I would like suggestions for design books

Discussion in 'Boat Design' started by Veloceruss, Oct 5, 2019.

  1. Veloceruss
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    Veloceruss Junior Member


    I live aboard full time and want to build a performance version of the PT11 dinghy. I have some naval architecture background and mechanical engineering. I have the Bethwaite book and love it. Can anyone suggest other books for performance dinghy design? I will probably draw it by hand so a book with manual calculations would be a good start.

  2. Squidly-Diddly
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    Squidly-Diddly Senior Member

    "performance" meaning what changes? materials up-graded?

    length and beam adjusted? For that I'd suggest downloading free Student Edition of Autodesk Inventor, and play with it so you will get effortless 110% accurate hard numbers for your new dimensions, not to mention hard numbers for otherwise very hard to figure specs like displacement at various waterlines.

    Bigger sails?
  3. Veloceruss
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    Veloceruss Junior Member

    I would change construction to strip planked for weight savings and because I prefer it. The dinghy would be strictly a fun, sailing dinghy, not a yacht tender. Freeboard would be reduced to save weight. Don't mind getting wet. I will probably reduce rocker and go for a finer entry. N12 designs look great to me but I'm considering a narrower BWL and little flare. Sail area would be about 70 square feet. Just my initial ideas. I'll look into Autodesk Inventor. Thanks.
  4. Alik
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    Alik Senior Member

    Uffa Fox books are very good. Start from basics!
  5. messabout
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    messabout Senior Member

    Try to get a copy of The Nature of Boats by Dave Gerr. There is a mountain of essential information about small boats in C.J. Marchaj's book: Sailing Theory And Practice.

    Your initial ideas include narrow beam, fine entry angle, and reduced rocker. All those features have some limited merit but can be argued against with some validity. For example, narrow entry and reduced rocker will let the boat plunge more easily. If the boat plunges, like sticking her nose down while on the face of a wave, then the CLR moves far forward and the rudder will become ineffective. That leads to the real possibility of a broach, swimming session, and possibly damaged rigging. If you reduce rocker too much the boat will not tack readily because the forefoot is too deep in the water. If the bottom at the transom is at or too near the waterline the transom will drag when the boat is heeled even a little bit.

    You can use Autodesk, Solidworks, or several other design programs to do the arithmetic for you. While I appreciate that kind of convenience, one does not learn as much, or intuit as much, as if he were drawing the lines and doing the calculations the old fashioned way. I agree that the use of a handheld calculator is acceptable .

    If you have an engineering education you will already be entirely capable of doing the math so that you can determine such things as Center of buoyancy, center of lateral resistance, center of gravity, prismatic coefficient etc. Layout your drawing with an even number of spaces so that the stations will be an odd number. Measure the immersed areas of each station, use Simpsons rule to determine the total volume of the underwater parts thus the displacement of the boat.

    For ball parking the elementary numbers such as displacement at whatever Wl beam and draft, you can make some educated wild guesses. You can do the exploratory math in seconds. Then move to the next iteration until you are satisfied that the basic design is worth the trouble to draw it up.

    Choose a waterline beam at the largest immersed section near or at the middle of the boat. Let us say that the waterline beam is 40 inches wide at that location. You are thinking maybe 4 inches of draft. The area of the immersed section is 40 x 4 = 160 square inches. Your sailing dinghy will probably have a prismatic coefficent of somewhere between 0.51 to 0.56 Multiply the mid area 160 times the coefficient....say 0.53 to get 84.8...........now multiply that number by the length of the waterline. let's say the WL, not the boat, but the WL , is 140 inches long. Multiply the 84.8 times 140 to get 11,872 that is the SWAG displaced volume in cubic inches. You are in salt water so the weight of a cubic inch of water is 64/1728 = 0.0370 pounds per cubic inch. Multiply that number by the volume you just discovered and you get 11,872 x 0.037 = 439 pounds of total displacement. That is a ball park estimate but a reasonable place to start your exploration of the design that you might want to build. Of course you will want to do the real measurements from the scale drawing that you are making. The more rigorous calculations will be pretty near the wild guess method result.

    Be sure to use a large scale for your drawing so that measurements are more nearly accurate. I suggest three inch scale (three inches to the foot) for a small boat. If you can get your hands on an architects scale, with the various scales on it, that will be most helpful. You may use the more sensible metric methods with a bit of adjustment to the coefficients and ratios, which I leave to you.

    Have at it Velocerrus.
  6. TANSL
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    TANSL Senior Member

    @Veloceruss, do you want to learn about naval architecture or just a few concepts to design your boat?
    By the way, perhaps AutoCAD is more comfortable than Inventor for this boat.
  7. Heimfried
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    Heimfried Senior Member

    I don't think the area resulting of beam times draft (40" * 4") is the area of the immersed section but the area of the circumscribed rectangle. Only if you multiply it by the ratio cB/cP (block coefficient divided by prismatic coefficient) you will get the area of the immersed section. Did I miss something?
  8. Veloceruss
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    Veloceruss Junior Member

    I would like to study naval architecture from some basics up. I did have an engineering course based on Marchaj's book but that was many years ago. I won't be able to build the boat until we get to New Zealand....unless I can find the right wood in Gambiers and a place to build it. What I plan to do is study and come up with a preliminary design and post it here for comments. I have thick skin. Then I will build a model from balsa wood I have stashed on board.

    I will buy Marchaj, Geer and Fox books. Going back to the US to pick up boat parts so can bring some cargo. Any other book ideas?

    PS. I owned a Flying Dutchman for 13 years. Also owned a Laser 28 for 25 years with well over 1,000 races. I love planing boats. But just a performance feel in light wind is great too. Uproar (our home) is a Farr designed First 42s7 and even overloaded with cruising gear, has a delightful feel sailing.
  9. wet feet
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    wet feet Senior Member

    I wish you well with your attempt to understand more about boat design.It won't just help with life around boats it makes you think in all sorts of ways.The books mentioned will give you a good all round appreciation of boats and the design process.I doubt that any other than the Bethwaite book will be of much use for designing a fast small boat.There really hasn't been much put into print on the topic of high performance small boat design.I would suggest visiting the websites of development classes and maybe finding a bit about their history-I know some have a number of images of designs through the ages and you will see how the shapes have evolved.For a small boat I would suggest adding Cherubs and lowrider Moths to the N12's that you already know something about.I would also advise being cautious about narrowing the waterline beam of a N12 design until you have sailed one,they can be very twitchy little boats and some experience will be the best way to learn what to change.A Laser will seem like a barge by comparison,even if it might be faster in some conditions.
  10. Dolfiman
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    Dolfiman Senior Member

    Thanks to members, we had interesting exchanges about dinghy design in the thread here below : hull shapes versus the trade-off stability/performance/ability of the helmsman (I share Wet Feet concern about too narrow design, needing higher skills through training), ...with links about historical and current developments. Not a book, but can be informative and helpful for you as it was for me :
    Dinghy 13 ft with bi-convex sections option https://www.boatdesign.net/threads/dinghy-13-ft-with-bi-convex-sections-option.62176/page-2
  11. Doug Halsey
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    Doug Halsey Senior Member

    For manual calculations, the classic Skene's Elements Of Yacht Design is great!
    bajansailor likes this.
  12. bajansailor
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    bajansailor Marine Surveyor

  13. messabout
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    messabout Senior Member

    Heimfried you did not "miss something". The difficulty is that I did not explain clearly enough. The method that I described is usually employed during an argument at the yacht club bar. It is also a quick and dirty method to decide the general dimensions of a prospective ( or imaginary) dinghy. Yes, use the immersed area of the circumscribed rectangle or trapezoid if the boat has a flat bottom. If the section has some curvature it is necessary to do some arithmetic to arrive at that actual immersed area. Apply the probable Cp to whatever that area is and you have the more or less average area of the whole lot of sections. This is a long way from doing an actual design but it does provide a starting point.

    Of course we would have had to do a guestimate of the total weight of the boat and its contents. That gives us something to aim at. You can work backwards by calculating the number of cubic inches or cubic centimeters needed to support a certain weight in pounds or kilograms. Divide by Wl length and you will be able to determine the required main section area after applying the probable Cp.

    This is a fun, perhaps reckless, way to brainstorm while thinking about little boats.
    bajansailor likes this.
  14. Tad
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    Tad Boat Designer

    The most up-to-date book on small sailboat naval architecture is Principles Of Yacht Design(4th Edition) by Larsson and Eliasson.

  15. wet feet
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    wet feet Senior Member

    I agree with this point,but even this excellent book has very little content that helps with small planing dinghies.It is an area that has been neglected for a long time.The Uffa Fox books were mostly written sixty or more years ago and a lot has changed since then.Not least our understanding of the features that make a good all round performance dinghy.We also have materials and construction techniques that were unknown back then.I suppose the root of the problem is that there just aren't that many people working in the field of fast dinghy design as it would be hard to make a living at it.The development classes seem to be driven by enthusiastic amateurs or the proprietors of small boatbuilding businesses.
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