Beam and depth increase

Discussion in 'Boat Design' started by wudenbote, Aug 23, 2008.

  1. wudenbote
    Joined: Aug 2008
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    Location: Middleville Mi

    wudenbote Junior Member

    Fellow wooden boat lovers: Thanks for all the input I have received...it is much appreciated. I will continue to bug all of you until my project is completed. The boat I am going to build is a modified Pacific dory with an OAL of 20' 8". My question is this...I would like to increase the beam and depth by 20%. Am I correct in assuming that stability would not be an issue if I increase both the beam and depth and thickness of the frames by 20%? Using a percent figure should keep all relationships the same. The boat will be conventional marine ply over hardwood frames. At present I am building the strongback. Thanks for all the help.....and I ALWAYS finish my beer :)
     
  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Sounds like a really bad idea - basically, you are suggesting increasing the depth by the same amount as the size of the frames, which means the inside diameter of the frames stays the same, and the frames extend another 20% out. That is both unnecessary and weight adding.

    You cant just apply an abritrary percentage to a design. You have stability issues by raising the centre of gravity,, and you are not increasing the length of the boat by 20% ..... and it goe on and on.

    Get another boat design with the interior room and beam that you need - but modifying a design in this manner is a really bad idea.
     
  3. terhohalme
    Joined: Jun 2003
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    Location: Kotka, Finland

    terhohalme BEng Boat Technology

    rwatson,
    I don't think you got it right? Wasn't it a 20 % growth in molding and siding of frame? Sounds reasonable.

    Your judgement of stability issues is based on what?

    Increasing of beam by 20% will make her metacentric height (stability) about 20% higher. For:

    BM = I/D

    and I will grow 1.2^3 times and D will grow about 1.2^2 times. There is still room for a 20% rise of gravity...

    Here BM is hight from bouyancy to metacentric, I is second moment of water plane and D is displacement.

    wudenboat, you knew the new design is then about 1.4 times heavier too? You should use 16% thicker plywood (yes, hard to find).
     
  4. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Oh yeah - sounds good easy.

    I roughed up a quick sketch of the result.

    Doesnt look like too many problems to me.
     

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  5. terhohalme
    Joined: Jun 2003
    Posts: 512
    Likes: 38, Points: 28, Legacy Rep: 506
    Location: Kotka, Finland

    terhohalme BEng Boat Technology

    OK, now I know how the pacific dory looks like.
     
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