changing the shape of the boat

Discussion in 'Sailboats' started by tamkvaitis, Jan 3, 2007.

  1. tamkvaitis
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    tamkvaitis sailor/amateur designer

    Has anybody been trying to design boat wich could change its shape. I am tolking about aft part of the boat. transom, could be made fuller then going downwind and less fuller then going upwind. Comments?
     
  2. CT 249
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    CT 249 Senior Member

    Yep.

    Rucanor Tri Star (I think that was the name) tried to do something similar in the 84/85 (I think) Whitbread Round the World race. It had a hinging stern that dropped down to extend the waterline and flatten the sections aft. The idea was banned by IOR (it was probably always of very dubious legality).

    Since then, the idea has returned in the Hugh Welbourne supermaxi Bols, which has apparently been a notable failure.
     
  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I have been thinking about a hinged area in the after part of a sailboat to make a quite flat underbody when motoring, and also to increase volume aft, sailing at 5 knots with the outboard stored midship, planing at 15 knots with the engine on the transom.
     
  4. Cliff Pope
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    Cliff Pope Junior Member

    Boats change their shape when they heel, and also when the internal ballast, eg the crew, is shifted backwards or forwards.
    And centreboards of course radically alter the underwater profile.
     
  5. tamkvaitis
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    tamkvaitis sailor/amateur designer

    shape of the boat doeasn't change in these situations
     
  6. Tim B
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    Tim B Senior Member

    I think Cliff meant the effective underwater shape. He is right, of course. A yacht, in particular will appear to be highly assymetrical below the waterline when heeled.

    This is one reason for trying to increase the effective length of the waterline at heel, as well as the obvious advantages for reducing wave drag.

    As far as a controllable shape-changing hull is concerned, I think you'd be better served using smart materials in keels and rudders instead. To have a notable effect from changing the hull the movement would have to be quite considerable.

    Mind you, there have been all manner of wierd and wonderful hull-shapes over the years...

    Tim B.
     
  7. Cliff Pope
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    Cliff Pope Junior Member

    Of course it does. Boats don't have a fixed underwater profile, half the subtlety of designing a boat is surely the variation in section at different angles of heel or trim?
    Look at the way a crew trims a dinghy for different angles to the wind - running, centreboard up, crew aft. Reaching, crew balance the boat just enough so that the boat rides on the chine. Close-hauled, centreboard down, more weight forward gives the forefoot more grip. It all depends on the design of boat.
    One with long overhangs will get "longer" as weight is moved to one end. Those with an inward sloping transom will get "shorter".
    A boat with a lot of bulk in the chines will get "wider" as it heels. One with tumblehome will get "narrower".
    I'm not saying that is the only way of altering a boat's size, far from it, but the effect is there and has been known, probably for centuries.
     
  8. tamkvaitis
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    tamkvaitis sailor/amateur designer

    I am reading F.Bethwaites book, and I was thinkinga about that the ultimate sailing boat should look like. I know that it is unreal to create ultimate sailboat, but I am just dreaming ;) As I am looking at 40-60 ft I am thinking about canter. To create ultimate downwind performance the boat should have huge amount of flat areas which would help to plane easily, but these flat surfaces would create very uncomfortable motion then going upwind. So I thout about changing the shape of the boat then sailing.

    Talking about my ultimate sailboat idea now I think it should have these features:

    canting keel - it is the most eficient way to create righting moment and keep the boat self righting.
    wing mast - The rig is more eficient using wing mast. Actualy I am thinking about freestanding mast, but a lot of calculations should be done, to predict the potential weight of mast ans structure suporting it.

    I am stil thinking about other features.
     
  9. Tim B
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    Tim B Senior Member

    When you start designing, you take a decision to optimise for upwind, offwind, or downwind performance. Open 60s etc. are predominantly downwind boats, sportsboats tend to be offwind or upwind. A dinghy is really the only boat you can expect good performace around the whole polar due to the massive change in righting moment from the crew hiking (remember that each crewman probably weighs almost as much as the boat, more in some cases).

    Tim B.
     
  10. tamkvaitis
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    tamkvaitis sailor/amateur designer

    I understand this thing about dinghies, thats why I started talking about changing the shape of the boat
     
  11. water addict
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    water addict Naval Architect

    It has been done before. There was a boat in the 80s (my memory aint what it used to be so I do not remember exactly) that had two triangular wedges that folded down as planing surfaces under power. I am almost certain that Robert Perry had it in his design review section of Sailing magazine.
     
  12. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Do you remember if it worked OK?
    I think it's most interesting for quite small boats.
     
  13. water addict
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    water addict Naval Architect

    Unfortunately, I did not hear anything about performance. Sorry.
     
  14. gonzo
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    gonzo Senior Member

    Yes, trim tabs.
     

  15. Doug Lord

    Doug Lord Guest

    IMOCA 60 "trim tab"

    -------------------
    Heres an excellent thread on SA with participation from the guy who designed the "trim tab" flaps on Bols:
    Imoca 60 with planing trim-tab - Sailing Anarchy Forums
    Address:http://www.sailinganarchy.com/forums/index.php?showtopic=48072&st=0&
     
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