A New Sketch -- Better? Or not?

[Bruce Taylor]

If I have properly digested the advice I received on the last hull I posted, this one should be a little better.

I've increased deadrise slightly, and slackened the bilge (especially at the bow).

I've narrowed the waterline at the bow, creating a bit of flare.

John Hardiman suggested letting the "flat" part of the keel run further aft before it kicks up toward the transom. I had trouble accomplishing this without producing odd kinks and bulges where I didn't want them. So, I reexamined Uffa Fox's designs (Wake, Defiant and various dinghys) and decided to drop the forefoot and move maximum rocker forward. At the same time, I broadened the aft sections as much as I could and let the keel rise toward the transom in a long leisurely curve.

This had the effect of raising the LWL by about an inch. However, the shape of the new LWL looks better (to me, anyway). To compensate for the increased draught, I added a bit of freeboard. The result looks tolerably well balanced to my eye. But my eye has been wrong before!

CB is a bit aft of center. I haven't done a curve of areas yet, but will try to find time this evening. I'll also draw a waterline at 20 and 30 degrees of heel, as John suggested.

As usual, any comments, positive or negative, will be received gratefully.

http://www.imagestation.com/picture/sraid34/p0de03c549ee73bfdbd3fb4703553edd8/fd318276.jpg

[ 10-07-2002, 03:16 PM: Message edited by: Bruce Taylor ]

[shamus]

Bruce, your LWL is a guess at this point isn't it?
This is about where I get to when I draw a hull, and then usually give up when all the math starts. You're showing admirable tenacity.

[NormMessinger]

Indeed he is. I've worked at it a bit but I've never fibured out how to use Carlson's software. It does, however, I believe, calculate the waterline based on loading or what not. That's the easy part. The hard part is figuring out how to design your own lines.

--Norm

[Bruce Taylor]

Shaun, for playing around with shapes I made a multichine mockup of the boat in Carlson's Hulls, which, as Norm says, calculates LWL automatically.

However, being somewhat obsessive, I also calculated displacement manually, by drawing a grid of one inch square on top of the sectional view at a given waterline, and counting the squares in each section. I divide the number of square inches in each section by 144 to get square feet, then multiply by 64 or 62.4 (weight of water) to get the total displacement. There might be an easier way that I don't know about.

The arithmetic is basic, but tedious. Fortunately, if there's one thing I'm good at it's mindless drudgery.

The numbers I got were quite close to the ones given by the Hulls software.

On John W's advice, I made a curve of areas. I'm not sure what to do with it, though! John, why did I do that?

It has a nice neat bell shape...very regular except for a "sag" around stations 9 & 10 (from which I infer that I should lower the keel a bit in that area to boost sectional area).

According to Gerr, the CB is located directly under the the peak of the curve. That corresponds exactly to the CB calculated for me by Hulls.

[ 10-07-2002, 10:44 PM: Message edited by: Bruce Taylor ]

[John E Hardiman]

Originally posted by Bruce Taylor:

CB is a bit aft of center. I haven't done a curve of areas yet, but will try to find time this evening. I'll also draw a waterline at 20 and 30 degrees of heel, as John suggested.

WHOA There....the "fun meter" should never get to 30 degrees on this type of hull (i.e. open small boat) unless you really like to swim. She'll round up in a minute. I think I suggested 5-10 degrees to look for heeled balance and CB and waterline with the crew in place. With the broad quarters and fine bow, at 20+ degrees of heel the hull will be way out of balance.

Now there are some round-the-world ocean racers that design the hull to be balanced with the boat on it's ear, but that is almost a pure wedge shape with double rudders and daggerboards. Kind of like two log canoe hulls that share the same stem with skin between the keels. Lifting most of the hull out of the water is just the same as putting guys out on the board. Very, very light for their length and beam and an entirely different idea on how to sail.

[Bruce Taylor]

WHOA There....the "fun meter" should never get to 30 degrees on this type of hull Oops. 30 degrees wd. put the gunwale under, wouldn't it? redface.gif That would create a bit of drag, he he.

If I have a minute this aft. I'll do those tilted WLs.

[johnw]

This is looking good. I think you could even add some beef back to the stern for stability. At the point where the rail is about three inches from going under, you will be more worried about stability than about induced drag. I'd just harden up the bilge a little at that point and keep the greater deadrise you've drawn. Where your actual waterlines are will depend on where you're sitting at any given moment, and I suspect these lines reflect more where they will be when you're rowing then when you're sailing, unless you sit on the center thwart when you're sailing. That's fine, because if you draw the lines with the weight farther aft, you'll see a more stable hull.

So what are you going to name her?

[Alan D. Hyde]

That's where the 45% angled transom could help, too...

Alan

[Buddy Sharpton]

How about reviewing directional stability for a bit. There is a lot more immersed lateral plan forward than aft. Will there be a skeg to even it out? Does seem to be to need deeper sections aft ,perhaps through paring away some of the fullness above the water line and shifting it below the water line. Otherwise, while fast it may be hard to prevent at lot of heavy weather helm on many points of sail. It's not so much the displaced volume, but the immersed lateral plane. I concur with the sail it flat approach demanded by this hull. I'd really try to keep the rudder axis vertcal or damn near so for maximum efficiency. As pretty as raked transoms look, with outboard rudders a lot of the rudder deflection is used to move the bow up and down instead of just left and right and that adds drag.

[shamus]

I'm back Bruce, not getting much time on this thing. Thanks for the explantion- thats cheating using software. I haven't seen Carlsons Hulls, but have played with a free version of Hullform, which did similar things including curves of areas and stability stuff. Problem is I don't really know what I'm looking at half the time. I use Acad for doodling, and you can measure sectional areas easily with that. How about some of these designers giving us all a tutorial here- what are you looking for in the curve of areas?

Now a probably dumb question- how does Carlsons Hulls draw a waterline if it doesn't know what you're building it of? and how much you weigh etc?
-you tell it , right?

[Bruce Taylor]

I'd just harden up the bilge a little at that point Consider it done, John.


So what are you going to name her?Dunno. I'll sleep on it.


That's where the 45% angled transom could help, too...Alan, I'm not sure what you mean, here. How could raking the transom improve performance?


Does seem to be to need deeper sections aft ,perhaps through paring away some of the fullness above the water line and shifting it below the water lineBuddy, thanks for your comments & the info on rudder axis. I knew there was some reason I went for a vertical transom, LOL. I'll bring the keel down a bit at stations 9 & 10. I can add fullness below the WL by firming up the bilges, but can't see how I might simultaneously "pare" it above the WL (short of creating tumblehome in the aft -- which I'm quite sure I don't want). ]

The other thing I might do is add a half foot LOA (stretching the aft a bit). I'll need legal-size paper, though!

"Sail it flat" is a given, here (which is partly why I'm leaning to ketch rig w/ low CE).


thats cheating using softwareThat must be why I felt compelled to count up the little squares.


how does Carlsons Hulls draw a waterline if it doesn't know what you're building it of? and how much you weigh etc?You tell the software what displacement you have in mind (the weight of the dinghy plus, say, two wet people, and a six-foot-long muskie), and the software shows you where that would put the LWL. So, if I enter 500 lbs, the LWL comes in at, say 4.5". At 700, it's more like 6.5". Change the areas of the sections below WL, and you change the displacement.

The number it gives is very close to what I get when I do the work by hand (small discrepancies can be explained by deficiencies in my multichine model and by inconsistencies in my square counting.)

Carlson's Hulls is free, by the way. Well worth the short time it took me to figure it out (the documentation is terrible, but once you get the hang of the program it's very easy to use).

[ 10-09-2002, 07:08 AM: Message edited by: Bruce Taylor ]

[John E Hardiman]

Originally posted by shamus:
How about some of these designers giving us all a tutorial here- what are you looking for in the curve of areas?
Ok, I'll bite.... But like all things in life it comes with a story. So pour a dollop of Nelsons Blood and sit down.

I have always wanted to build ships. My Mother saved the crude drawings I did as a 5 year old and never doubted. I followed my parents around the world and learned and saw and read everything I could and remembered. I finished high school early and went to a junior college to get the math I would need. I learned to weld, burn, and machine. I applied to the best schools for Naval Architecture and by grace of God went to some of them. As you may know, the best schools put you work or to sea between terms to salt you. I learned to crank through the higher math and just how stupid computers can be (iron core memory?...punch cards?..paper tape?... the hexadecimal number "Q"? ;) ). I learned the hows and whys of electronics, mechanics, reciprocating steam engines, structure, rules, NDT, and economics. I graduated and spent another year learning my trade in a repair yard from the calculating desk (everyone should have to do 6 months of weight calculations) to the guy who drew with chalk on the hull and said "cut here". Then I had the extreme fortune to get asked to join a design team developing advanced concepts, not in the lab, but actually building something that advanced the art. For the past 17 years, I've thought, fought, cursed, built, destroyed, developed, argued, teamed, and critiqued with some of the best engineers it has ever been my pleasure to work with, and also some of the worst. And I have come to the following conclusions...

1) Naval Architecture is the last of the "guild" technical professions. You have to serve some sort of apprenticeship before you can fully understand what can and what can't be done with the tools and medium available. However an apprenticeship in one facet does not entitle you to address all facets. For some things in Naval Architecture I am one of the best at what I do, for other things I'm passable, as a boat carpenter I suck.

2) A true hydrodynamicist and designer is a blend of innate talent, nurtured by education, and developed by experience. Leave out any of those three things and you have something less. I have met plenty of people who I (if it was my choice) would not allow to touch anything that the lives of seamen depend upon. They just lack the necessary "insight". They probably think the same about me.

3) Do not become enamoured by the math and computers and by the same token do not dismiss them. They are tools that you must understand and use, no more, no less. One must chose the right tool for the job; i.e. a power planer is a carpentry tool, a backsaw is a cabinetry tool.

4) Finally, the only thing that a Naval Architect or Designer can offer you is an opinion. Maybe a better informed, or more experienced opinion, but still only an opinion. Do not be bullied by esoteric theory, crushed by computer generated "data" or the weight of published books, or gulled by eloquence.

That said and done, what does one look for in a sectional area curve? For this type of sailing hull (for that is what it has become, I would say go compare it to one of the mid '80's Santa Cruz CA boats in the 30-50 foot range and notice the sections, and proportions; say an Olsen 30 or one of Bill Lee's, major difference at a glance is the rake of the stem) the maximum section (i.e. the peak of the sectional area curve) to be ~0.60-0.65 LWL aft with the center of the area under the curve (i.e. the CB) ~ 0.55-0.60 LWL aft. The center of the water plane should be aft of the CB to prevent hobby-horsing. Other than that it should "look right" and "balanced". If it was so easy to describe you could easily program a computer to draw the correct shape first time, but you can't. This is why I paraphrased this answer with my tirade above.

[plimsol]

John, Thank you for a beautiful tirade.
Having worked a little with a N/A computer program, I am not so sure that a computer program gives you a sense of what the shape is compared to drafting it on paper. This is not a Luddite tirade, just a feeling that one's attention is distracted from the shape, to the crunching of numbers and fighting with the program. Perhaps this is the Art and Science aspect of Naval Architecture.
The beauty of the computer is that one can explore alternatives quickly and easily. They remove the drudge of relofting and can do calcualtions in an instant. Yet, the numbers are meaningless without practical experience.
Cataloging and organizing the designs ofL.H. Coolidge and L.E. Geary has given me an appreciation of the body of work they turned out, without the aid of computers and plotters. The calculation sheets of Coolidge are precise ledgers of finely drawn numbers, which eventually gave him confirmation of what he designed from experience.
" I though of my unknown ship. It was amusment enough, torment enough, occuption enough".- Joseph Conrad.

[Bruce Taylor]

Wonderful stuff, John H. I'm glad I opened this bottle.

WB magazine is a marvellous resource, but in years of reading it -- dwelling on the design reviews, mulling on the study lines, staring long and hard at the construction sequences -- I learned less about design than I've picked up in the past few weeks from submitting specific questions to the generous and knowledgeable people who frequent this forum.

Speaking of WB, I'd really enjoy a series of articles about the basics of small craft design. What is needed is a garrulous and worldly N.A. -- the design equivalent of Bud C. Macintosh -- who is willing to lead amateurs through the entire process of developing a plausible shape. I'm sure there are other readers who'd like to see such a series.


The center of the water plane should be aft of the CB to prevent hobby-horsing. Other than that it should "look right" and "balanced".I'll try to digest this. It raises the spectre, in my mind, of Francis Chichester launching Gypsy Moth and discovering, to his horror, that she was "a rocker!"

Plimsol writes:


The beauty of the computer is that one can explore alternatives quickly and easily. They remove the drudge of relofting and can do calcualtions in an instant. Yet, the numbers are meaningless without practical experience.Of course, hand-calculated numbers won't mean any more to the inexperienced designer.

Actually, I think programs like Carlson's might be useful as a way of helping people like me develop better intuitions. The effects of your changes, as you say, are immediately obvious. Squeeze the hull here, and the CB moves back...puff out the hull here and the WL moves up, etc.

For instance, I lost a lot of bouyancy, at one point, when I hollowed the bow too much. I could have learned that after doing a long series of manual calculations (without a planimeter!). However, playing with my computer mockup quickly gave me a sense of how far I could go in that direction w/out seriously unbalancing things.

The lesson would have been no more valuable if it had been acquired with greater loss of time.

Having said that, I think drawing by hand has helped me to "feel" the shapes.

Of course, I won't know till I push it into the water whether I've actually learned anything worth knowing!

It gives me some comfort to borrow the intuitions of people like John H., Bainbridgeisland and John W. And mmd, if only he would get his mangy arse over here smile.gif (just kidding...Michael's been busy with real work).

[Buddy Sharpton]

What I meant by paring away above the waterline to put more displacement/bouyancy below the waterline in the aft sections would require the outboard regions of the sections becoming more sloped and less vertical, less wall sided, less like a burdensome non planing hull and with more flair, like a dinghy that this boat is. As you increase deadrise to get a deeper lateral plane in the aft sections, the waterlines will narrow, the sides slope( slope may not be the right word as these are not flat panel sides but you get the idea) at a greater angle in order to meet the same deckline at the sheer. Absolutely the opposite of tumblehome where you are getting wider waterlines for the same deckline above.

[Alan D. Hyde]

Bruce, with the raked transom, buoyancy aft increases as the vessel's immersion increases.

It also helps the stern to rise to waves.

Somewhat akin to flare forward helping a fine bow to rise to the waves.

Alan

[A. Mason]

Hi Bruce, it is interesting that you should mention the lack of articles on how to design a hull in boating magazines.

During the 1920s, such information was a regular monthly feature of the magazines. The articles were kind of like a serial textbook on naval architecture. My father's early knowledge of good design was learned from regularly reading these articles.

Somewhere in the 1930s the emphasis shifted somewhat to regular monthly "build your own boat" designs, with far less emphasis on the design side and more emphasis on the construction side.

Through the mid 1950s, these monthly featured designs included not only the lines but also the offsets, construction views, etc. - everything an amateur builder needed to construct the boat.

Since the 1960s, boating magazine reviews have tended to be limited to a sail plan, accommodation plan, and maybe some photos of the prototype.

Actually, it wouldn't be very difficult for WoodenBoat to include some technical articles. They could easily be based on the old "historical" articles from "The Rudder" [WB owns the old publication files].

There has been far more changes in construction materials than in the theory and procedure of designing a seaworthy boat. Since it appears that there is a lot of interest in older wooden designs, reprinting selected old articles would seem appropriate, with the possible inclusion of some comments by a contemporary naval architect on any apsects that may have changed over the last 80 years.

Personally I've thought it's been sad that such information is no longer written about in consumer publications. Enlightenment of what is really involved in designing a boat would benefit everyone.

Anita

[Bruce Taylor]

Buddy, thanks for clarifying. I'll chew on that for a while.


Bruce, with the raked transom, buoyancy aft increases as the vessel's immersion increasesAh, I see what you're getting at. Alan, raking the transom "into" the existing bouyancy would just subtract from bouyancy at all waterlines all the way up to the top of the transom. Nothing to be gained there. Raking it "out" wd. increase length and weight, and only begin to add bouyancy when the transom is immersed -- i.e. when the boat is overloaded or at extreme heel. Rather than rake "out" I'd be inclined to simply stretch the LOA, which could add plenty of useful bouyancy in the aft, I think.

What you say about stern seas is true, of course.

Anita, thanks for the historical perspective. That's quite interesting. WB does publish those design reviews, and they are most interesting. I read, and reread them, constantly.

It would be fun to see a series, though, giving the reader a gentle introduction to such scary subjects as block coefficients and D/L ratios, and explaining why the Jumblies should never have put to sea in a sieve.

Just an idea.

[ 10-09-2002, 12:59 PM: Message edited by: Bruce Taylor ]

[Alan D. Hyde]

Bruce, the rake gives you more buoyancy when you need it, but when it's not needed, there's no additional wetted surface as there would be if you merely lengthened the boat.

Alan

[Bruce Taylor]

You may have something there, Alan. While plumb transoms are exceedingly common in fast sailing dinghys -- Windmill, Thistle, Finn, Albacore, Wayfarer, One Design 14, Lively, and many others -- this is an undecked multi-purpose boat, and a bit of reserve bouyancy could hardly be unwelcome.

On the other hand, John W. has suggested that I slightly increase displacement below the LWL (and, hence, wetted surface) in the after sections. And Buddy has raised the matter of keeping the rudder vertical (a subject to which I had given no thought whatever).

Hmmm.

[shamus]

Thanks John Hardiman for biting. Your 'tirade' sounds in essence a bit like Chappelle and others, who say much the same thing. I guess nothing's changed much then. People like me probably shouldn't try to draw boats at all, but it's hard to keep from fiddling with ideas. The upside is that nothing I ever draw is likely to get built. Wish I could say the same about other amateur efforts- there's a magazine here pushing the most horrible looking sailing dinghy you ever saw, including stories about some idiot taking one five miles offshore. Well for all I know it could be seaworthy, but it sure doesn't look it to me.
Bruce- not meaning to hijack the thread here- looks like you're getting close to shopping for wood then?

[Meerkat]

FWIW, I've just put a page for John Welsford on his "Tread Lightly" design where he summarizes how the design came about. The link at the bottom about the story of a typical builder (nothing to do with design - sorry) will be up sometime tomorrow.

http://www.jwboatdesigns.co.nz/new_designs/TreadLightly.html

[Meerkat]

I thought this might be of interest:

For unballasted small craft design, try this: Start the design as a flat-bottomed double-ender, making LOA/beam = 4/1. Other types can be developed along the lines of Matryoshkas (Russian dolls). A dory can be created by taking the double-ender and replacing the stem aft with the triangular 'tombstone'. On this basis, a 16ft double-ender becomes a 15ft 3in dory (LOA). A skiff can be created by removing a quarter of the overall length of the double ender from the stern and plugging the hole with a transom board; this makes the LOA/beam 3/1. The punt can be developed from a skiff by lopping 1/8 LOA fore (again measured from the original double-ender), with the resulting hole plugged by a bow transom. A dinghy is a punt shortened still further from the bows. A pram is a still more shortened dinghy. These figures are not absolute. The trick for each development is to stay in the confines of the same sheerlines of the original double-ender - that is, the beam for the pram is still 4 ft if the original Double-ender is 16 ft. Oh, I nearly forgot. Make sure that sheer line of the original D-E is a NATURAL curve, that is part of a circle arc. For hard-chined hulls, the circle arc segment is set on the LOA as chord. The (maximum) height of the arc is the hypotenuse of a right triangle of which the long leg equals the half-Breadth at Beam (1/8 LOA). The short leg equals the Profile height at Beam. The acute angle between the long leg and the hypotenuse equals the flare angle. (Gardner, John 'The Dory Book' page 43 I found this and a bunch of other design tips at:
http://home.clara.net/gmatkin/therules.htm

[Bruce Taylor]

Bruce- not meaning to hijack the thread here- looks like you're getting close to shopping for wood then?I always enjoy a well-orchestrated thread-jacking. Take the helm, I say.

Shopping for wood? No, no, no. Winter is closing fast around here. I might build a little model this winter, if I have time, but I won't set up the moulds until next spring.

Meerkat, thanks for posting those links. I spent a long time at that "Rules" site. John W's account of Tread Lightly is most interesting, so far. I hope the other pages will be up soon.

[Stephen Hutchins]

Just at a glance, it looks like she is gonna float higher by the bow than what you have drawn. I think your last design was a better all around car topper. If you go with this one, consider a sail stepped well forward in the boat.

[Bruce Taylor]

I hope you're wrong, Stephen!

I think the hull is more balanced than it looks "at a glance," but I may be fooling myself.
So far, the calculations I've been able to perform have been encouraging. CB is slighly aft of station 7, the curve of areas has a very respectable shape.

In any case, I'm adding some volume at the back & bringing the keel down almost to the baseline at stations 9 & 10, and moving the forefoot up an inch. In effect, I'm backtracking slightly toward the earlier hull I posted.

[ 10-10-2002, 09:23 PM: Message edited by: Bruce Taylor ]

[Meerkat]

I posted John's story of "Tread Lightly" that he derived his inspiration from today. Pages for his other designs are in the works.

Bruce - too deep a forefoot?

[ 10-11-2002, 03:31 AM: Message edited by: Meerkat ]

[Bruce Taylor]

Thanks, Meerkat. I read and enjoyed it.


Forefoot too deep?I'm raising it an inch, while lowering the keel in the back. Less Foxy, more boxy...probably safer.

[Scott Rosen]

I'm curious, Bruce, why did you draw a perfectly plumb transom?

If you shortened the lwl by a few inches, raked the transom so as to keep the loa the same, then you could have less wetted surface, which would make for easier planing, but you would still have the benefit of the existing lwl when sailing to windward with even the slightest heel. This is, after all, a fast, slippery planing hull. Might as well get the most of it.

You wouldn't lose any usuable interior space, and you could play around with the bouancy a bit to optimize speed.

Just a thought.

In case it doesn't come through from this post, I like the lines a lot. I think it's time to start building the model.

[Bruce Taylor]

Scott -- Alan raised the same question, and it's a good one. My first drawing had a raked transom, but it didn't properly clear the water on heel, as John W. pointed out. I removed the rake in order to help keep the transom clear without having to deepen the V too much (losing displacement) or add too much rocker & inconstant deadrise (warping the run). Since most of the fast dinghies I like seem to have dead plumb transoms I reasoned that there couldn't be much of a penalty for that "wetted surface".

That was my thinking...whether it holds water (or holds water out) is, of course, a separate question.

I'm almost ready to make a model. I have to finish a final drawing, reverting to some features of the previous hull I posted.

[Dave Hadfield]

Bruce, I'm quite looking forward to a photo of your new canoe-cruiser in the water next spring but today I saw a photo that struck me.

It's in Jennings new book about canoes. (Wonderful resource!) There's a picture of a Rushton Vesper sailing canoe there. It's ketch-rigged with bat-wing sails. Beautiful boat. A bit more of a canoe than you're designing now, but perhaps worth a look.

[john welsford]

Trace your curve of areas onto a piece of stiff paper, then balance it over a pencil across the width, the point of blance is the centre of bouyancy, not neccessarily the high point of the curve. Then you need to work out where you are going to put the weights, at a guess you will have ( if a sailing boat) two people aboard, one just aft of amidships, and one further aft again on the tiller. At a guess you will find that the centre of gravity is somewhat aft of the nominal centre of bouyancy which will result in the boat trimming stern down.
Bear in mind that Uffa Fox' designs were very specialised racers, very advanced and quite revolutionary for their time. They depended on a very mobile crew for trim and you will see pictures in his books of the crew stacking out rather further forward that we are accustomed to in our modern shapes which are typically finer forward and broader aft.
JohnW


Originally posted by Bruce Taylor:
Shaun, for playing around with shapes I made a multichine mockup of the boat in Carlson's Hulls, which, as Norm says, calculates LWL automatically.

However, being somewhat obsessive, I also calculated displacement manually, by drawing a grid of one inch square on top of the sectional view at a given waterline, and counting the squares in each section. I divide the number of square inches in each section by 144 to get square feet, then multiply by 64 or 62.4 (weight of water) to get the total displacement. There might be an easier way that I don't know about.

The arithmetic is basic, but tedious. Fortunately, if there's one thing I'm good at it's mindless drudgery.

The numbers I got were quite close to the ones given by the Hulls software.

On John W's advice, I made a curve of areas. I'm not sure what to do with it, though! John, why did I do that?

It has a nice neat bell shape...very regular except for a "sag" around stations 9 & 10 (from which I infer that I should lower the keel a bit in that area to boost sectional area).

According to Gerr, the CB is located directly under the the peak of the curve. That corresponds exactly to the CB calculated for me by Hulls.

[Bruce Taylor]

Dave -- One of these days I'm going to build a Victorian decked sailing canoe. There are some wonderful designs available, including Vesper and her younger sister Notus.

I considered a tandem sailing canoe, along the lines of Rushton's Ione, but it really wouldn't lend itself to casual cartopping. An undecked boat is what I need, I think.

That Jennings book is magnificent. I posted a note about it in Misc. a month or two ago.

John W. -- Thanks for getting back to me on that curve of areas. In the meantime, I reread the pertinent section of Gerr's book and found what I needed.

In my fourth (and, I hope, final) revision I've de-Foxed the lines a bit and addressed some of the issues people have raised.

I started making a model yesterday, so I should have a toy version of the boat to play with in a couple of days.

Am I right to think that I can use this model to experiment with weights and see how trim will be affected w/ different weight distributions?

I'll post a picture later, if I have time.

[ 10-18-2002, 09:31 AM: Message edited by: Bruce Taylor ]

[john welsford]

Here is another check that you can do, take your curve of areas, and put a box around it. As wide as your curve at its widest, and as long as its longest. Work out the percentage of the squares area that the area inside the curve covers and this is your prismatic coefficient. Now a boat need to have that P/Cf at a certain number for a certain speed to length ratio. Much easier to get precisely right in motorships intended to do the same speed all the time, but useful in working out whether your hull is in the ballpark for the type of craft you are intending.
For instance, a heavy displacement yacht that will never ever exceed hull speed should be around 52/53% a lighter boat that might get a little surf on a wave now and again 54/56% and so on up to a full planing powerboat at way over 60%.
Now when I am designing to a known performance envelope, I do a profile which gives me a waterline length, then I draw a midships section, I already know the P/Cf that I need for the performance envelope for the S/l ratio so I can measure the cross sectioanal area of my proposed midships section, multiply it by the P/Cf and get a trial displacement for that profile and midships section.
I already know the target Disp. as I have in my research for the proposal established all of the "other" statistics such as displacement to length, sail area to displacement, Bruce number, fred number, jack number and so on.
When the profile and the midships underwater section is right, you can draw a dummy curve of areas that will fit the P/Cf and that will tell you what underwater cross sectional area you require at each station.
By the way, I use an integrating planimeter ( electronic) the same as a surveyor or cartographer uses for the measurement of areas,but otherwise download my inspirations with a pencil so have developed systems and shortcuts to reduce the amount of rubbing out and rework I have to do.
Hope all that makes sense.

JohnW


Originally posted by Bruce Taylor:
Dave -- One of these days I'm going to build a Victorian decked sailing canoe. There are some wonderful designs available, including Vesper and her younger sister Notus.

I considered a tandem sailing canoe, along the lines of Rushton's Ione, but it really wouldn't lend itself to casual cartopping. An undecked boat is what I need, I think.

That Jennings book is magnificent. I posted a note about it in Misc. a month or two ago.

John W. -- Thanks for getting back to me on that curve of areas. In the meantime, I reread the pertinent section of Gerr's book and found what I needed.

In my fourth (and, I hope, final) revision I've de-Foxed the lines a bit and addressed some of the issues people have raised.

I started making a model yesterday, so I should have a toy version of the boat to play with in a couple of days.

Am I right to think that I can use this model to experiment with weights and see how trim will be affected w/ different weight distributions?

I'll post a picture later, if I have time.

[Bruce Taylor]

Thanks again, John. That not only made sense it did me a world of good. Gerr gives a method of estimating the prismatic coefficient from the area of the midsection, but the calculation he gives is cumbersome.

I calculated the prismatic coefficient, following your method, and got a P/Cf of 61-62% (at 680 lbs. disp., LWL 7"). I think this is in the right ballpark for a multipurpose hull like this one?

My scale model is almost done.

[john welsford]

Originally posted by Bruce Taylor:
Thanks again, John. That not only made sense it did me a world of good. Gerr gives a method of estimating the prismatic coefficient from the area of the midsection, but the calculation he gives is cumbersome.

I calculated the prismatic coefficient, following your method, and got a P/Cf of 61-62% (at 680 lbs. disp., LWL 7"). I think this is in the right ballpark for a multipurpose hull like this one?

My scale model is almost done.

[john welsford]

Originally posted by john welsford:
</font><blockquote>quote:</font><hr />Originally posted by Bruce Taylor:
Thanks again, John. That not only made sense it did me a world of good. Gerr gives a method of estimating the prismatic coefficient from the area of the midsection, but the calculation he gives is cumbersome.

I calculated the prismatic coefficient, following your method, and got a P/Cf of 61-62% (at 680 lbs. disp., LWL 7"). I think this is in the right ballpark for a multipurpose hull like this one?

My scale model is almost done.</font>[/QUOTE]Hi Bruce, Gerrs method cant really work as you need the rest of the hull to determine the P/Cf, what I do is I work out a statistical definition of the hull according to the typoe of boat, its use, its critical performance envelope etc which tells me whatP/Cf I need in order to achieve a boat with those performance characteristics, then design to that.
Your P/Cf of a little over 60% is about consistent with a boat like a Sydney Harbour 18 footer, a full planing hull . If you are doing a general purpose knockabout you need to be closer to 55%

John Hardiman, your thoughts?

John W

[John E Hardiman]

Originally posted by john welsford:
[/QB]
John Hardiman, your thoughts?

John W[/QB]Sorry Guys, just got back last night from a week on the road.

Ok, I've already made a comment on the newer thread on the model. On the topic of prismatic coefficient = displaced volume/(LWL x midships area)= Cp to use the US standard notation, as Cf is coefficient of friction and I don't want to get myself confused. Common wisdom circa 1965 from Henry & Miller looking at CCA rules and the Antelope tests says the following:
speed length ratio &lt; 1.0; Cp &lt; 0.55 for thin or &gt; &gt; 0.60 for broad waterplanes
speed length ratio 1.0-1.15; Cp 0.55-0.58
speed length ratio 1.25-1.35; Cp 0.58-0.61
speed length ratio &gt; 1.35; Cp &gt; 0.61 broad waterplanes and flat keels or &gt;&gt; 0.50 for thin and rockered

So at below hull speed, you want an easily driven fine hull or a broad heavy hull that can be driven hard. As you speed up for the same LWL and displacement the Cp rises until the hull becomes a skimming dish with Cp dependent upon the waterline shape and rocker (Cp high for broad shapes with flat keels, low for fine hulls with a lot of rocker). This makes sense as to hold displacement but increase driving power (i.e. waterplane transverse moment for a day sailer), the hull must become broader and flatter. If the crew is a significant amount of the hull weight, then you can add more rocker to lengthen and broaden the hull for the same displacement which dercreases Cp. As the weight of the crew decreases in proportion to the hull, the waterline must become fuller overall as the hull must provide more stability to allow the greater driving power. Thes hulls tend to very broad for their length to allow the maximum moment to be generated by the crew. (Of course keels, and ballast ratios begin to enter in here, talk to Bainbridgeisland, he has a better handle on this than I)

The ultimate examples of this is as John W said, a planing skiff on one hand (Cp &gt; 0.6, speed length ratio &gt; 1.5, LWL/Bwl ~ 1.5) similarly an older I-14( Cp ~0.4, speed length ratio &gt;1.3, LWL/Bwl 3.44) or look at the hollow, high deadrise clippers (speed length ratio ~0.5, Cp ~0.45, LWL/Bwl ~5) compared to a flat bottomed packet (speed length ratio ~0.5, Cp ~0.7, LWL/Bwl ~3).

This is why I compared the hull to the Santa Cruz boats of the '80s earlier. When light, the hull should plane if you can hold her up. When fully loaded you'll end up driving her hard with the same sail area to try and get any speed, so don't, just take what you get. As I said before the requirements contridict each other.

The real trick now is picking a good sail area to Bwl^2 for the given hull shape. I think this ratio should be about 6-8 for the "crew only" displacement with the ratio lateral plane(including rudder)/sail area of 0.08-0.09 at the same displacement. Rudder should be ~ 10% of the lateral plane, and the rudder+centerboard area/sail area should be approximately 0.05. Most of this comes from Henry & Millers book.

[Bruce Taylor]

John H. -- This is extremely useful info. I'll print it out & frown at it until it sinks into my solvent-soaked brain.

I've just spent an enlightening hour w/ John Teale's book, calculating predicted performance at different displacements and sail areas. He provides a very interesting (and simple) calculation for predicting speed of a dinghy.

"First, divide the Sail Area/Displacement ratio by 10. Next, take the Displacement/Length ratio away from 100 and add the two numbers."

He seems a little uncertain of the value of his method, but supplies a chart, plotting the results of his calculations against the Portsmouth stats for various dinghys.

I did the calculations at various displacements and sail areas and learned a great deal about the kinds of tradeoffs I have to make between burden and sailing performance.