Appropriate Scale for Modeling

Discussion in 'Sailboats' started by MastMonkey, Jan 10, 2011.

  1. MastMonkey
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    MastMonkey Junior Member

    I am interested in true twin keel designs. It seems designs incorporating them have had mixed success based on my reading of the subject here. There have been some suggestions that they could offer some performance advantages due to the more effective lateral resistance a twin keel offers as well as the potential to shape the keels with asymmetrical profiles. If I wanted to pursue some research into effective twin keel arrangement, determining details such as the most effective camber, location fore and aft, appropriate corresponding hull type, which scale model would be give me results that could be applied to a life size hull?
     
    Last edited: Jan 10, 2011
  2. daiquiri
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    daiquiri Engineering and Design

    Everything could be true about twin keels except that one. Twin keels will give less total lift and more total drag, when compared to a geometrically similar single keel of the same total surface, thus making it less efficient in generating lateral resistance.
     
  3. gonzo
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    gonzo Senior Member

    Twin keels have two advantages: One they draw less water, and two they keep a boat level at low tide. From the performance point of view, there are no advantages.
     
  4. MastMonkey
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    MastMonkey Junior Member

    Maybe a more appropriate question then would be to determine how to preserve the advantages of the fixed keel (i.e. shoal draft) without sacrifices made to performance. If a twin keeled vessel could equal the performance of a single keeled vessel, without a loss of performance, it would be advantageous. Per the performance, my concern was not to best the single keel, but to overcome the limitations of twin keels. I am under the impression that a couple of unique aspects, mentioned in my original post, of the twin keel arrangement could help it do so. That one keel is always vertical has potential for improved later resistance over a heeled singled keel vessel was my belief. I would like to conduct research similar to what Patrick Bray did into twin keels:

    http://www.brayyachtdesign.bc.ca/article_twinkeels.html

    But I am uncertain at which scale results of a model test would be accurate and scalable. My interests is in determining the ideal twin keel arrangement through experimentation. The article mentions that there is not much data regarding hull form testing of vessels specifically designed to be twin keeled vessels and it implies that there may be a combination of hull form and keel arrangement that can match a single keel for performance.
     
  5. Paul B

    Paul B Previous Member

    Perhaps you should ask the staff at the towing tank facility you plan to use for your experiments?
     
  6. BATAAN
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    BATAAN Senior Member

    3/4 inch to the foot is 1/16th full size and is easy to square, extract square roots from, and to cube. Speed in scale proportion is as to the prototype as the square root of the linear ratio. The linear ratio is 16. Sq root of 16 is 4 so the model will exhibit scale behavior at 1/4 the speed of the full size design.
    As to displacement, scale varies by the cube of linear ratio, on 3/4" scale as 16x16x16=4,096, so if your model trimmed to waterline weighs one pound the real boat weighs 4,096 pounds. A Lincoln penny scales out at exactly 25 pounds scale weight at this scale.
    I've taken the above from Weston Farmer, who gives a very detailed account of how to build scale flotational models from poster board centerline bulkhead and shaped bulkheads, 1/8" balsa planking and waterproofed with shellac and varnish, in his book "From my old Boat Shop". As he began NA practice in 1919 and worked until the late 70s he has some lessons for all who would know something about how powerboats go through water.
    The man was a virtual preacher for using cheap models to prove a drawing board design before any timber was purchased and said it saved his *** numerous times in stupid mistakes.
    Honest men make honest boats.
     
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  7. Paul B

    Paul B Previous Member

    I think every notable test tank in the world will tell you that a 1/16 scale model is pretty useless as a test model.
     
  8. BATAAN
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    BATAAN Senior Member

    I've never done it myself but who said anything about a test tank? That costs money.
    WF noted that a walking speed on 4 mph a scale speed of 16 mph is achieved.
    All you need is calm water and knowing what you're looking for in the wake/speed behavior. The towing line must be low, a needle puts it below the w/l and in the extension of proper shaft angle. Use a fishing rod to tow.
    Method works for boats 15 to 50 feet quite well for $10 materials and a few hours work.
    At many Yachtsman's Dollars per hour, much more expensive model, technicians hovering, a test tank is real cool if you're designing an America's Cupper or TITANIC but for most plain boats this is enough.
     
  9. Paul B

    Paul B Previous Member

    Your Rube Goldberg method does not tell the original poster anything about what he wants to learn.
     
  10. gonzo
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    gonzo Senior Member

    I think Bataan is answering the question of scale. 1/16 scale would give you a 2' model for a 32' boat which is adequate.
     
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  11. BATAAN
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    BATAAN Senior Member

    It's certainly not my Rube Golberg method as I'm far too dumb to figure it out.
    WF was a NA of 60 years experience and I merely quote him.
    The poster asked for an appropriate scale to judge his design with.
     
  12. Paul B

    Paul B Previous Member

    Adequate? To determine what?
     
  13. Paul B

    Paul B Previous Member

    Your answer to his question is about as useful to him as simply writing different configurations on slips of paper and drawing them randomly from a hat.
     
  14. thedutchtouch
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    thedutchtouch Junior Member

    i've heard of people using 1.5" to the 1'. same easy math as above, twice the size. 1 inch real = 1/8 inch scale model.
     

  15. BATAAN
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    BATAAN Senior Member

    Properly scale weighted 1/16 scale flotational model in a fishtank gives trim, stability and waterline info instantly and accurately. Towing same in flat calm and video taping the result gives more info on wake making and steering characteristics. This is how mechanics did it in the past and WF's 70 foot aluminum motoryachts are a testament to his using the technique, in support of his traditional design skills. Not every vessel design needs the ultimate NASA engineering approach to get a successful result. Some designers of the past were incredibly competent and I try to learn from them.
     
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