Help with increasing displacement

Discussion in 'Boat Design' started by LTDboatdesign, Oct 25, 2010.

  1. LTDboatdesign
    Joined: Oct 2010
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    LTDboatdesign Junior Member

    Hi everyone, I am currently designing a single chine double ended sailboat. I am in the process of finishing the hull design and I want to add more displacement to the boat. As of now she displaces around 8500lbs. i want to increase that by about 1000lbs. I am wondering if there is anything wrong with just moving the water line up 1.5 inches. that, by displacement would be about where i want to be. I could also change the underwater shape, but i want to see what you all had to say.

    thanks, Cody
  2. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    That's 12% heavier which is reasonable enough for the hullform of boat you describe not to worry too much about a major screwing up your coefficients.

    You don't give enough information for very specific advice. Just watch your scantlings and look hard at your stability under sail ( Initial slope of the GZ curve ).
  3. gonzo
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    gonzo Senior Member

    To increase the displacement, you add more weight to the boat. If you want a different hull shape, it may have the same, more or less displacement. The total displacement is largely a function of scantlings and equipment. I am not exactly sure what your specific problem is.
  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I understand that your problem is that you have nearly finished this design stage and have realized that the boat will have to displace more than was calculated at the beginning of the stage.

    You have two choices now: to either start another design iteration (that's the famous "design spiral"), by modifying hull lines and volume distribution in order to give the hull a more optimum shape for the new displacement, or you can just increase the draft and accept whatever you get in terms of fuel consumption, seakeeping etc.

  5. PAR
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    PAR Yacht Designer/Builder

    If you sailboat design has a fixed appendage, you can increase it's volume while retaining the basic hull form intact. This has a diminishing set of returns and naturally, some draw backs, but It works for modest amounts.
  6. philSweet
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    philSweet Senior Member

    In a similar vein as PAR's comment, if your boat is flat bottommed or low deadrise, you can increase dead rise, lowering the keel and keeping the chine where it is. This doesn't effect stability, but where the extra weight ends up may require you to adjust the stability.
  7. philSweet
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    philSweet Senior Member

    Are we talking about something like Monroe's Egret here? In your design, where does the chine lie at the bow and the stern. Can you post a sketch?

  8. alan white
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    alan white Senior Member

    Depending on the design, in addition to adding ballast or weight in any case, extending the transom aft a bit will also raise it to clear the water, allowing a more efficient flow of water past the lowest point of the stern. It also will slightly move the CLR aft so some small fore-aft adjustments to either rig or keel appendage would be required. This would work only if the waterplane profile at the chines isn't adversely affected (usually the single chine is already close to the waterline and lowering it can adversely "square up" the sides. Hopefully the chines would still clear the water's surface.
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