Multi speed/length relationship?

Discussion in 'Multihulls' started by RHough, May 17, 2008.

  1. rayaldridge
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    rayaldridge Senior Member

    Richard does have some excellent and accessible stuff to say about hull shapes on his website-- highly recommended.

    I agree with his take on maximum beam/waterline ratio, though I'd add the caveat that this is if we're talking about boats with similar SA/D ratios. There have been some boats with very fine hulls that performed well in light air-- like the Gougeon 32. But that was probably due to its huge rig and light weight, rather than its fine hulls. In anything but a zephyr, it tended to tip over.

    I drew my little beachcruising cat Slider with slightly better than 10:1 hulls, because any finer and I wouldn't have had enough displacement to be useful. I used dory hulls like Richard's Janus, for the same reason.

    Ray

    http://slidercat.com/
     
  2. u4ea32
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    u4ea32 Senior Member

    Multihulls are quite a bit different than monohulls.

    Monohulls are so incredibly constrained due to carrying all that lead, and having very little stability, with lots of windage, wetted surface, and wave drag, that its amazing they ever get over 1.34xsqrt(LWL).

    We have a small fleet of multihulls in Hawaii: a 36 foot stretched and turbo charged Formula 28 cat; a Reynolds 33 cat; a Farrier 33 tri; and a 44 foot 12000 lb monster P-Cat. Sometimes we also get some beach cats, like Hobie Fox etc. Each of these boats is very different, and each has its conditions where it beats the others SOUNDLY, by embarrassingly huge margins that no handicap system could ever equalize. So we just race boat-for-boat.

    The Reynolds is usually fastest in light conditions. The F-33 is usually fastest upwind in light to moderate sloppy conditions. Fox are blazingly fast downwind in most conditions. The turbo Formula 28 is almost always fastest. The 44 foot monster is just brutally majestic downwind and reaching in heavy air and huge seas.

    The long thin hulls mean wave speed does not matter much. Stability allows the power to be used, but each allows that power to be used in different conditions -- the fox, with the long sprit, cat handle huge power downwind, as the power limit is when the bows go under, and the sprit lifts the bows; the heavy 44 can keep the power fully applied in huge seas because its so stabile in pitch and yaw; the strecthed 28 can handle huge power in almost all conditions, but has less bow lift (so the Fox is sometimes faster) and less reserve bouyancy (so in survival conditions, the 44 is eventually faster); the F-33 has good power and low drag; the Reynolds has low drag (very light) but poor stability (due to windage of the tramp when it flies a hull, not due to lack of beam or righting moment).

    So simple equations are fun for sitting at the computer, but they don't tell the story at sea, on the race course, at all.
     
  3. BigCat
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    BigCat Junior Member

    DWL beam vs. draft

    Hi, Terho - Thanks for posting those charts. The beam / speed chart explains why Stars 'N Stripes went with 16 to 1.

    Does this mean that you don't get less resistance if the draft is less than the beam at the waterline? The chart doesn't show that range. It seems to me that cruising cats always are shallower than they are wide at the waterline, so it would be of great interest to know.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Best is hull waterline/2=hull draft. With round bilge you got then the best volume/wetted surface ratio.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    You serious? :D :D :D
     
  6. oldsailor7
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    oldsailor7 Senior Member

    YUP.!! :D
     
  7. oldsailor7
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    oldsailor7 Senior Member

    They say "A man convinced against his will, is of the same opinion still".

    This applies to so many monohull/keelboat sailors today.

    They seem unable to conceive that a ballasted monohull is like an athlete running with a ball and chain shackled around his ankle. :eek:
     
  8. DanishBagger
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    DanishBagger Never Again

    By that token, one could make an equally illfounded argument, that an unballasted multihull is like a top-heavy tricycle, or that a multihull is like a tall man trying to stay balanced on two slow water skis towed sideways.

    Anyway, of course it's nonsense, just like your statement. You're misusing the quote you posted.
     
  9. oldsailor7
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    oldsailor7 Senior Member

    Quote:- "Anyway, of course it's nonsense, just like your statement. You're misusing the quote you posted."

    OK. Then try this one.

    Multihulls capsize---Monohulls sink. Take your pick. :eek:
     
  10. DanishBagger
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    DanishBagger Never Again

    Fair enough. But a multihull seems to have a bigger risk at capsizing than a monohull has for sinking. Hmm.

    Anyway, it wasn't my intention to debate which one is "better". I was merely opposing your rhetoric (literally - I'm not putting you down - we all use rhetorics to a certain extent) attemps at "proving" that monohullers are just being backwards.
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    If that were the only thruth we would still be burning withess too :p
     
  12. rayaldridge
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    rayaldridge Senior Member

    Why do you believe this?

    Ray

    http://slidercat.com/
     
  13. DanishBagger
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    DanishBagger Never Again

    What makes you think otherwise, I may ask?
     
  14. RHough
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    RHough Retro Dude

    Okay ... we are way off topic ... but ...

    Multihulls only capsize due to operator error or structural failure.

    Monohulls only sink due to operator error or structural failure.

    The two are equal in this.

    The difference is that after operator error, the multi does not sink but provides a survival platform for the crew that abused it. The mono might decide it has had enough abuse and sink, leaving her crew to fend for themselves.

    There is not one case of a multi capsizing absent human error.

    The list of monos that drop their keel and kill crew is getting longer. Mutli's get safer and faster with each generation, monos are getting faster and less safe.
     

  15. Chris Ostlind

    Chris Ostlind Previous Member

    Pulling apart the slippery folds of the Hornet's nest...
     
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