what "toe-in" means, if I'm understanding you. In any case, I added a rubbing strip partway down the board to support it at the proper angle.

Yes, I am asking if your redesign of the Eightball lateral plane involved correct fore-aft location and angle of the leeboard, as perhaps some mislocation might have skewed your experiment. Also, I am questioning if there is some Observer-Expectancy Effect (http://en.wikipedia.org/wiki/Observer-expectancy_effect) in your test.

Endstopped foils are just more efficient than surface-piercing foils. The science is really not in dispute. I can give you a real-world example that seems pretty unassailable. The Stiletto 27 catamaran was originally produced with a central swing-up foil. This was a very fast boat, much advanced for its time, built of Nomex, very light and well-designed in almost every respect. However. a few were produced with in-hull daggerboard, and these boats were markedly faster than the ones with a surface-piercing foil.

Actually, the research I saw about surface-piercing foils involved hydroplane research which evaluated much higher velocities than we see here with dingy sailing. I don't dispute the research at high velocities, but what about the lower velocities? Is there research data "science" whether the effect is significant at these lower velocities?

Can I read this Stilletto 27 data somewhere, or is this anecdotal?

Pardon me for being skeptical of anecdotal 'evidence'. Just asking.

But if performance is part of your design brief, leeboards, in my opinion, are less than optimal.

It is sloppy to simply say "performance" as if it is a binary thing.
There are many types of performance, speed reaching, speed running, and there is also the issue of performance measured in ergonomics, where cruising space is ruined by a centerboard trunk, and/or the issue of shoal draft. Sailing is a holistic activity involving, cost benefit trade offs of many aspects of performance and the quality of the sailing experience, and being vague about 'performance' implies a bias and sometimes sounds dogmatic.

boat fan

03-12-2011, 10:22 PM

Making the end/top plates tilt would complicate things, one scheme would be the foil in two parts that slide on each other, the plates attached to each of the foil parts.

Bolger`s Insolent 60 ?

htom

03-13-2011, 08:40 AM

Ohhhh I'd never heard of that one! Yes, like that.

brucehallman

03-13-2011, 10:42 AM

Yes, the unfinished Bolger design Insolent 60 has folding foil shaped keel with and end plate. (Having a mind blip.) There is another Bolger design with this folding mechanism which was actually built. It worked (of sorts) and the issue was that the folding mechanism needs to be very stoutly engineered, and built, which is not easy considering that the end plate is made of a solid chunk of lead. In the example of the one which was built, it worked when extended, but the folding arms of the geometry articulation mechanism jambed. The result is that the foil worked when extended, but did not rotate when retracted. This caused drag in shoal conditions requiring the retraction of the keel. A few years ago I diagrammed this folding foil keel/ballasted end plate mechanism, and confirmed in my mind that it worked on paper, and confirmed in my mind that the detailing of the structural pieces was workable.

DGentry

03-13-2011, 11:25 AM

slidercat

03-13-2011, 12:20 PM

Can I read this Stilletto 27 data somewhere, or is this anecdotal?

Pardon me for being skeptical of anecdotal 'evidence'. Just asking.

It is sloppy to simply say "performance" as if it is a binary thing.
There are many types of performance, speed reaching, speed running, and there is also the issue of performance measured in ergonomics, where cruising space is ruined by a centerboard trunk, and/or the issue of shoal draft. Sailing is a holistic activity involving, cost benefit trade offs of many aspects of performance and the quality of the sailing experience, and being vague about 'performance' implies a bias and sometimes sounds dogmatic.

You back up your opinions with little other than anecdotes, so this seems a bit funny. But if you're actually interested, Google is your friend. Here's a piece on Sailing World that refers to the process of modifying a Stiletto.

http://www.sailingworld.com/gear/getting-into-yacht-rodding

There are plenty of other references on the net to these modified boats being faster than the originals. One piece I read somewhere opined that the hot-rodding of Stiletto 27s occurred when the F-27 tri was introduced and proved faster than the Stiletto 27 cat. Hardcore racers decided to cut their hulls and convert to daggerboards, whereupon the Stilettos regained dominance.

In the context of discussing leeboards versus daggerboards, I think most reasonable people would define "performance" as lift to drag ratio, unless they're being deliberately obtuse, or generalizing wildly for the sake of argument. The ventilation of a board that is not endstopped degrades this performance. I guess you don't have to believe this, if you really don't want to.

"Speed running...?" Really? You know, most folks lift their boards when running, if conditions permit.

I think you could make the argument that leeboards are superior to centerboards downwind, because when lifted, there is no slot to cause turbulence. But not Slider's daggerboard, which when pinned in the up position seals off the slot completely.

brucehallman

03-13-2011, 07:00 PM

I guess you don't have to believe this, if you really don't want to.

You mentioned " The science is really not in dispute." earlier. I was hoping to get to read this science. As I said already, the science I have seen pertains to hydrofoils. And hence at much higher velocities. Is there science pertaining to the low velocity of sailboats? I am guessing the mathematics is akin to to the drag equation, were the drag varies in proportion to speed exponentially. So, I am just wondering if the low speed 'end capping' is the sensible reason when the research is high speed? I grant there seems to be some genuine effect involved, but I am trying to understand which. There might be other reasons, including observer-expectancy effect. Or, not. Which science were you reading when you said the science is really not in dispute? Can I read it too? Thanks.

slidercat

03-13-2011, 08:37 PM

You could try Frank Bethwaite's High Performance Sailing. Excellent book by a guy who was something of a Renaissance man.

boat fan

03-13-2011, 08:51 PM

You could try Frank Bethwaite's High Performance Sailing. Excellent book by a guy who was something of a Renaissance man.

Astute reading Mr Aldridge.

Frank Bethwaite is highly regarded here in Australia and internationally.

Inventor Sailor Scientist and more , all rolled into one.

http://www.google.com.au/url?sa=t&source=web&cd=1&ved=0CBUQFjAA&url=http%3A%2F%2Fwww.bethwaite.com%2F&rct=j&q=bethwaite%20design&ei=J3Z9TaTTN8fIcbDo1LkG&usg=AFQjCNHJ3yrrn7AquRPeRfX8m7ikZQy_zw&cad=rja

brucehallman

03-14-2011, 09:40 AM

You could try Frank Bethwaite's High Performance Sailing. Excellent book by a guy who was something of a Renaissance man.

Thanks for the tip, looks like I need to read page 241. Amazon has "look inside the book" disabled, so I need to find a hard copy. My basic question is to quantify the effect. My hunch is there is some trade off, where leeboards have turbulence at the top end interface with the water degrading performance opposed by daggerboards which have less area resisting lateral motion due to their depth being measured from the bottom of the hull versus being measured from the waterline like a leeboard. And this would also vary with velocity, I am curious to see the mathematics of this trade-off.

James McMullen

03-14-2011, 09:59 AM

slidercat

03-14-2011, 10:46 AM

I agree with you, James, but to be fair, Dave Gerr makes this very argument in The Nature of Boats. His first chapter is about Dutch yachts, which he considers to be the progenitors of modern yachting. He's claiming that leeboards of a given chord and depth will have more working area exposed than a centerboard of the same chord and depth.

I think he's wrong, because it isn't just area that matters, it's effective area. The part of the leeboard that is masked by the hull is not operating in clean water, as the centerboard is. The bow wave is washing past and through that part of the leeboard, and as Bethwaite demonstrated experimentally, that kind of turbulence increases the drag of a foil enormously.

Bruce, being immune to anecdotal evidence, will not find this persuasive, but Thomas Firth Jones, in Low Resistance Boats, recounts the building of a small daysailer based on Bolger's Fieldmouse pram. (At the time he was greatly influenced by Bolger's writings and says that had Bolger been a used car salesman, he'd have been a millionaire.) Anyway, Jones scaled the boat up to 9.5 feet and strip-planked her. Barrel of Monkeys had a leeboard. Jones says "It didn't work on either tack." After the first season, he cut the bottom and built a case. As an experiment, he at first used the leeboard as his daggerboard, on the principle that changes should be limited to one at a time so that results can be correlated.

"The improvement was unbelievable. She steered much easier and made much less leeway. I'm certain that no one who has ever tried a good sailboat, first with a leeboard and then with a daggerboard, would ever go back to a leeboard. There simply is no comparison."

He explicates his viewpoint by noting that in racing yachts, genoas are deck-sweeping, in order to cut losses at the bottom of the sail. The reason why non-sweepers still work pretty well is the low density of air. But water is 835 times more dense than air. He says that except for very slow craft, losses in foils not endstopped by a hull are "catastrophic." He points out that Dutch leeboards attempt to mitigate this by making the boards narrower at the waterline than below, but says that the primary way the Dutch yachts deal with the inefficiency of leeboards is by making their hulls slow.

It's a shame Bruce doesn't have a boat with leeboards that he'd be willing to experiment with. He might be converted.

brucehallman

03-14-2011, 11:08 AM

"leeboards measured from the waterline" sounds like a fallacy to me. The side of the hull itself in either sort of boat is providing area to resist lateral motion. Why would a leeboard hanging a few inches away from the hull have some magical extra advantage over the hull itself in the part that is shadowed? I think that is a post hoc rationalization.

The same logical falacy would apply in reverse, with a leeboard close to the face of the boat, why might it have an "end stopping" problem? That is why I have been interested in reading the science and have been trying to steer away from opinion.

brucehallman

03-14-2011, 11:15 AM

It's a shame Bruce doesn't have a boat with leeboards that he'd be willing to experiment with. He might be converted.

I have had a couple leeboard boats, and it is a problem in my life that I own too many boats (13 and counting) so I gave one away and the other got used up from age and I sent it to the dump.

It would take more than one boat to test this though. I am just trying to tease out the "everybody knows" anecdotal from the quantified. To really get good data you would probably need design a watertank test.

I am questioning whether the hydrofoil data that James is hanging his hat on is relevant at the velocities we are working with. I have a strong hunch that the effect is depended on the square of the speed, so it may exist at our speeds but be negligibly small and outweighed by other factors including observer-expectancy bias.

slidercat

03-14-2011, 11:47 AM

It would take more than one boat to test this though.

Maybe, but then you could speak from personal experience, as James and I are doing. Plus, if you are correct and the effect is negligible, you would have a much stronger argument. So far, you have put forward no evidence, anecdotal or otherwise, that leeboards are as effective, performance-wise, as end-stopped foils.

I'm not saying that leeboards are bad-- there are advantages to them in practical terms, especially for boats in which high performance is not necessary. But until you introduce evidence of their comparative efficiency, I'll have to go with my own experiences, and the experiences of other sailors and designers.

As to "observer-expectancy bias," I expected that the leeboards would work fine on my 8-Ball. When they did not, and I built a daggerboard case, the difference was not subtle at all.

brucehallman

03-14-2011, 12:09 PM

That boat was a double sized Gypsy, originally designed for possible OSTAR use. The keel has since been replaced, the rig modified, and a new cabin added, IIRC. I think there are some youtube sailing videos of it out there, too.

Yes, it was Gary Blankenship's boat, Bolger design 459. Originally very deep draft with a fin keel, and later retrofitted with the prototype of a swinging ballasted end stopped foil. Gary reported that he had problems with the workmanship of the fabricator of the steel parts, and the swinging mechanism jambed. Certainly, there are some mean stresses involved in those moving parts. Back a few years ago I penciled out the parts to see how they work. The whole assembly shown in this diagram are shrouded in a foil shaped sheathing.

For some pictures of design 459 isometrics see here: http://flickr.com/photos/hallman/3272257317/

Here is my sketch working out detailing of the swinging arms of the retrofit keel, (also on Insolent 60)

http://www.hallman.org/bolger/Insolent60/iso.gif

brucehallman

03-14-2011, 01:02 PM

As to "observer-expectancy bias," I expected that the leeboards would work fine on my 8-Ball. When they did not, and I built a daggerboard case, the difference was not subtle at all.

I am guessing you had your leeboard experiment positioned wrong on the Eightball, probably too far aft. Additional testing could quantify it.

James McMullen

03-14-2011, 02:58 PM

Bruce, I think I am actually hanging my hat on a variety of factors, including my own experiences, on theoretical constructs I find plausible and sufficient, and on the observed fact that there isn't a single dedicated shoal draft racing boat that chooses to use leeboards instead of a centerboard, bilgeboard or daggerboard configuration. I will not deny that leeboards have some qualities that may well recommend them for particular uses, particular designs, or particular skippers. These overwhelmingly tend to be for reasons other than wringing out upwind performance.

brucehallman

03-14-2011, 03:05 PM

...there isn't a single dedicated shoal draft racing boat that chooses to use leeboards.

I cannot think of any dedicated shoal draft racing boat. Every racing committee of which I am aware routes their race courses away from shoal draft areas. Which dedicated shoal draft racing boat(s) are you thinking of?

James McMullen

03-14-2011, 03:05 PM

I would also say that a complicated pivoting linkage and mechanism like that moving ballast keel sketch is just begging for problems when it's below the waterline. Saltwater corrosion, marine-growth fouling and simple jamming are all potential problems that are very inconvenient and hard to fix while afloat. Especially for a 60' boat! Seems like a lot of potential failure points--as Gary Blankenship found out.

James McMullen

03-14-2011, 03:06 PM

I cannot think of any dedicated shoal draft racing boat. Every racing committee of which I am aware routes their race courses away from shoal draft areas. Which dedicated shoal draft racing boat(s) are you thinking of?

Amigo, there are hundreds of different designs of CB and DB equipped racing dinghies. Hundreds! Shoal draft is important for beachable and trailerable boats whether or not you're actually mostly sailing in primarily shoal-draft waters.

brucehallman

03-14-2011, 03:29 PM

...problems ...Especially for a 60' boat!

No dispute with that. Except if your design spec is for a very fast folding 60 foot boat that stores compactly on a 20 foot trailer. (Like with the I60.) Then a folding ballasted keel is the only way to go.

Seems like a lot of potential failure points--as Gary Blankenship found out.

No need to exaggerate. IIRC, Gary reported a singular failure point, attributed to the steel fab shop. His ballasted swinging fin keel worked fine for him, except that it did not articulate as it swung, (causing needless drag in the upward position). Not a fatal problem, but only a nuisance while motoring into and out of the marina.

In Gary's case, he needed movable ballast because he needed to retrofit his deeply drafted OSTAR racer for use from open ocean, to shoal draft Florida waters.

brucehallman

03-14-2011, 03:33 PM

Amigo, there are hundreds of different designs of CB and DB equipped racing dinghies. Hundreds! Shoal draft is important for beachable and trailerable boats whether or not you're actually mostly sailing in primarily shoal-draft waters.

Got it. I grant you that those choose dagger boards. Might the explanation also be that they are interested in the ease of handling? Shifting a leeboard from side to side and tending it underway is a nuisance compared with lifting a daggerboard. Racers are also interested in time saved due to simplicity of handling.

dredbob

03-14-2011, 04:18 PM

Not to confuse things here, but I do know of at least one racing fleet that uses leeboards primarily or completely. That is the ACA sailing canoes.
The serious racers almost always choose a single leeboard that is not shifted from side to side, as opposed to a pair on either side.

Bob

brucehallman

03-14-2011, 04:37 PM

DGentry

03-14-2011, 05:16 PM

Got it. I grant you that those choose dagger boards. Might the explanation also be that they are interested in the ease of handling? Shifting a leeboard from side to side and tending it underway is a nuisance compared with lifting a daggerboard. Racers are also interested in time saved due to simplicity of handling.

Having raced a whole lot of ridiculously complicated, dedicated racing sail boats, including a few with twin bilgeboards, I do not agree with the gist of this.

Bruce, are you just trying to convince us that leeboards have their place? I believe most of us already accept that concept.

But, I daresay you aren't going to convince us that leeboards are just as efficient as bilge/center/daggerboards - especially when we can see that millions and millions of dollars in scientific research are being spent to produce even as little as 1/3 of a knot of superior boatspeed in racing sailboats - and none of that research is pointing to leeboards.

As for sailing canoes - there are a number of different classes. Some use basic "paddling canoe" hulls, and those are used with leeboards. Dedicated sailing canoes, which evolved using whatever they wanted, do not use leeboards.

brucehallman

03-14-2011, 06:12 PM

Bruce, are you just trying to...But, I daresay you aren't going to convince us that leeboards are just as efficient as bilge/center/daggerboards - especially when we can see that millions and millions of dollars in scientific research

I am just trying to tease out the reason why. As before, the zillion dollar racers are racing per their own contrived race rules designed over the years with an incidental purpose of being expensive to price ordinary people out of their club.

Honestly, I wonder if it is the 'wanna-be' like those rich guys that points developers of mainstream boats to make the choices that they do. Trying to marketing a boat that doesn't have the influences of the rich-guy boats is suicide. How else to explain the overwhelming prevalence of the Marconi sloop? You can't tell me that Marconi sloops are the best choice for 95% of the sailing public. The Marconi sloop is the way it is because of the boat market mimicking racers, to customers who are not racers.

You mention "millions of dollars in scientific research". I like reading that kind of stuff. Where can I read it? Have you actually read it, or are you saying that it must exist behind the curtain somewhere.

I am questioning whether 'some of this the big guys do it, therefore it must be scientific', amounts to the Wizard of Oz behind the curtain?

DGentry

03-14-2011, 06:39 PM

You mention "millions of dollars in scientific research". I like reading that kind of stuff. Where can I read it? Have you actually read it, or are you saying that it must exist behind the curtain somewhere.

I am questioning whether 'some of this the big guys do it, therefore it must be scientific', amounts to the Wizard of Oz behind the curtain?

Are you being disingenuous? It's astoundingly easy to find this stuff, especially nowadays when it's only a click away, rather than a drive to a major university library. Simply google "America's Cup" if you want to read up on research that has been, and is being, done and dollars being spent. Maybe try googling "tank testing," too, or "hydrodynamics." After you're done with that, in a few years, go over to SailingAnarchy.com to see the latest and greatest in racing developments, as they happen.

Honestly, I wonder if it is the 'wanna-be' like those rich guys that points developers of mainstream boats to make the choices that they do. Trying to marketing a boat that doesn't have the influences of the rich-guy boats is suicide. How else to explain the overwhelming prevalence of the Marconi sloop? You can't tell me that Marconi sloops are the best choice for 95% of the sailing public. The Marconi sloop is the way it is because of the boat market mimicking racers, to customers who are not racers..
Well, some of that is a bit off subject, but I agree that a lot of boats are probably not designed with leeboards in large part because leeboards are not mainstream. It seems likely though, to me, that performance considerations might often have come into play, as well.

brucehallman

03-14-2011, 06:50 PM

Are you being disingenuous?

I asked you if you had read the research, and you don't answer and instead ask if I am disingenuous?

Sorry, email doesn't convey context much. This goes back to earlier discussion where I tried to Google to find research on low velocity surface piercing foils (AKA, leeboards). I did find that the surface piercing foil research seemed to all be high velocity stuff from hydroplanes. Do you know of low velocity surface piercing foil tank research (http://www.google.com/search?q=tank+testing&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#sclient=psy&hl=en&client=firefox-a&hs=cII&rls=org.mozilla:en-US%3Aofficial&q=%22tank+testing%22+%22surface+piercing+foils%22&aq=&aqi=&aql=&oq=&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=23685193ae1ce66f)? Google can be like a needle in a haystack sometimes. Has there been scientific research of low velocity surface piercing foils, leeboards?

See the second paper in this google search (http://www.google.com/search?q=low+velocity+surface+piercing+foils&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a), which says "The surface piercing foil will not experience cavitation at low speed". Cavitation seems to the problem with surface piercing foils, right? Without cavitation, how much of a problem is there?

DGentry

03-14-2011, 07:37 PM

Have you actually read it, or are you saying that it must exist behind the curtain somewhere.

Context is hard in text, yes. To me, with the later Oz reference, this was perceived as snarky, rather than sincere, and not in need of a direct answer. In any case, I'll say that I have read some stuff, and that it was dry and long ago. Relevant to this discussion, at least as far as I care to go into it anymore, we'll just go with the magic curtain. I absolutely do not claim to be an authority on what research has been done or what it says. I'm currently happy enough to read in Sailing World, the A-class website, the BMW Oracle site, etc, etc that some research or testing was done, and to believe that they did it, and then see the real world results.

If you can't narrow down answers on the net, perhaps there's a naval architect or university you could ask your questions of.

brucehallman

03-14-2011, 10:30 PM

If you can't narrow down answers on the net, perhaps there's a naval architect or university you could ask your questions of.

The question is: Why are leeboards deficient?

The consensus answer here is "we know they just are", "everybody knows they are deficient", "I read it a long time ago", "I had a bad experience with leeboards, so they are deficient", "the big boy racers don't use them, they must be deficient".

All those answers remind me of the Wizard of Oz, "never mind the man behind the curtain". Pardon me if I am skeptical of Conventional Wisdom.

I will try to find a copy of the Bethwaite book, maybe it has the answer. It is a little expensive to buy, I will try the library.

slidercat

03-14-2011, 10:59 PM

See the second paper in this google search (http://www.google.com/search?q=low+velocity+surface+piercing+foils&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a), which says "The surface piercing foil will not experience cavitation at low speed". Cavitation seems to the problem with surface piercing foils, right? Without cavitation, how much of a problem is there?

You're confusing cavitation with ventilation. Two different phenomena. There are other issues as well, including some you'll find out about if you ever read that Bethwaite book. Here's one-- foils run in chaotic turbulence, as is found in both leeboards and outboard rudders, have much more drag than they would if run in clean water. This is why serious racing boat usually have inboard rudders, despite the practical disadvantages. It's also why gybing boards have not proven to be as effective as their proponents hoped.

Bethwaite demonstrated this by towing surface-piercing foils behind a powerboat at sailing speeds.

brucehallman

03-15-2011, 09:39 AM

You're confusing cavitation with ventilation. ... foils run in chaotic turbulence

I Googled it and see the difference, though neither do I see ventilation as being a problem for low speed leeboards, (as with an eight foot pram). I don't recalling seeing or hearing ventilation at low speeds with the leeboards I have used. If low speed ventilation is causing the "lack of end stopped foil" problem, LOESF, shouldn't it be visible? I am trying to understand the physics of why leeboards are deficient.

I thought that well functioning foils did not experience chaotic turbulence (IOW, they experienced laminar flow instead of chaotic flow), and that non-functioning foils did experience chaotic turbulence.

Perhaps the problem is that a few inches of the foil near the water surface has interrupted lamilar flow due to introduced turbulence from the air water interface. That might be occurring, but I have a hard time visualizing that this seemingly negligible drag difference would amount to a difference you reported as being "night and day".

slidercat

03-15-2011, 10:17 AM

brucehallman

03-15-2011, 11:08 AM

There's nothing magic about foils. They can't turn turbulent water into smoothly flowing water. They have to operate in the environment they find. By the way, the NACA 4 digit series I use in my boats is a turbulent flow section, not a laminar flow section, but that's another subject entirely.

I'll make one last try. Bolger understood the concept of tip losses. That's why he put fences on the bottom of his rudders sometimes. So if he thought it was worthwhile to do something about these losses, when they occurred at the bottom of a foil, then why can't you see the value of doing something about these losses at the top of the foil?

I think you misunderstand Bolger and rudder endplates.

Water flows across the bottom edge of a rudder decreases its effectiveness. Water does does not flow across the top of a leeboard.

I also think you might be mixing up the issue of effectiveness of the rudder, with the issue of the drag of the rudder.

It seems obvious that end plates on rudders increases the rudder drag. (Consider that both wetted area is increased and it goes from having one turbulent tip, to two turbulent tips when you add an end plate.) Added drag from an endplate is deemed worthwhile in exchange for added effectiveness in a shoal draft rudder.

Bolger's endplate were nothing more than a device to make a shallow rudder as effective as a deep rudder might be when in a shoal draft boat you could not afford a deep rudder.

Also, I am not sure you understand foils. Put differently, a foil with "laminar flow" is the same thing as saying that the foil is "streamline". On a molecular level looking at a 2D slice of water, streamline flow involves layers of water flowing in parallel streamline lines to the adjacent layers. Parallel lines are 'laminar'.

slidercat

03-15-2011, 12:57 PM

Sigh.

Bruce, clearly your knowledge of foils is very superficial. Google "foils" and "turbulent flow sections" and "laminar flow sections." The NACA 4 digit foils I'm using in my boats, NACA 0009 for boards and NACA 0012 for rudders, are known as turbulent flow sections. These are rather ancient in technological terms, designed way back in the early 20th century. Modern highspeed boats often use laminar flow sections, which are faster but have drawbacks for certain applications. The 6-series foils are examples of laminar flow sections.

Put differently, these are different shapes.

I tried, but I'm done.

dredbob

03-15-2011, 01:44 PM

Would you know if the ACA racing canoes are prohibited from using dagger boards?

I checked the rules and for the Open Canoe classes, only leeboards are allowed.

Bob

brucehallman

03-15-2011, 01:48 PM

Google "foils" and "turbulent flow sections" and "laminar flow sections." Put differently, these are different shapes.

Googled. (http://www.google.com/search?q=%22foils%22+and+%22turbulent+flow+section s%22&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#sclient=psy&hl=en&client=firefox-a&hs=clu&rls=org.mozilla:en-US%3Aofficial&q=foils++%22turbulent+flow+sections%22+%22laminar+ flow+sections%22&aq=f&aqi=&aql=&oq=&pbx=1&bav=on.2,or.r_gc.r_pw.&fp=f65aa1d3f8dcd1fc)

I will look elsewhere for an explanation of why leeboards are deficient. If it has to do with a "lack of end stopped foil" problem, what is the physics of this problem? It isn't the 4 Digit NACA foil issue, that is a diversion it seems.

Canoez

03-15-2011, 01:58 PM

brucehallman

03-15-2011, 02:20 PM

Bruce, I think that James hit on this much earlier in the thread. It all happens at the air/water interface for the foil. It has little to do with the part of the foil that is fully in the air or fully in the water. As my college fluids professor remarked when we were discussing, of all things, small boats, he said that planning for the shape of hulls is difficult because at the infinitely thin air/water boundary interface the flow behaves very much like supersonic flow does for jet fighters, but that the shape is designed for relatively low speed flow (i.e. NACA 1000 series sections in the case of a foil). I don't fully understand the phenomenon, but he said that this thin section can rob a significant amount of energy from even a well designed hull.

Thanks, that insight is helpful. While cavitation or ventilation aren't making sense to me at these velocities, I can imagine that shock wave at the air-water boundary might. I wonder if anyone has experimented with diamond shaped board (http://en.wikipedia.org/wiki/Swept_wing#Supersonic_behavior) at the water interface, and transitioning to a foil shaped board below water? I can see that the bulge of water in front of a leeboard is analogous to a shock wave, and that bulge obviously takes energy out of the equation. Making the edges at the waterline 'knife edged' (diamond shape the board) would reduce a shock wave bulge in the air-water surface. This begs the question: why isn't the shape of the entire hull analogous to the shape of a board?

Canoez

03-15-2011, 02:42 PM

Laminar flow again.

Things don't necessarily scale the way you might think - has a bit to do with Reynold's number and flow. We were trying to model the dive planes for the SSN 21 project but were working with about a 3" chord length for something that is like, 30' chord length. We had to make these strange features to adhere to the surface to "trip" the boundary layer on the model to make it behave like the full scale article.

To be truly efficient, your diamond shape would need to be able almost "ripple" to move to be just at that infinitely thin irregular surface that is the air-water interface. I think the practicality of making that work is why things are the way they are. Also to expand on the previous leeboard/centerboard discussion, the centerboard doesn't have that that air/water interface to rob energy- at least not in the water flowing over the hull. You've got the effects that happen at the juncture of the centerboard and the hull, but that's all underwater. (usually!) The University of Virginia did some work on a strake-dillet design at the junction of sail and hull to improve the losses at that juncture, too. I only saw pictures of the mock-ups and don't know the ultimate results.

Nicholas Scheuer

03-15-2011, 02:56 PM

For what it's worth, amidst this "highly technical discussion" (anything involving Reynolds numbers) let me offer a couple of anecdotal facts.

The single Shearwater Yawl that is equipped with NACA 0009 assymetric foils for leeboards used to sail circles around my Shearwater when sh was fitted with original style (from the builder) flat leeboards. That boat was both faster and more weatherly than my boat, buy a significant margin.

However after my boat was fitted with a pair of assymetric LAMINAR FLOW FOILS designed by Tom Speer, a Hydrodynamic Engineer (and tried on his sailing canoe) my boat is virtually equal in performance to the NACA foil Shearwater in both speed and weatherliness. In fact, since the other Shearwater incorporates some refinements in her rig which my boat lacks, I might arguably claim my Laminar Flow foils to be a bit superior.

An improvement in sailing performance using the same pair of boats in a variety of winds should be fairly convincing.

Moby Nick

Canoez

03-15-2011, 03:08 PM

Well, yes that should be true, all other things being equal. If you have truly laminar flow, you aren't shedding little downstream vortices which would rob the hull of energy as you were with the flat leeboards. These downstream vortices are an indication of drag that is robbing you of energy and therefore speed. (Note some symmetric NACA foils would be considered laminar flow foils - it just depends on the "fluid".)

If you then took a laminar flow board that was a centerboard, it should be even faster still as you're not losing energy at the air/water interface as well that is present in a leeboard.

As was also noted earlier, fences, winglets and foils at the end of a foil (airfoil or hydrofoil) also improve flow to reduce wingtip vortices as seen in this image (Ever note the little winglets at the end of the wing of a newer 747?):

http://cdn-www.airliners.net/aviation-photos/photos/5/0/1/1091105.jpg

brucehallman

03-15-2011, 03:23 PM

NACA 0009 assymetric foils for leeboards

Hmmm, am I understanding this right? Airplane wing-like. The port side leeboard gives lift to starboard, and the starboard leeboard is the mirror image giving lift to port?

Canoez

03-15-2011, 03:30 PM

Not lift and not airplane like - simply a symmetrical streamlined shape that provides for laminar flow at the designed hull speeds. (bottom sketch)

http://www.aerospaceweb.org/question/aerodynamics/sports/sphere-airfoil-comparison.jpg

If the flow is truly laminar, there is no flow separation behind the foil as shown.

James McMullen

03-15-2011, 03:43 PM

The angle of attack created by leeway also creates lift. The rudder angle and the foil or keel work in concert, whether or not they are attached to one another as in a full keel, or two (or more) separate foils. This is one of the reasons why lee helm is so very detrimental--it's creating an angle of attack the increases lift to leeward--very unfortunate!

A symmetrical foil is a compromise to allow use on either tack. Whether the extra expense, penalty weight and complication of twinned asymmetric bilgeboards is worth it is debatable. It may indeed be worthwhile for leeboards to attempt to minimise their known deficiencies.

I don't remember whether or not it specifically mentioned leeboards or not, but this book (http://www.amazon.com/Physics-Sailing-Explained-Bryon-Anderson/dp/1574091700) I thought was very interesting for us math geeks. It gives the equations and formulas as well as the theories behind wave resistance and vortex creation and parasitic drag and all that for both sails and underwater appendages. I do remember there was a whole bunch of stuff about keels. You'll need to have a calculator at hand when you read it if you can't do cube roots in yer head though. :D This should answer an awful lot of questions along with their theoretical underpinnings for you, Bruce.

The Physics of Sailing Explained by Bryon D. ANderson

http://ecx.images-amazon.com/images/I/51J7HZ2WTML._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA300_SH20_OU01_.jpg

brucehallman

03-15-2011, 04:09 PM

The angle of attack created by leeway also creates lift. It may indeed be worthwhile for leeboards to attempt to minimise their known deficiencies.

It seems obvious that if you are going to have a left and right leeboard anyway, that the foil shape might as well also be left handed and right handed. James, I still don't know what the physics behind these "known deficiences" might be. Canoez suggestion that it might be akin to a supersonic boundary at the air-water interface is the most plausible physical reason I have heard yet.

If the supersonic boundary model is good, then you would be seeing a pressure wave develop on the leading and trailing edge of the leeboard. Indeed, IIRC, you do see this pressure wave on leeboards. Like with supersonic airplane wings, this supersonic pressure wave is "ideally" addressed with a diamond shaped wing. Might a diamond shaped leeboard (at the air-water boundary zone) mitigate the "known deficiency" of leeboards? It might.

Nicholas Scheuer

03-15-2011, 04:27 PM

To me it is a "No Brainer" (subject of another thread in the bilge) that leeboards might as well be assymetric, as there will almost always be a pair.

I've fund that my assymetric Laminar Flow leeboards (dramatically reduce leeway by the simple test of sailing a one or two mile close-hauled course toward shore, sighting along mast and forestay toward an object on shore, then maintaining that object "in my sights" as we sail along. Usually there is a bit of leeway, but WAY LESS than in the old days using flat leeboards.

As for a semetric NACA foil generating lift due to leeway, LEEWAY is what I was trying to ELIMINATE.

As for a NSACA foil being "laminar flow", someone will have to explain that. In my experience, laminar flow foils have their thickest chord much farther aft than NACA-00 foils. Laminar flow foils also feature sharper curves at the leading edge.

My Speer foil does also feature a slight hollow curve on the high pressure (outboard) side.

Moby Nick

boat fan

03-15-2011, 04:40 PM

Might a diamond shaped leeboard (at the air-water boundary zone) mitigate the "known deficiency" of leeboards? It might. Morphing that ( diamond ) shape to the submerged foil shape will cause considerable "air ingestion " or cavitation.
The boat`s non uniform vertical motion , as it moves over the water`s surface complicates the situation considerably.

brucehallman

03-15-2011, 05:07 PM

Morphing that ( diamond ) shape to the submerged foil shape will cause considerable "air ingestion " or cavitation.
The boat`s non uniform vertical motion , as it moves over the water`s surface complicates the situation considerably.

Perhaps true. Assuming that James' "end stopped foil" hypothesis is the true reason for the alleged deficiency of leeboards.

John Meachen

03-15-2011, 05:46 PM

I think you misunderstand Bolger and rudder endplates.

Water flows across the bottom edge of a rudder decreases its effectiveness. Water does does not flow across the top of a leeboard.

I also think you might be mixing up the issue of effectiveness of the rudder, with the issue of the drag of the rudder.

It seems obvious that end plates on rudders increases the rudder drag. (Consider that both wetted area is increased and it goes from having one turbulent tip, to two turbulent tips when you add an end plate.) Added drag from an endplate is deemed worthwhile in exchange for added effectiveness in a shoal draft rudder.

Bolger's endplate were nothing more than a device to make a shallow rudder as effective as a deep rudder might be when in a shoal draft boat you could not afford a deep rudder.
.
The point made in this post is probably correct as far as drag from skin friction is concerned but almost certainly wrong when the matter of induced drag is considered.If there is any amount of flow migration from the high pressure side to the low pressure side of a foil,there is usually a vortex resulting.Anybody who has ever stirred a tub of water will have some idea of the energy required to cause such a circulation and it can only come from one source-the energy the rig is producing-and if you subtract this energy from the total amount available it is inevitable that the boat will be travelling more slowly.The subsequent post relating to winglets on aircraft wings are concerned with the same phenomenon and the high percentage of recent aircraft with such features incorporated after extensive CFD and/or wind tunnel testing should be an irrefutable statement as to their effectiveness.The bean counters would never allow the inclusion of inefficient features.

brucehallman

03-15-2011, 05:59 PM

...almost certainly wrong when the matter of induced drag is considered.If there is any amount of flow migration from the high pressure side to the low pressure side of a foil,there is usually a vortex resulting.

I can easily visualize pressure differential on one side of an end plated rudder than the other sides. (port, bottom and starboard) In principle, there should be water drawn around the plate edges when this pressure attempts to equalize, and vortices when this happens. Isn't this induced drag?

boat fan

03-15-2011, 07:35 PM

James McMullen

03-15-2011, 08:03 PM

boat fan

03-15-2011, 08:12 PM

That would be great to see.

Poly tarp sails would be ok , as long as they were identical.

In the end ,it`s all about using the boats , after all.

I wished I was over your side of the pond.......

Nicholas Scheuer

03-15-2011, 08:26 PM

That PD Racer bet might be worth taking, provided the leeboard boat can utilize assymetric foil leeboards, while the daggerboard may use a symetric foil that is not equipped to pivot on a logitudinal axis.

Everyone MUST be aware by now that a PD Racer kicks up WAY more disturbance at the square bow than any "surface piercing" leeboard might.

Moby Nick

DGentry

03-15-2011, 08:45 PM

Having done essentially that very experiment, I'll take that bet. I'm willing to publicly put my money where my mouth is right here. I'll put up $100 right now on the DB boat beating the leeboard boat. Let's make it a best 4 out of 7 races to dampen out noise and get a more accurate sample size.
(And I'll bet another $50 that it doesn't take the whole series for the DB to come out on top!)

Who's up for it? We just need someone to step up and build a couple of otherwise similar boats. PD Racers? Goat Island Skiffs? Any takers?

Sounds fun, but, realistically, if there's an appreciable difference in racing skills, the better skipper could/should win every race, no matter which boat he or she is in. In racing, there are just way too many variables besides foils.
Better to move back to the two boat testing idea, side by side with identical settings.

James McMullen

03-15-2011, 09:36 PM

Swap skippers and boats, even swap rigs to even out differences in sail shape and cut. Do a whole series to get a good sample size. My money is on the DB.

But hey, I'm not the one who needs convincing that the "conventional wisdom" is some big conspiracy of cluelessness. Since both scientific theory and the organic evolution of small boat design both seem to favor the non-surface-piercing foil, the smart money is on the null hypothesis.

brucehallman

03-15-2011, 10:50 PM

Since both scientific theory and the organic evolution of small boat design both seem to favor the non-surface-piercing foil, the smart money is on the null hypothesis.

It is hard to believe that water tests have not been done. That is what I would like do (or see). Tease out exactly why, (or not) leeboards are deficient. Design some experiments, and drag them through the water with a scale measuring the forces.

slidercat

03-15-2011, 11:19 PM

Ray , I think your claim that lee boards are "an abomination " may be a little strong , IMHO.

It would be nice to see two identical Bolger Light schooners , or Michalak Lagunas racing , boat for boat.Swap crews too.
One fitted with the Michalak lee board , the other with a Bolger style dagger board.

Personally , I would have to give slightly shorter odds to the dagger board boat as the favorite ,but I`m not sure that one would be much slower than the other.

You're probably right. Did I call leeboards an abomination in this thread, or was that in some other rant?

I would love to see a couple of Lagunas with those mods go at it. The leeboard on the Laguna is pretty clever. I don't think Jim Michalak gets enough credit from Bolger acolytes. I seem to remember Bolger referring to Michalak as his go-to guy when engineering issues came up that were above his paygrade. Heresy, I know, but I tend to like Michalak's designs a little better than Bolger's, though maybe that's just my inner engineer trying to get out.

Here's a candidate for a trial:

http://slidercat.com/blog/wordpress/wp-content/uploads/2011/03/slipper02.jpg

Right now, the only leeway prevention devices are the rudders, and the boat still goes to windward sort of okay. I've actually considered putting a central foil on the little boat-- which would be just like a leeboard, but without the boundary layer issues, if it were far enough forward to be in clean water.

slidercat

03-15-2011, 11:36 PM

Sounds fun, but, realistically, if there's an appreciable difference in racing skills, the better skipper could/should win every race, no matter which boat he or she is in. In racing, there are just way too many variables besides foils.
Better to move back to the two boat testing idea, side by side with identical settings.

Have you seen that old test, sponsored by a philanthropic organization interested in helping third-world fisherfolk? They were trying to determine the best rig for a sail-powered fishing boat. The test is 30 or 40 years old, judging by the somewhat old-fashioned rig they called Bermudan, but they got two identical catamarans (on the theory that these boats were more sensitive to small differences in rig efficiency) and tested three "archaic" rigs against the Bermudan-- gaff, sprit, and lateen. To their great surprise, the sprit rig was by far the most weatherly.

Anyway, the two boats were sailed side-by-side in the Solent, so that conditions were the same for both crews. The crews were exchanged to level out differences in skill, and while the sailors were encouraged to add sail-control bits, they were also instructed to take a sort of reverse-racing attitude to the other boat-- no covering, for example.

So it would be possible, I think.

Probably a much simpler test would be to have two identical foils, one end-stopped by a solid chunk of wood to simulate a hull, and one supported on either side by small pontoons of the same area as the simulated hull, so that it operated in surface-piercing mode. You could tow both behind a power boat and see which developed more drag.

boat fan

03-16-2011, 12:26 AM

Hello Ray .

Actually , you are right , you called lee boards an abomination on another thread...

All agreed about Michalak`s work too.
I actually thought that his Laguna gave a very credible account for itself at the Duckworks events. Its performance got noticed .

I guess that was my point really , not disputing that dagger boards are the better way , just saying you can end up with a half decent boat , with care , either way.

How is the new cat project going ?

coucal

03-16-2011, 01:59 AM

Frank Bethwaite's tow testing for surface-piercing rudders was complicated by the presence of a fine-grain turbulance layer from the hull forward.

But he was adamant that there is an adverse surface piercing effect for foils.

Fully immersed bodies (he argues) recover much of their energy; the energy spent in the acceleration of water around the front of a submerged foil is recovered as "..it slows and stops, its pressure rises, and this increased pressure forces the tapered after part of the shape forward. In this way, the tapered after part of the (fully immersed) streamline shape recovers almost all the energy which the forward half expended in accelerating the water 'backwards'. The greater the energy recovery, the less the drag"
From 19.4 Modern Foil Development, High Performance Sailing

In 19.6 The Drag of Surface Piercing Foils in Wake , he argues that this pressure energy is lost in surface piercing foils due to the escape of energy at the surface of the water as spray (or the formation of waves as the water is pushed upwards & outwards at the top of the foil).

brucehallman

03-16-2011, 10:01 AM

Frank Bethwaite's ......argues that this pressure energy is lost... as the water is pushed upwards & outwards at the top of the foil).

If that is true, then a surface piercing foil could be developed which mitigates this leading edge pressure wave. This is remarkably similar to the problem with leading edges of wings in supersonic flight. The ideal shaped wing for supersonic flight is knife edged fore and aft, and diamond shaped. I would be fascinated to see a test of this in water, where about 12 inches of the leeboard (at the air-water boundary line) is diamond shaped, and the deeper portion of the board is foil shaped to give left hand or right hand lift to windward for each side's leeboard. I could imagine this could give an advantage to a leeboarded boat, relative to a neutral lift single centerline foil.

slidercat

03-16-2011, 10:44 AM

Frank Bethwaite's tow testing for surface-piercing rudders was complicated by the presence of a fine-grain turbulance layer from the hull forward.

That's true, but isn't this the situation with a leeboard, or for that matter, a transom-hung rudder? They both must operate within this turbulent boundary layer.

What struck me about this research is that the differences in drag were far from subtle-- drag increased to several times the calculated drag. If I recall correctly, calculated drag was supposed to be one-sixth of a pound, but actual measured drag was over a pound.

slidercat

03-16-2011, 10:54 AM

All agreed about Michalak`s work too.
I actually thought that his Laguna gave a very credible account for itself at the Duckworks events. Its performance got noticed .

I guess that was my point really , not disputing that dagger boards are the better way , just saying you can end up with a half decent boat , with care , either way.

How is the new cat project going ?

I really like the Laguna. John Wright stopped by the house on his way to the EC a couple weeks ago, and showed me his boat. Very impressive. The leeboard was pretty brilliant-- something only an inventive engineer would think of, I guess. The board is pivoted low on the hull, and the top is restrained by an outwale-- makes much more sense than the usual approach. If I were going to put a leeboard on something, I'd definitely steal that idea.

New cat is progressing slowly. I'm trying to get a free day to start cutting bulkheads.

Dan St Gean

03-16-2011, 01:23 PM

Ray,
Why not conduct a super easy and low cost experiment with Slipper? You could build in a daggerboard trunk on one hull using no more than a pound of material, and you could build a Michalek leeboard using the same board. In fact, other than the small hassle of the leeboard mount possibly becoming obsolete, you're out nothing. On top of it you could do some GPS tracks to windward before any board as a baseline, leeboard, then daggerboard. Mighty fine thought expeiment IMHO. One other thing to think about is that there have been numerous cats with no board--but most were v hulled. Teh more recent examples have been the Dart, Escape playcat, Wave and Getaway. All these seem to rely on rudders as foils for their windward bite more than their hull shape (although it does play a part). What would be more interesting is your dory shaped hulls should be poor windward foils--really revealing the efficiency of good rudders OR the need for a dagger/center/leeboard.

Dan

brucehallman

03-16-2011, 02:51 PM

... a leeboard, or for that matter, a transom-hung rudder? They both must operate within this turbulent boundary layer.

How thick is this boundary layer? I can see for sure that the water behind the hull has been stirred up. But the water to the side of a boat? Perhaps an inch or so, but depending on the leeboard mount detailing, it is imaginable that the leeboard could be extended to relatively clean water out to the side.

I still am imagining the competitive advantage potential with the use of a matched set finely shaped leeboard asymmetrical foils, one of left hand lift and the other of right hand lift.

Nicholas Scheuer

03-16-2011, 03:22 PM

Anyone desiring to judge the maximum benefit of a leeboard should try an assymetric foil (even if it's only one). It will significantly reduce drag, while minimizing leeway.

Of course such a leeboard will take at least twenty times the hours to construct, compared to a flat one, because true chord curves are critical.

IMHO a flat leeboard is just an exercise in antiquity.

Moby Nick

brucehallman

03-16-2011, 03:42 PM

an assymetric foil ...will take at least twenty times the hours to construct..,

Perhaps it could be quicker. If you had a series of CNC cut templates, you could quickly rough out the shape using a angle grinder with 36 grit disks aiming to overcut by 1/4 inch, then skim on some Bondo for the final shape, sanding it with finer grit to exactly match the templates. This could be pretty quick work, assuming you got the templates right using CADD, this could go relatively quick.

Nicholas Scheuer

03-16-2011, 05:14 PM

My Shearwater's leeboards were made with fixtures designed to guide a hand router over the desired chord of the wooden core, minus the planned thickness of the fiberglass sheath. After the fiberglass sheath had been applied, the final shape was sanded using another set of CAD-designed templets for the high and low pressure sides. None of this was quick. I did get a pair of leeboards shaped exactly like the CAD drawing.

The first board has a Balsa core applied to both sides of 1/2" marine plywood. The second board has a Cedar core applied to 1/2" plywood. The Cedar weighs little more than the Balsa, and was not only easier to work with, but soaked up less Epoxy resin.

Moby Nick

brucehallman

03-16-2011, 05:20 PM

...shaped exactly...

Exactly? What kind of tolerance is appropriate. +/- ten millimeters? one millimeter? a hundred microns? Does the theoretical and optimal foil shape vary with anticipated speed of the boat? Hence, does the optimal foil shape have to be a compromise based on "anticipated speed"?

Nicholas Scheuer

03-16-2011, 05:56 PM

I believe my foils, having a chord length of 20", are correct within one milimeter. Furthermore, the finish is very smooth, having used "filler primer" and lots of hand long-board sanding.

I know nothing about how foil shape relates to speed. I needed a pair of new leeboards, and did not want to build another set of flat ones. My Shearwater is 28-ft LOA, displaces just under 3000-lbs, and has a maximum speed on a reach of right near 7 knots.

Moby Nick

slidercat

03-16-2011, 11:25 PM

Dan St Gean

03-17-2011, 09:48 AM

Dan, I imagine that would be interesting for Bruce. But considering my personal experience with leeboards I'm just not willing to spend any time on an experiment that seems pointless to me. If there were any real controversy on this matter, it might make a more interesting experiment for me, but I doubt you can find a single naval architect who believes that leeboards are as efficient as daggerboards. That doesn't mean they aren't a good compromise sometimes.

Right now I'm thinking either LAR keels or a centerboard in one hull. We went out again today and this time the GPS was working. The tracks are nothing to write home about, so I'll have be doing something. I'm tending toward the centerboard.

Just sayin'. Unless you have a pretty well shaped centerboard, it's likely to be no better than the leeboard unless employing flaps to close the slot. Drag is drag. Why not a dagger? You've had good experience with it in Slider in exactly the same waters. I personally like the kick up feature of centerboards--I'm going to have to decide on what to do with my Tamanu cat pretty soon.

Dan

slidercat

03-17-2011, 10:07 AM

Dan, you're probably right. One of the issues is structural. The boat is very lightly constructed (cartoppable) so my thought was that a centerboard would be less strain on the structure in the event of a grounding. Slider's built like a tank-- probably a good bit heavier than she needs to be. Maybe I just need to make the daggerboard a bit delicate, so it snaps off instead of the case rupturing. Have you seen these guys:

http://www.flyingfoam.com/QuickOrderDetails.html

I was thinking about them when Nick was talking about the effort he had to put into his assymetric leeboards. For a smallish boat, you could order a 72" span which would give you 2 36" cores, glass them onto ply stubs, and there you go. Effortless perfect foils.

James McMullen

03-17-2011, 10:09 AM

Just sayin'. Unless you have a pretty well shaped centerboard, it's likely to be no better than the leeboard unless employing flaps to close the slot.

Not my experience. Mind you, I'm not saying that a centerboard won't benefit from being carefully shaped or that flaps to close the slot aren't beneficial. But reducing drag is not necessarily the same thing as increasing lift, and it is the degradation of lift that is the primary disadvantage of a surface piercing foil.

You can get lift from a symmetrical foil or even just a completely flat plate due to nothing more than angle of attack. There's plenty of boats that have no particular shaping at all--just a steel plate, or a big timber keel in the case of some very traditional boats. They'll go to windward too, though of course they are generally not as efficient at doing so. An asymmetric foil is an attempt to increase lift even further than the component associated with angle of attack. For sails this is quite important. For underwater appendages there are many other structural and operational constraints that seem to limit the practical utility of it compared to the expense and complication.

slidercat

03-17-2011, 10:48 AM

Surely someone must have tried assymetric bilgeboards. I wonder why they aren't more common, since Nick's Shearwater is much more weatherly with assymetric leeboards. Racers wouldn't mind fiddling with boards at every tack, if they could get an extra degree or two out of it.

In fact one of my design heroes, Thomas Firth Jones, did a pretty goofy thing, putting a single daggerboard in the float of his Trimaran Hummingbird. When racing, one of the crew was detailed to pull the board and trot across the decks to drop it in the other case on the far float. When he came to draw his little Night Heron tri, he used a single case in the cabin. He rationalized this by saying Hummingbird was a long-distance cruiser that might stay on one tack for days, but Night Heron would be used mostly in restricted waters where tacking was more frequent. I think maybe it was a case of sadder but wiser.

OOps. Google is my friend. Scows use assymetric bilgeboards very effectively. Now I'm wondering why the idea hasn't spread to sportboats.

Oops again. here's an interesting piece on twin keels and their various incarnations:

http://www.docstoc.com/docs/50407974/Twin-Keel-Yachts-%E2%80%93-Development-o

It reads a bit like a promotion for the UK firm Hunter, but lots of interesting info, all the same.

Dan St Gean

03-17-2011, 11:37 AM

Ray,

I've seen them before, but I think I'll be bookmarking that site. Thanks! My next rudder will be an order form them + a bit of carbon.

As for daggers with intentional weak spots...they work as advertized as long as the boat's case is stronger than the board AND the board is still strong enough to do it's job. I sailed on a redesigned Tennant Tourissimo 10 that had breakaway daggerboards. They must have worked great because we flew upwind at 10-12 and off the wind up to 18. No breakage. However the owner forgot to pull one up and entering his tidal mud mooring, and it snapped off just like it was supposed to. Reattaching it and fairing it might be a pain though, so some sort of kick up sounds good to this shallow water sailor.

Dan

Nicholas Scheuer

03-17-2011, 11:46 AM

I know of one builder who has assymetric Bilge Boards in a racing boat on the market. I'll have to hunt around for the literature, but the boat is built in Minnesota.

I chatted over the phone with the builder a couple of years ago when discovering his literature and he said he was using NACA00 foils. He also said there was no question about the bilge boards promoting superior performance.

One obstacle to any of this is the "conventional wisdom" that anything other than a flat centerboard is not worth the bother due to a sailboat's relatively slow speed. Peter Duff, builder of my boat, is one who told me that.

Anyone who "pushes the envelope", in any discipline, will encounter plenty of contrary "conventional wisdom".

Moby Nick

Nicholas Scheuer

03-17-2011, 03:09 PM

FOUND THE LITERATURE! She's the Victory-38 Scow (V38 on her sail) and a brochure can be secured at www.VictoryByDesign.org (http://www.VictoryByDesign.org)

A bullet point list of features says "Sideboards: draw 5-ft (profile looks like a Spitfire wing), easily retractable, asymetrical". Another area of the literature says "Carbon Fiber Sideboards: The carbon fiber elliptical design provides not only lift, but gives a high strength-to-weight ratio."

There is not a lot of text in this brochure.

Moby Nick