Planing Trimarans

Discussion in 'Multihulls' started by Doug Lord, Sep 30, 2006.

  1. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I'm out!
     
  2. Dan S
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    Dan S Junior Member

    Doug, why did you go back and delete this post? I was typing out a nice response only to find that you had gone back and deleted it completely. Are you second guessing your own comments now?
     
  3. Doug Lord

    Doug Lord Guest

    planing tri

    Just felt I was wasting my time-no point in keeping it going......
     
  4. Dan S
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    Dan S Junior Member

    Right....
     
  5. ActionPotential
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    ActionPotential Junior Member

    Lift bow?

    Kite - maybe. Very long bowsprit and a huge gennaker does not lift bow.
     
  6. Doug Lord

    Doug Lord Guest

  7. RHough
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    RHough Retro Dude

    Did someone say that a planing trimaran was not possible? Or did they just say that the examples cited (until now) are not planing?

    Now that you have brought up YPE as a planing tri ... how much faster would it have to be to get up on foils?
     
  8. ActionPotential
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    ActionPotential Junior Member

    I don't think anyone said that a planing trimaran was not possible, just that the examples cited are not planing.
    A trimaran can plane if it can be balanced to keep both amas out of the water. Or it can plane on its leeward ama if both the other hulls are flying.
    Re: YPE
    "two of the hulls travel along the same path. The third hull is for the crew of two and should fly to windward"
    Since 2 of the hulls travel along the same path aren't they just one hull with a gap in it?
    Looks like a proa to me, or if the windward 'hull' always flies, a monohull.
    It's certainly fast enough to get up on foils but would the foils be any smaller in area than the leeward hull(s)? Are they just buoyant foils?
     
  9. ActionPotential
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    ActionPotential Junior Member

    What we were discussing was a trimaran planing on it's centre hull and whether it is planing if an ama is supporting some of the weight. Even if we do classify YPE as a tri, which is it's centre hull, which are its amas?
     
  10. RHough
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    RHough Retro Dude

    The same logic that makes YPE a trimaran, would make a boat on two foils a catamaran (i.e. foiling Moth).

    The same logic that makes a foiling Moth a mono would make the YPE a proa.
     
  11. CT 249
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    CT 249 Senior Member

    Surely the definition of planing isn't when ALL the boat is supported by dynamic lift - the portion of the hull immersed in water is still displacing SOME water, therefore according to simple physics it is still generating SOME lift - and therefore not all the boat is supported by dynamic lift.

    Isn't an America's Cup and skiff NA correct when he says "The naval architecture definition is clear. If the centre of gravity is above where it would be if the boat is floating at its static waterline, it’s planing”?

    Re the claim that "On the other hand a hull like the seahugger Moth with a moderate beam to length ratio(10/1) has characteristics of both types: it begins to plane at around 10-15 knots because it's moderate beam to length ratio stalls the onset of serious wavemaking resistance and it's flat hull sections promote planing."

    That's now how it works according to the man who created the narrow Moth. He says "Moths now don't plane, but just exceed hull speed by sveral times... ". He regards a planing type boat, even when it has a very similar shape and ratios but is much beamier, as "different" to the Moth because only one of them (not the Moth) is a planing hull and notes that "the lift in a moth is unimportant". Another Moth designer, also a champ in cats, says that the modern narrow Moth is NOT a planing hull but operates in the fashion of a catamaran's super-slim hull. The NA who has lent his computer and VPP etc to the designers of narrow Moths that have won several worlds is not sure whether Moths plane at all. Some other Mothies say narrow Moths DO in fact plane....some don't. There's world champs on each side but it's probably fair to say that most Moth designers favour the idea that modern seahugger Moths do NOT plane.

    The Mothies aren't alone; years after designing Avenger, the man who "invented" the planing dinghy, Uffa Fox, told naval architects that the sailboats of the ‘50s, even at their fastest, were “not really in the “planing” range at all! As Julian Bethwaite says, planing is “virtually impossible to define – and the more you look into it, the more impossible it is to define”.

    It's interesting how planing is seen as such a key to speed, when most modern skiffs are designed with less of an idea about maximising planing lift than old boats - compare the narrow skiff seahugger Moth to the scow Moth. And a speed sailboard just proves the problems of planing......they "plane" (in the true sense ie that they are rising above their static waterline) at about 2 knots, but you can still be up to your bum in water and being passed by Optimists.
     
  12. ActionPotential
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    ActionPotential Junior Member

    By that definition a trimaran, as soon as the leeward ama supports any weight, the boat is planing.
    Discussion was around the centre hull of a trimaran and my opinion is that if we can have both amas clear of the water and then raise the GC above it's static position then the centre hull is planing.
     
  13. CT 249
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    CT 249 Senior Member

    Not necessarily, surely, because it must be the static waterline when heeled - otherwise any boat with any volume in the bilge would be "planing" when it heels as someone steps aboard, even when tied to the dock. A scow Moth would be "planing" if you heeled it without the rig.

    So the C of G of the two conditions - moving and static - must be compared keeping the angle of heel identical. There's an illustration in one of the books mentioned in this thread showing the movement of the C of G as a boat planes, I think; down as the boat picks up speed and the wave train causes it to sink, then up when planing begins.
     
  14. ActionPotential
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    ActionPotential Junior Member

    In the case of a trimaran if the centre hull is lifted by the heeling force rather than hydrodynamics it is not planing but tending towards hull flying instead, regardless of the hull shape/dimensions. I believe that this will occur long before hydrodynamics lift the hull. Still happy to say it is planing if hydrodynamics can lift the centre hull withboth amas clear.
     

  15. CT 249
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    CT 249 Senior Member

    Sure, the centre hull is lifted by the heeling force and that's not planing. The point I was trying to make was that the two effects - the shift in the c of g of the boat due to heel, and the shift in the c of g of the boat due to dynamic lift - are separate and that therefore (once you allow for that, which you have to do on most boats) the definition of planing given abo ve (ie a boat is planing when the dynamic lift causes it to rise above its static c of g) still applies.

    Of interest, the Irens ORMA 60s, certainly those of some time ago (I'm not sure about the current ones) had amas that were designed flatter than the main hull, with more thought to dynamic lift - so maybe we have to consider dynamic lift from the amas as well and therefore

    It seems a moot point to me, as we all know planing multis have been tried heaps of times and never been much of a success due to the small/tiny stagnation point etc. Since width, not length, is what counts in generating planing lift, a long skinny multi is an innefficient shape for planing.

    I thought, from conversations to those who have asked Farrier about it, that the idea is that the F boats' main hull generates enough lift that it reduces the disadvantage of having the fat centre hull (necessary for accomodation) rather than to actually improve performance over the optimum slimmer hull. But I may have remembered this incorrectly.
     
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