Lighter than she was designed for.

Discussion in 'Boat Design' started by BTScow, Sep 19, 2009.

  1. BTScow
    Joined: Dec 2006
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    BTScow Junior Member

    What happens you make a boat lighter than what she is designed for?

    My initial though goes to the stability of the boat. Because the boat floats higher, does it mean the Center of Gravity is lower? Then, what of the various ratios; the ballest/Disp ratio goes from 40% to 55 or even 60%. Performance will go up, but at what cost?

    Many Thanks,

  2. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    depends how it sheds weight...

    Assuming it is a sail boat in question; Remove some ballast and the CG will move up meaning less stability when heeled - not a good call. Remove some weigh upstairs may result in the CG going down that will result in a stiffer boat which can also be a pain.
    Be careful not to weaken it structurally by changing or omitting some materials.

    Best to consult the designer of the boat about your intentions or let another NA check things for you
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Most designs do their best if floating "on their lines", meaning trimmed to sit level on the designed waterline. This is the place where the vast majority of her hydrostatic and dynamic calculations have been made.

    A boat that is lighter, of course will not make as big a "hole" and this lack of displacement will dramatically affect how she responds to influences from wind, wave, sea and crew.

    In other words, if you do manage to build a boat substantially lighter then she was intended, you should ballast her down to where she belongs. On the other hand, if you know you can build a boat substantially lighter then the plans, then the boats lines should be reworked around this new target displacement. The result will be a boat that makes a smaller hole in the water, but one that it fits into it precisely.

    It should be noted that a sailboat that is built significantly lighter, but had ballast added low, to pull her down will likely need a NA look at the rig and all of it's attachments, as these will likely now be undersized.

  4. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    Depends on the boat - if it's a multihull then great, the lighter the better. Your wife will find enough stuff to weigh it down in any case.
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