Which naca profiles for sailingboats

[skaraborgcraft]

Always a lively debate. I will agree there is a potential performance increase in the use of NACA foils, but im also on the side of simplicity. Flat metal plates work as leeway prevention,but not as efficient as a NACA foil. As Ed has pointed out, rig and sails can make more of a difference to windward performance than plate alone,as can all the other aspects of hull shape,crew ability etc etc ad nausism. I have just made a set of foils,rudder and daggerboard for a dayboat,theres about 30% flat sides and the rest is tapered,thicker at the nose and shallow angle all the way to the back square edge. Took about 2 hours to do both with an electric plane and a disc sander. Im guessing it wont stall as soon as a flat plate, but it also wont have as much lift as a true NACA foil, but as its going in my own dayboat with a balanced lug rig i was not prepared to spend the extra hours creating a true foil. That does not mean i dont respect the years of work gone into foil research,but as an owner/builder,its up to me to decide on what i think is performance worthy in terms of time invested.
Another project will have a 0012 drop keel and a 0009 rudder section,but that project will be built as a performance boat. Cheers

[James McMullen]

James,

By choosing a low aspect lug rig, you've lost more to windward than my high aspect bermudan and choosing a flat plate.

My Tirrik will outpoint Rowan by 10 degrees.

Ed, I think you're totally exaggerating. I haven't sailed with a sloop-rigged Tirrik any time I can remember, but I've sailed in company with all sorts of sloop-rigged boats and I don't get particularly outpointed by any that aren't something like an Etchells or a Melges. 10 degrees is huge! And I don't just always sail in lapstrake character rowboats either, I ride the trapeze on a 505 or drive an eight-ton 36' yawl on occasion too, so I'm not just a single rig partisan. Rowan's got a high peaked and pretty high aspect lug foresail, and she points awfully good for a rowboat. I credit the foil for much of that.

I choose the lug rig for a sail and oar boat for a great many reasons--particularly handling and handiness--but that doesn't mean I don't enjoy wringing all the performance I can out of this boat. I am never content with low-performing boats of any shape or sort, be they paddled or rowed or sailed. Since tank testing has proven that a properly shaped foil will provide better performance than a flat plate, I think it's very much worthwhile to add it into the mix of all the other compromises and choices you have to make when chasing down an optimax solution for my own particular variety of boating.

I don't expect anyone to take my claims on faith or purely as an academic thought experiment. Rowan and I will be at Howard Rice's Small Boat show in Port Townsend on May 19 and again at the PT Wooden Boat Festival In September for any of you relatively local guys who would like an easy opportunity to check her out.

[Geoff C]

Here's what little I can contribute from an engineering point of view:

A typical flat plate will generate lift up to an angle of ~6 degrees depending on aspect ratio and other factors, then it will stall and give high drag. A NACA 0010 foil will go to ~12 degrees before it stalls, and an asymmetric foil will go to about 18 degrees. These numbers are for comparison purposes only, not hard and fast values.

To visualise why a flat plate will stall early, consider that there is a component of flow that travels from the high pressure region under the foil, up across the nose (leading edge) of the foil to the low pressure region above it. The thicker the foil, the easier it is for this flow component to travel across the front edge of the foil without separating. This is why NACA type foils are not sharp at the leading edge, and why thicker foils stall at a higher angle of incidence.

If the boat in question is set up with a moderate amount of weather helm, a flat plate center/ dagger/ leeboard often may not see more angle than it takes to stall it, assuming it is making a reasonable speed. The rudder, however, will be operating at a much higher slip angle as it works to counter the sails' attempts to make the boat round up into the wind, and will therefore generate most of the lift to windward. The rudder therefore should have a thicker chord than the board.

In terms of finish and profile accuracy, invest most of your time on the front 1/3 of the foil, since flow will inevitably become turbulent and separated as it passes along the surface.

[IanHowick]

My problem with the NACA-knows-best school of thought is based on where those foils come from.

NACA designed aerofoils (up to the 50s) for a medium which is compressible and of a very different density and viscosity to water. Wing designs for high-speed flight in a powered aircraft in more-or-less stable air would suggest that slavishly copying designs for low-speed sailing in choppy and moving water, each operating at quite different Reynold's numbers, are not optimal...

Andy

The NACA 00XX series was figured out a long time ago (1930's) when aeroplanes were small and slow. Airfoil deign has come a long way since then, but those old sections for small slow aeroplanes are in the range of Reynolds numbers for sailing boat foils, hence their widespread use.

Neil Pollock produced a foil section which is parallel sided with shaping on the leading and trailing edges that is much easier to shape than a naca section and very nearly as good. It saves the issues of tapering the thickness as the chord reduces towards the tip of the foil - thickness can stay the same, just the parallel sided section between the leading and trailing edge shaped sections is smaller. Here is a reference to the formula for calculating it: http://www.boat-links.com/foils.html (scroll down to section 2.3 Parallel Sided Foils)

Michael Storer uses these Pollock parallel sided foils, you could get the template by spending $20 on his OZ racer plans.

I've seen some research (by Frank Bethwaite, I think) referred to that compared foils shaped leading and trailing edge by hand and eye versus shaping by hand with the aid of good templates, all other things kept equal. I recall a measurable difference around a race course of perhaps 5%. Perhaps someone has a reference to this research?

Ian

[John Meachen]

[QUOTE



I've seen some research (by Frank Bethwaite, I think) referred to that compared foils shaped leading and trailing edge by hand and eye versus shaping by hand with the aid of good templates, all other things kept equal. I recall a measurable difference around a race course of perhaps 5%. Perhaps someone has a reference to this research?

Ian[/QUOTE]

I have seen many races in which the difference between placing well or not so well was half the length of a foredeck after 90 minutes racing.A 5% performance gain of just one element of the boat would be well worth having.Those with the willingness to learn how to use the free Xfoil program can develop their own foils.It takes a bit longer to learn to make those foils unless you have access to a CNC router.

[John B]

Foils are pretty. Its like looking at the front guard of an xk120.
Don't you just love lying down in the boatyard starin up at those lovely rudder profiles
... or, is it just me.

[Daniel Noyes]

Ian[/QUOTE]

I have seen many races in which the difference between placing well or not so well was half the length of a foredeck after 90 minutes racing.A 5% performance gain of just one element of the boat would be well worth having.Those with the willingness to learn how to use the free Xfoil program can develop their own foils.It takes a bit longer to learn to make those foils unless you have access to a CNC router.[/QUOTE]

Foils are pretty. Its like looking at the front guard of an xk120.
Don't you just love lying down in the boatyard starin up at those lovely rudder profiles
... or, is it just me.

the above are two excelent reasons to insist on precisely shaped NACA type foils for your sail boat...
1: You are involved in high stakes, cut throat, international calibre competition at the highest levels where a boat lenght of speed over many miles traveled is important.
2: Foils make you feel good about your boat.

I am someone who is a big fan of excelent performance, heck I take the extra 3 munites at the ramp to hook up a stay set because I find the lightness and stiffness of a stayed rig increases boat speed considerably. When designing the Ipswich Bay 18 I went with a Aluminum plate centerboard and a foil shaped rudder.

I have discussed the merits of foil vs. plate type boards with my father a ME grad of MIT and fuel controlls engineer for jet engines for 30+ yrs, he has also designed and built his own hover car and utility class racing boats, this is a guy who knows fluid flow in all it's intracicies and he has greatly informed my understanding of the subject.

[skaraborgcraft]

Im curious to know why James used the same NACA 0012 section for the board and rudder foil? Is your rudder somehow constrained to swing through a small arc that this profile wont stall? Cheers

[James McMullen]

Well, I was assured by my computer programmer friend that 0012 was the ne plus ultra and Holy Grail of all small boat foils, and besides, he already had the algorithm plugged into the CNC. Plus, there is such a thing as putting too fine a point on something. It's just a freakin' rowboat after all.

[Nicholas Scheuer]

The thicker the foil cross-section, the higher the number; 0012 is thicker than 0009, and the thicker section is more resistant to stall.

[kenjamin]

...It's just a freakin' rowboat after all.

Did you get hit by lightning or something??? Do you have a fever? Did you recently fall and hit your head? Whatever it was that caused you to make such a rash statement, I hope you overcome it and get back to your old self again soon. Who the heck am I going to argue with if you go mainstream on us???

[wtarzia]

Is there a good site from which you can print out foil shapes for use as templates? --Wade

[slidercat]

Wade, if you look around on Duckworks, there's a spreadsheet that allows you to plot and print out a profile of the 4-digit NACA foils.

Another possibility is to buy a model aircraft wing blank of foam then reinforce and glass it. That's what I did for a centerboard for my little cartop cat. The foam is just crappy styrofoam beadboard stuff, but the shape is extremely precise, and the blanks are pretty cheap. I got 2 wings, about 12 inches by 36 inches 0012 for less than 40 bucks. I just glassed the blank to a plywood piece that the pin went through, but you could probably make them adequately strong for general use by milling a couple of slots into them and filling the slots with carbon tow, then bagging the glass onto the blank.

[Nicholas Scheuer]

Thanks for the ip on Styrofoam Wings, slidercat! I may try one on a boat sometime.

ONE CAUTION, FOLKS!!! Styrofoam can be sheathed in EPOXY RESIN and fiberglass, BUT NOT POLYESTER RESIN. Polyester will disolve Styrofoam.

[wtarzia]

Very interesting, Ray, thanks. -- Wade

[Chris Ostlind]

the above are two excelent reasons to insist on precisely shaped NACA type foils for your sail boat...

1: You are involved in high stakes, cut throat, international calibre competition at the highest levels where a boat lenght of speed over many miles traveled is important.
2: Foils make you feel good about your boat.

Even the chunkiest cruising boats use foils with aero sections and they sure as hell aren't racing. Take some time and make a list of the boats in production that use shaped foil boards fore and aft. Now make a list of the production boats that use flat plates to do their fluid work. Give us an idea as to how the world's best boat builders continuously slap flat plate appendages on their boats when they must know by now that is all they need and the flat plates are a whole lot easier and cheaper to build. Somehow, I'm missing the connection between effective shaped foils in wide use by a huge variety of craft and the argument here that a Sanford and Son solution is just peachy. What's next guys...? poorly executed joinery that is filled with thickened epoxy and a smile in the direction of the customer while he is told that it works just as good as a properly made joint? Has America really stooped to that sort of process as a justification? And you wonder why we are running down the path to becoming a Second World nation. Bondo Nation

[John B]

How come we haven't done tubercles yet.
I say they're worth considering on a rudder because they delay stall and they're no good on a centreplate because you can't get the board up anymore.

[Nicholas Scheuer]

What's a "tubercles"?

[JimConlin]

What's a "tubercles"?

[Nicholas Scheuer]

Well, learn sumthing new every day. However, that's as far as it will go for me; Too "high performance" for boats I'm interested in. As for inability to drawing up a daggerboard, wouldn't sizing the slot to fit a tubercle, then having half of a terbucle exposed at the bottom of the slot fix that obstacle?

Would you offer a pronumciation key for "tubercle"? Do I start with "tube" (long U) or "tub" (short U)?

[Daniel Noyes]

Even the chunkiest cruising boats use foils with aero sections and they sure as hell aren't racing. Take some time and make a list of the boats in production that use shaped foil boards fore and aft. Now make a list of the production boats that use flat plates to do their fluid work. Give us an idea as to how the world's best boat builders continuously slap flat plate appendages on their boats when they must know by now that is all they need and the flat plates are a whole lot easier and cheaper to build. Somehow, I'm missing the connection between effective shaped foils in wide use by a huge variety of craft and the argument here that a Sanford and Son solution is just peachy. What's next guys...? poorly executed joinery that is filled with thickened epoxy and a smile in the direction of the customer while he is told that it works just as good as a properly made joint? Has America really stooped to that sort of process as a justification? And you wonder why we are running down the path to becoming a Second World nation. Bondo Nation

OK, Ill play

The rush to put foil shaped apendages on any and every production boat is a symptom of the deeper problem America is having, the desire for flash and eye appeal rather than simplicity and substance... why use a perfectly good plate when a foil is sooooo much more curvascious... and every one knows the Americas Cup boats use foils so isnt it only logical that every trailer sailer afloat must also use perfectly formed foils???
In fact the introduction of the foil into daysailing hulls corresponds nicely with a drastic drop in the quality of wood work in our sailing hulls.

More to your point Sir... I would contend that some of the finest floating wood work in the world is and has been, built around flat plate appendages as opposed to foils!

[skaraborgcraft]

Well, I was assured by my computer programmer friend that 0012 was the ne plus ultra and Holy Grail of all small boat foils, and besides, he already had the algorithm plugged into the CNC. Plus, there is such a thing as putting too fine a point on something. It's just a freakin' rowboat after all.

Fair play.I was begining to think that maybe you were reluctant to build another NACA section jig,but you cheated anyway and done it on a cnc! My understanding was the board should be/can be a smaller NACA section than the rudder,
as it turns through less of an arc than the rudder,which is why i was curious to your use of the same foil size. Obviously it works for you. I am glad that,as Ken has picked up on, your admission that you own just a "freakin rowboat",which if was true,you wouldnt need any foils at all,or maybe an 0012 section skeg? The real question is,if you had to make your foils by hand,with no cnc to play with, would you bother creating NACA sections or something else? Cheers

[Gerarddm]

My answer to your question is yes, I will do mine by hand as required. A small sail&oar boat needs all the help it can get going to windward.

[Canoeyawl]

If you look back in history just a little ways you can note that the difference between aircraft that fly, and aircraft that don't, is the foil.
Flight or no flight - That is is a big difference.

[SHClark]

I have to weigh in.
Making nice centerboards and rudders is one of the fun boat building projects.
The parts are relatively small and can be changed out easily, so it is one area where you can experiment and learn what works.
Also because you drag both sides of a centerboard and rudder through the water, they are often a bigger chunk of the wetted surface of the boat than you might expect, so having a nice smooth finish on them is relatively important. If you are going to lavish attention on your boat, this is a good place to do it.

Almost any traditional design can be improved by putting modern foils under them and modern sails above them.
This will manifest itself in a few degrees of pointing and a few tenths of a knot of boat speed.
You will also side slip less each time you tack.
Put together, it can be several minutes per sailing to windward.
You are betting against the house when you claim otherwise.

However, if your idea of sailing to windward is to lounge in the stern sheets with the tiller over your shoulder and dream of topical islands as your poorly set sails either luff or stall, you probably shouldn't bother.

Minimum effort:
The most critical thing is the leading edge. This should absolutely not be square with sharp corners. If your board is relatively thin, like less than 3/8 of an inch, a half round is probably desirable. Most metal foils are only about this good. As your board gets fatter, a more parabolic profile is better. In any event you should endeavor to have this be smooth and lack big gouges and nicks. Racers all are fanatic about their leading edges.
Most traditional boats have centerboards that are much larger than they actually need to be, so the low hanging fruit is in the front 3" or so. The trailing edge bevel should be long enough to put the water back together nicely. This depends on how quick you want your boat to go and how fragile a trailing edge you will accept and also how free you wish to stay from modern materials. If you accept epoxy and fiber, your trailing edges can be 1/16 of an inch and strong enough to cut catamaran hulls in half. If not 1/4" is probably as fine as you are likely to want. A 3" bevel is possible with a 10" table saw, so that's a pretty easy thing to achieve. If your centerboard or rudder start to vibrate when you get going along nicely, your trailing edge is probably too thick you could quiet it down by thinning it down. Not being warped is pretty important and smooth transitions are also nice.
If you want to build nice pieces, the UIUC airfoil database has most every thing you need. http://www.ae.illinois.edu/m-selig/a..._database.html If you really want to dig in you will find that foil design is a wonderland of Reynolds numbers and low drag buckets in which you can happily spend many winter evenings. You can also down load Xfoil, and engage in the real science of reverse engineering your foil based on operating conditions. If you want to cut to the chase, the 000 series has been used for years quite happily. 10-12% thickness to chord ratios are almost never wrong. For rudders, I prefer the 63 series, which has a small cusp in the trailing edge. Rudders tend to be more forgiving if they are a bit fatter, so 12-18% is pretty much the range. This is often driven by structural requirements of rudder shafts etc. These seem to be more stall resistant than many others, which is kind of nice when things get loaded up.

Most traditional centerboards and rudder designs have very inefficient profiles. You can reclaim interior space and improve the weight of your helm by using more modern designs. Unless shoal draft is a key concern, most any boat will sail better with a deeper more vertical board and a narrower deeper rudder. You need some confidence to make these changes, but they will make a big difference. From my standpoint, I like to sail boats that sail very well, and I like think about what is happening in the air and water that makes the boat sail. So it is always worth the trouble to get this stuff right.
SHC

[JimConlin]

Thanks, Steve.

[Daniel Noyes]

Most traditional centerboards and rudder designs have very inefficient profiles. You can reclaim interior space and improve the weight of your helm by using more modern designs. Unless shoal draft is a key concern, most any boat will sail better with a deeper more vertical board and a narrower deeper rudder. You need some confidence to make these changes, but they will make a big difference. From my standpoint, I like to sail boats that sail very well, and I like think about what is happening in the air and water that makes the boat sail. So it is always worth the trouble to get this stuff right.
SHC

"inneficient" profiles? they are profiles that put light loads on the centerboard trunks that house them, and trunks have long been a weak point in traditional built hulls.
also the low angles trad boards are set at are similar to the delta wing configuration in a jet that is good at providing lift through a wide range of air speed and the delta wing is less likely to stall due to it's profile... sounds like a pretty efficient set up to me...
and Yes shoal draft is always a big plus.

I think you are flat out wrong in asserting that the use of a foil section will in any way improve helm ballance... infact adding high aspect ratio foils to a traditional hull will mean that the center of lateral plane will likely move forward drastically creating extreme weather helm, unless you shift the pivot point of the center board aft to compensate for the more vertical foil, this will probably take a redesign of some structural members etc...

I too enjoy the act of sailing the boat, being part of the interaction between wind water and rig, I think a great deal about water flow, wake formation, etc... and have never once been able to reliably attribute a short coming in performance to the lack of 1/4 inch of camber on either side of a flat plate type centerboard.

[John Meachen]

"inneficient" profiles? they are profiles that put light loads on the centerboard trunks that house them, and trunks have long been a weak point in traditional built hulls.
also the low angles trad boards are set at are similar to the delta wing configuration in a jet that is good at providing lift through a wide range of air speed and the delta wing is less likely to stall due to it's profile... sounds like a pretty efficient set up to me...
and Yes shoal draft is always a big plus.

I think you are flat out wrong in asserting that the use of a foil section will in any way improve helm ballance... infact adding high aspect ratio foils to a traditional hull will mean that the center of lateral plane will likely move forward drastically creating extreme weather helm, unless you shift the pivot point of the center board aft to compensate for the more vertical foil, this will probably take a redesign of some structural members etc...

I too enjoy the act of sailing the boat, being part of the interaction between wind water and rig, I think a great deal about water flow, wake formation, etc... and have never once been able to reliably attribute a short coming in performance to the lack of 1/4 inch of camber on either side of a flat plate type centerboard.

Its good to see an acknowledgement that the moderate loads generated by old style planforms are the result of inefficiency.The analogy of delta wings may not be appropriate as we are considering foils that operate in the Reynolds numbers more usually encountered by gliders and not many gliders have delta wings.
I find it interesting that Mr Clark uses 63 series foils for rudders,I ought to investigate them further.My last rudder used the SD8020 section after comparison with other 10% thickness/chord sections.

[Y Bar Ranch]

Its good to see an acknowledgement that the moderate loads generated by old style planforms are the result of inefficiency.

The underwater stuff ends up exactly balancing the sails, so that argument doesn't hold. The inefficient planforms will, however, have more drag. And therefore more force.

[SHClark]

Dan:
I was pretty careful to say the weight of the helm. This is distinct from the balance of the boat as in the amount of weather helm a boat has. The rudder design can make the helm lighter without having any effect on the amount of weather helm a design has.
Let me explain, the rudder has to generate a certain amount of side force to keep a normal design on course.
This is centered at the center of effort of the rudder blade, which for purposes of simplicity we say is 1/4 of the way back from the leading edge at the height of the geometric center of the immersed blade. What determines how heavy the helm feels is the distance between that center of effort and the axis of rotation defined by the gudgeon or rudder post. The length of the tiller is the other variable.
The more leverage the tiller has relative to the rudder center of effort, the easier it is to turn the rudder. If the balance of the boat is the same, it takes the same amount of side force, all that changes is how hard the helmsman has to work to keep the boat on course. So you can make a boat with a great deal of weather helm seem much less like a back breaker by making the rudder deeper and narrower. You can also "balance" a rudder by putting some area forward of the axis of rotation, however you have to be careful not to overdo because you can make a rudder that keeps turning once it has started turning. 15% forward of the axis of rotation is pretty much the limit.
The structural issues are fairly straight forward. Of course many traditional centerboard trunks aren't really well supported, but the force a centerboard can generate are determined by the stability of the boat. So what changes as you add a deeper centerboard is the length of the moment arm. The arm gets longer, but the load goes down with it. So the ft/lbs on the bearing points can be pretty similar. One should probably do a bit of math before building a 5' deep daggerboard for the dory.
For low Reynolds number foils, such as the ones we are talking about, tests and practice demonstrate that deep plan forms are superior at generating the necessary side force. This can be trumped by lots of other legitimate issues in the design compromise. Maintaining shoal draft is probably the #1 reason to go with low aspect ratio foils.
SD8020 are nice shapes, I use them of my International Canoe dagger boards. Rudders and NACA 63010.
SHC

[slidercat]

Fair play.I was begining to think that maybe you were reluctant to build another NACA section jig,but you cheated anyway and done it on a cnc! My understanding was the board should be/can be a smaller NACA section than the rudder,
as it turns through less of an arc than the rudder,which is why i was curious to your use of the same foil size. Obviously it works for you. I am glad that,as Ken has picked up on, your admission that you own just a "freakin rowboat",which if was true,you wouldnt need any foils at all,or maybe an 0012 section skeg? The real question is,if you had to make your foils by hand,with no cnc to play with, would you bother creating NACA sections or something else? Cheers

It's really not that tough to make NACA profiles by hand. As someone pointed out above, it can become complicated if the board is tapered or given an even more complex plan form, such as an elliptical tip/ However, it's my feeling that a simple rectangular planform works pretty well, and needs only one profile template per foil to shape. Tip losses will be greater of course with a less than optimal shape, but they will be minor compared to the drag losses associated with unshaped or minimally shaped foils. Consider also that a tapered or elliptical foil will also have less area for the same depth and root chord. There are practical aspects as well. A rectangular daggerboard, for example, will fill the slot fully whether up, down, or in-between-- and my suspicion is that drag from an open slot is more significant than tip losses for a rectangular foil. There are probably ways to increase the efficiency of rectangular foils-- a fence (or wing) at the bottom, for example. I suspect that such a foil would be more efficient (for the same area) than a tapered or elliptical foil.

As a notably unskilled and impatient boatbuilder, I try to put my time and energy into things that make the boat sail better. To paraphrase Fernando, it's better to be good than to look good. I didn't put much effort into fairing Slider's hulls above the waterline, so you have to step back if you're offended by lumpy planking. But I put a lot of time into fairing the hulls below the waterline. Same deal with the foils-- the stocks and tillers are crude, but the rudder blades themselves are done as well as I could manage with my limited skills.

It's true that the best wooden boards are laminated from trips of solid timber with alternated grain, but mine were made from laminated plywood, shaped and glassed. Four years later, the trailing edges are a bit wavier than they were at first launch, but performance is not affected.

For my money, the most interesting, beautiful, and well-crafted wood boats built in the last 50 years have been multihulls. I don't think many were built with flat plate foils.

[slidercat]

Dan:
I was pretty careful to say the weight of the helm. This is distinct from the balance of the boat as in the amount of weather helm a boat has. The rudder design can make the helm lighter without having any effect on the amount of weather helm a design has.

A perfect example of this is the barndoor rudders found on traditional catboats. Folks might recall that I tried barndoor rudders on Slipper, my little cartop cat, in an effort to simplify the sailing and setup of the boat. The helm was very heavy, because of the leverage generated by the trailing edges being so far aft of the pintles. Higher aspect rudders were much lighter (and much more responsive.)

Folks who sail boats with kick-up rudders will discover that the kicked-up rudders make the helm much heavier until you get them down again. The area hasn't changed, just the way the area is arranged.

[kenjamin]

Now that we have generally agreed that NACA shaped foils are pretty much a good thing, could we start arguing about the angle at which the NACA profile should be applied?

My vote would be red for the centerboard and black for the rudder.

When trying to get a handle on this NACA foil thing, it seemed to me that the angle that the cross section should be applied is the angle where the board or rudder will be used most frequently. To me the angle of the centerboard in the figure below is about where it will be most often but when (in Adobe PhotoShop) I tried to apply the designated NACA0008-34 in the red direction, the darn thing got so thick that I would have needed to increase the width of the centerboard slot by quite a lot. So to keep the slot as narrow as it is in the plans and still apply the NACA 0008-34 cross section in the red direction, I stretched the cross section along its length and left the width the same. This seems to work fine for me but I can't help but wonder what you guys think. I'm sure there are guys that will tell me that I'm over-thinking the whole thing and it really doesn't matter that much. Let me just say right here and now that to me it does matter. Never thought I'd find myself in a position to race a boat that can do 5 knots at best but I'm looking forward to the opportunity and trying to embrace this whole race thing. When I race other SCAMPs I want to know I have the fastest foils I can shape. Isn't that why they call it racing?

[AndyG]

There are symmetrical NACA profiles which are listed with different thicknesses making them suitable for different needs: usually "can I get this spar in that wing?". I suspect (if the board is thick/strong enough) you've not done any harm here.

Andy

[SHClark]

A link to walk you through the process.
http://www.onemetre.net/Design/FinArea/FinArea.htm
This is based on a ballast model. In order to make it work for un-ballasted boats you have to turn the righting moment arm on it's side to account for the shift in crew weight.
You can be less than fully accurate with much of this stuff, for example the center of buoyancy when heeled will be somewhere to leeward of center line. You can guess that it's about 1/2 way to the lee side waterline. You can approximate the center of effort heights and depths. If you are so minded, excel spreadsheets can be created to explore the sensitivity of these factors.

Of course the days are getting longer and the nights by the fire with a slide rule are getting shorter. So patience for this kind of stuff may have to wait until next year.
SHC

[Daniel Noyes]

Its good to see an acknowledgement that the moderate loads generated by old style planforms are the result of inefficiency.The analogy of delta wings may not be appropriate as we are considering foils that operate in the Reynolds numbers more usually encountered by gliders and not many gliders have delta wings.
I find it interesting that Mr Clark uses 63 series foils for rudders,I ought to investigate them further.My last rudder used the SD8020 section after comparison with other 10% thickness/chord sections.

Nope the low amount of stress on a trad centerboard has nothing to do with less efficiency or less lift, if it did the boat would simply slide sideways!!!

I figured it was more than obvious that the loads on a traditional board are light because as Mr. Clark was mentioning in the previous post trad boards are usually only lower part way so...

your wrong in thinking that the board is not generating lift, the load is lighter because the same amount of force is acting on a shorter lever arm than it will on a long deep board lowered to a 90 degree angle to the trunk.

The analogy to delta type wings is very appropriate as the viscosity of water is far greater than air and it's compressability far less, simply put, what works best on a slow moving airplane in the air is not necessarily the best form for most water applications.

[Daniel Noyes]

Of course the days are getting longer and the nights by the fire with a slide rule are getting shorter. So patience for this kind of stuff may have to wait until next year.
SHC

+ 1

[Nicholas Scheuer]

My objective for TRUE NORTH's leeboards was to have the optimum foil crossection at the most effective angle for minimizing leeway, which is vertical, or nearly so considering that they are gravity-actuated going down. The foil they present when angled back for shoals is no worse that having a 0012 foil progress toward a 0010 foil.

A more important consideration, to me anyway, was what sort of tip profile to have that least disrupts waterflow? Any sort of square-cut tip is out of the question. I repeat, ABSOLUTELY out of the question. I ended up using a well-rounded leading edge (on the order of 12"R and a square corner at the trailing edge. (see my sketch on a page-1 post) Furthermore the leading edge around that 12"R was tapered s well as rounded. In a conversation over the phone with Peter Duff I described my idea of an ideal leeboard tip as looking like a Spitfire wingtip instead of like a P-51 Mustang wingtip. I later decided that a square profile at the trailing edge was better for leeboards, though one sees lots of high-performance rudders with "Spitfire" profiles.

[John B]

I read that the spitfire wingtip profile in yacht rudders can be more prone to tip vortex production than the style that you describe Nicholas. IE , a radius leading into what is basically a rightangle to the trailing edge.
I had an interesting conversation while sailing on saturday. A friend of mine has modified his rudder on his 50 ft yacht( from circa early/ mid 1960's)
The yacht had a modern spade rudder retrofitted about 10 or 15 years ago and it had a spitfire wing tip shape. Anyway , he's squared that off, clipped it like a P51, and added a small endplate. His feeling is that he now has better tracking and more 'bite' and for the same or slightly less wetted surface.
Plus the rudder is now shallower than the keel, that always concerned him.

[Hwyl]

Thanks, Steve.

Ditto, nice to see you on here.

[John Meachen]

Nope the low amount of stress on a trad centerboard has nothing to do with less efficiency or less lift, if it did the boat would simply slide sideways!!!

I figured it was more than obvious that the loads on a traditional board are light because as Mr. Clark was mentioning in the previous post trad boards are usually only lower part way so...

your wrong in thinking that the board is not generating lift, the load is lighter because the same amount of force is acting on a shorter lever arm than it will on a long deep board lowered to a 90 degree angle to the trunk.

The analogy to delta type wings is very appropriate as the viscosity of water is far greater than air and it's compressability far less, simply put, what works best on a slow moving airplane in the air is not necessarily the best form for most water applications.

Having accepted the force transmitted to the hull is a function of the depth at which the reaction to the lateral forces of the rig is generated,why would you want less than the maximum possible?For performance most small boats only lift the board when they are overpowered by the rig.
I have no problem understanding the difference between the characteristics of water and air and normally use a range of Reynolds numbers to guide my choice of section a useful online calculator can be found here: http://www.engineeringtoolbox.com/re...ber-d_237.html.

[Daniel Noyes]

Having accepted the force transmitted to the hull is a function of the depth at which the reaction to the lateral forces of the rig is generated,why would you want less than the maximum possible?For performance most small boats only lift the board when they are overpowered by the rig.
I have no problem understanding the difference between the characteristics of water and air and normally use a range of Reynolds numbers to guide my choice of section a useful online calculator can be found here: http://www.engineeringtoolbox.com/re...ber-d_237.html.

Say X amount of force/lift is needed to counteract the weight of the wind in the sails and keep the boat from sliding side ways, both the traditional type low aspect center plate and the modern high aspect center board/foil will be generating X amount of force/lift, otherwise the boat will not sail it will be pushed sideways.

The traditional low aspect centerplate will carry it's lateral area closer to the hull, say the board draws 2 ft and the center of area is 8"-10" below the keel, that means the lever arm that force X is acting on is relatively short and the amount of twisting stress/torque applied to the centerboard box is small.

With a high aspect foil that draws say 4-5ft of water the center of area may be 2' below the keel, with the same X amount of force acting on the foil the toque imparted to the centerboard box is many times greater than on the low asapect foil.

[zauberberg]

Daniel, but isn't this the davantage and the design goal of the modern high aspect hydrofoil: Producing the same target lateral force to prevent side way slip as the traditional board, but due to its higher aspect ratio at
- a bigger righting moment, resulting in more upright, efficient sailing and at
- a lower induced drag, resulting to higher speeds, but of course
- at the cost of a mechanically heavier loaded structure, both of the foil itself as well as of the supporting structure?

[James McMullen]

Fair play.I was begining to think that maybe you were reluctant to build another NACA section jig,but you cheated anyway and done it on a cnc! My understanding was the board should be/can be a smaller NACA section than the rudder,
as it turns through less of an arc than the rudder,which is why i was curious to your use of the same foil size. Obviously it works for you. I am glad that,as Ken has picked up on, your admission that you own just a "freakin rowboat",which if was true,you wouldnt need any foils at all,or maybe an 0012 section skeg? The real question is,if you had to make your foils by hand,with no cnc to play with, would you bother creating NACA sections or something else? Cheers

1. I built those boards seven years ago. The research I did at the time indicated that 0012 was a good choice.
2. The "just a freakin rowboat" comment was meant to imply that more than a few people are taking this stuff too seriously and getting way too heated about it. I don't think there's any question that the science comes down clearly on the side that foils are better, but there's also a good argument to made that a lot of fat, old greybearded, occasional sailors aren't nearly skilled enough to take any practical advantage of this.
3. When I don't have a CNC to play with, I shape the foils by hand using templates and careful attention to detail. I go to the extra effort here for the same reason that I go to the effort to cast and machine my own custom bronze fittings. I have no need or interest in settling for the sub-excellent compromise of a flat plate when I have the means to simply do it right.

[John Meachen]

Carried to the logical conclusion you have a foil in the water,one above the water and a large righting moment.


How far you may wish to follow the path of maximum performance depends on location and preference.The important thing is to enjoy the ride.

[Hwyl]

Is that Steve Clark's (SHClark) Cogito?

[John Meachen]

Is that Steve Clark's (SHClark) Cogito?

No,not Cogito-an A class rather than a C class.I found them quite fascinating when I saw a fleet racing off Travemunde a few years ago and by coincidence it was at Travemunde last year that i had a brief conversation with Steve Clark while he was rigging his IC.

[Canoeyawl]

Here are some "left overs" from an A cat rig

[kenjamin]

Machined one side of my SCAMP's offset centerboard yesterday. Had made a template of the NACA 0008-34 foil shape and transferred that to two rails of 3/4" pine plywood for the router jig. I had made two other (smaller) templates to use at different places on the board but ended up just using the one. Also gave up trying to apply the foil shape on the diagonal because it would have been an even bigger hassle. As it was it took a couple of hours to machine the one side and what a mess it made. I came inside the house to get a drink and happened to look in the mirror and I was completely covered with mahogany sawdust and there was a half a pocket of sawdust in my shirt pocket. Still, I have to say it worked great. The shape looks super slick and with a little light grinder work, it cleaned up nicely. I also machined in the pockets for my little lead bricks that I'm placing inside the middle of the board. I took pictures but haven't downloaded them yet. I'll post them tonight.

[kenjamin]

Here are some photos as promised:

Here's the basic set up.



It's far from perfect but I can clean it up with my grinder and a bunch of sanding.



Still a lot of work to do.



Inside I routed out pockets for my lead bricks. The tiny white holes were for the registration of the pockets.