Maximum beam and COB

Discussion in 'Boat Design' started by cthippo, Dec 14, 2011.

  1. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    I'm getting ready to design a variant of the Raptor for a friend. Mine is 17' 3" by 26" and he wants one a bit longer with about the same beam, but more legroom. The idea I'm playing with is trying to separate the center of buoyancy from the point of maximum beam by about 2 or so feet so that when he's sitting in the cockpit his knees are at the widest point of the boat.

    Is there any reason this can't, or shouldn't be done?
     
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  2. KJL38
    Joined: Jun 2008
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    Location: Tasmania

    KJL38 Junior Member

    For the most efficient paddle stroke you need to plant the paddle as close as possible to the center of the boat so it may not be the most efficient. It all depends on priorities.
     
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  3. messabout
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    messabout Senior Member

    You can pull this off by fattening or thinning the sections at one or the other end of the boat. That ploy would change the curve of areas to a different shape which may or may not be optimum. This would let you move the CB to suit the situation

    There are some designers who claim that a long skinny boat needs a couple of sections, at the middle of the boat, to be identical which would tend to make the sides parallel. In this case you have not manipulated the CB but have merely provided knee space aft of the CB
     
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  4. Petros
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    Petros Senior Member

    There is no problem doing this, as long as you match the CB with the CG. Almost every shape conceivable has been used in kayak hulls, small changes result in small changes (or not detectible) changes in how it behaves.

    I think your kayaks are a bit wide for my taste, anything over 22" is wide to me. Realize too that making longer will also increase the righting moment (it displaces more water as it rolls over).
     
  5. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    That's because your rear end isn't 19" wide. I won't even try to get into anything less than 24" wide, and that's usually a tight fit.

    I haven't drawn anything up yet, but I'm thinking that something like a pointy teardrop shape with the maximum beam shifted slightly forward might work. I'll have to draw it up in Freeship and see how the hydrostatics look.
     

  6. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    Here's what I've come up with so far...

    Dimensions are 18' 6" x 26" with a design draft of 6" at a displacement of 300 lbs. This design, named the Black Crowe for the friend I developed it for, is a little slimmer hull on a little longer boat, but the resistance numbers from KAPER look almost the same as the Raptor.
     

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