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  1. #1
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    Jul 2004
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    Default Why exactly are small yachts slower than large ones?

    I'm talking about displacement hulls here , not planing designs.
    I am totally familiar with the formulae here (mainly the empirical ones) and it is a given that boats with longer lwl's are faster than those with shorter lwl's.

    I am also familiar with the scaling factor , ie. to a certain extent why model boats heel in the slightest winds compared with their up scaled cousins. To a degree this is to do with both form stability and where their ballast is located.
    What I may fail to comprehend is the basic that is operative as any design is scaled , ie what happens to beam , displacement and wave theory etc.
    A small boat can have the same sail area to displacement ratio as a large one.

    So I hope I understand the principles as in real life and in boat design.

    What i would like to understand is how does this work at the basic hydrodynamic level?
    Would somebody please explain with formulae if necessary but in a way i can understand the basics.

    It seems something like a Volvo boat (mono's anyway) can play be different rules, even when not planing.

    TIA

  2. #2
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    Apr 2009
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    Default Re: Why exactly are small yachts slower than large ones?

    Depending upon how seriously you are interested, there are a number of authors who have written books for the popular market. CA Marchaj for one. As well as a couple by him, I have one by Fabio Fosatti translated from Italian.

    It is quite complex with many factors coming into play but at a basic level the predominant resistance above slow speeds is the wave making resistance which is a factor of (approximately - i.e other hull & foil design factor can't be ignored) the waterline length. Basically the hull generates waves (the energy to do this comes from the boat) and those waves have a certain speed which is dependent upon their wavelength. It takes an awful lot more energy to drive a boat at a faster speed than the waves it generates. This is known as hull speed. A simplistic way to think about it is that once you're at that speed you are climbing a wave that you've created. Planning hulls break out of this by rising above that wave.

    Very simplistic, but I hope it gives you a general idea.

  3. #3
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    Default Re: Why exactly are small yachts slower than large ones?

    Quote Originally Posted by lpdsn View Post
    Depending upon how seriously you are interested, there are a number of authors who have written books for the popular market. CA Marchaj for one. As well as a couple by him, I have one by Fabio Fosatti translated from Italian.

    It is quite complex with many factors coming into play but at a basic level the predominant resistance above slow speeds is the wave making resistance which is a factor of (approximately - i.e other hull & foil design factor can't be ignored) the waterline length. Basically the hull generates waves (the energy to do this comes from the boat) and those waves have a certain speed which is dependent upon their wavelength. It takes an awful lot more energy to drive a boat at a faster speed than the waves it generates. This is known as hull speed. A simplistic way to think about it is that once you're at that speed you are climbing a wave that you've created. Planning hulls break out of this by rising above that wave.

    Very simplistic, but I hope it gives you a general idea.
    Yes. I understand to some extent the fact that a boat has to displace the mass of water equal to the boats mass as it moves.
    Obviously a larger boat must displace a greater mass but also has more power available from a greater sail area so I assume the lwl is effectively correlated to weight.
    Again a cat uses a different set of physics.

    However it can't be a linear relationship.
    (Stops rambling)

  4. #4
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    Default Re: Why exactly are small yachts slower than large ones?

    I think you've misunderstood what I'm trying to say. It is not the weight of water displaced, it is the wavelength of the waves generated by that displacement of water that is the main factor.

    LWL is waterline length. At hull speed it determines the wave length of the waves generated by the hull. The longer the wavelength the faster the waves travel so in the same way the higher the hull speed is. It's certainly not linear but I can't remember the exact forumla of wavelength to wave speed off the top of my head. Generally hull speed has a relationship based on 1/SQRT(LWL) but it is only the main factor and not the only factor.

    There's not a fixed relationship between weight of a boat and length. I remember a pontoon neighbour a few years ago had a boat only 1' longer than mine but with a design displacement about 190% of mine. Different boats different design approaches.

    Cats don't use a different set of physics (in fact they illustrate that lwl is not the only factor). Planning boats do.

  5. #5
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    Default Re: Why exactly are small yachts slower than large ones?

    Its all about friction.

  6. #6
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    May 2001
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    Default Re: Why exactly are small yachts slower than large ones?

    Quote Originally Posted by lpdsn View Post
    I think you've misunderstood what I'm trying to say. It is not the weight of water displaced, it is the wavelength of the waves generated by that displacement of water that is the main factor.

    LWL is waterline length. At hull speed it determines the wave length of the waves generated by the hull. The longer the wavelength the faster the waves travel so in the same way the higher the hull speed is. It's certainly not linear but I can't remember the exact forumla of wavelength to wave speed off the top of my head. Generally hull speed has a relationship based on 1/SQRT(LWL) but it is only the main factor and not the only factor.

    There's not a fixed relationship between weight of a boat and length. I remember a pontoon neighbour a few years ago had a boat only 1' longer than mine but with a design displacement about 190% of mine. Different boats different design approaches.

    Cats don't use a different set of physics (in fact they illustrate that lwl is not the only factor). Planning boats do.
    Just a diversion, a submariner once told me that they arrived at the speed of a target, by measuring the bow wave seen through the periscope. This was before bulbous noses were being used (not sure how this affects hull speed).

  7. #7
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    Default Re: Why exactly are small yachts slower than large ones?

    Quote Originally Posted by jakeroyd View Post
    ...I am totally familiar with the formulae here (mainly the empirical ones)
    Or not, obviously. Or you wouldn't be asking the question, or for that matter making sweepingly general remarks that bear no relationship to the hydrodynamics that underlie this issue.

    Start here (one example, but it is a good summary of some of the key principles). You'll find some of the content you're interested in, starting at Acrobat page 157, or document page number 143, onwards.
    http://www.homepages.ed.ac.uk/shs/Cl...E%20Tupper.pdf
    Last edited by Scala; 18-11-17 at 23:09.
    Graham. "Scala" now sold. Boatless.

  8. #8
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    Default Re: Why exactly are small yachts slower than large ones?

    Quote Originally Posted by alant View Post
    Just a diversion, a submariner once told me that they arrived at the speed of a target, by measuring the bow wave seen through the periscope. This was before bulbous noses were being used (not sure how this affects hull speed).
    Sounds plausible, although they had bulbous bows before they had submarines even though they didn't understand exactly how they worked. The idea of bulbous bows is that the trough of the first wave (from the bulb) partially cancels the peak of the second wave (from the bow) so overall less energy goes into the generated bow wave.

  9. #9
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    Default Re: Why exactly are small yachts slower than large ones?

    large boats are faster because of the S/V - G tto LWLratio, although this may seem awfully technical, it is very easy to understand in an empirical manner with simple observation.
    This is best done from a small sailing dinghy moving as fast as you can make it go. Look behind and you will see a yacht with a superior S/V-G to LWL ratio bearing down on you with the crew waving glinting hands in the sunshine, seemingly holding a magical talisman of speed.
    The hand glints as a direct result of the superior ratio and directly affects the performance of the boat.
    Expressed in it's simplest terms it is as follows
    Space/volume of gin to the size of the boat. It is a simple fact larger boats carry more gin, this enables the crew to perform feats of seamanship such as would put Odysseus himself to shame. Fine tuning can be achieved by varying the tonic/ice ratio.

  10. #10
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    Default Re: Why exactly are small yachts slower than large ones?

    Quote Originally Posted by solent clown View Post
    Space/volume of gin to the size of the boat. It is a simple fact larger boats carry more gin, this enables the crew to perform feats of seamanship such as would put Odysseus himself to shame. Fine tuning can be achieved by varying the tonic/ice ratio.
    It's the same effect that causes the wind speed to increase retrospectively once safely ensconced in the club bar. Towards the end of the evening it is possible for a Westerly Centaur to be sailed faster than a Volvo 70.

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