Transom clearance question

Discussion in 'Boat Design' started by k_skit, Sep 17, 2009.

  1. k_skit
    Joined: Aug 2006
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    Location: Moscow, Russia

    k_skit Junior Member

    Hi!

    I am trying to design a sailing boat for myself.

    The basic figures are to be:
    LOA= 8m
    LWL= 7,7-7,9m
    B= 2,54m
    Loaded Displacement= 2,6t
    SA= 40-42 sq. m.

    Disp/LWL ratio about 140 gives a chance for this boat to reach planning (at least theory tells that :) ).

    Now I am trying to develop the hull. I am amateur and have no ability to use towing tests or flow simulation.

    Could somebody tell me how to settle the distance between lowest point of transom and LWL in order to have a longest possible LWL and still avoid separation at the stern wave? Is there any rules on thumb?

    Hope somebody will help me...
     
  2. PAR
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    PAR Yacht Designer/Builder

  3. k_skit
    Joined: Aug 2006
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    k_skit Junior Member

    Dear PAR,

    I am fully agree that any novice should read this book first.
    I have studied two editions of it (edition 2 and 3).

    But I have not found the answer to my question.

    Still thank you for the attention.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Displacement? Plane? Target S/L? you question is a bit ambiguous.
     
  5. k_skit
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    k_skit Junior Member

    To PAR:

    The total displacement is 2,6 ton (fully loaded);
    The target Speed/Length ratio is 1,4
     
  6. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    In a nutshell, no, there are no rules of thumb. The key is the slope of the buttock lines aft--you should make them as horizontally flat and straight as possible. The lowest point of the transom should be as close to the water as you can make it, but not be submerged.

    Your later statement abount displacement/length ratio = 140 as being an indicator of planing probably does not really mean anything. Planing when--upwind or downwind? One of my boat designs, Bagatelle, was designed with a displ/length ratio of about 60, and she could never plane upwind, although occasionally started to plane going downwind. A more important factor for determining planing ability, besides hull shape, is the Sail Area/Displacement ratio. You need a very high ratio because planing requires a LOT of energy--sail power. If you look to the open class around-the-world racers, they all have very high SA/displ ratios, up around 50 or so. Most conventional sailing designs are in the region of 20 or less. And even then, the open class boats cannot plane upwind, only downwind and reaching.

    I hope that helps.

    Eric
     
  7. PAR
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    PAR Yacht Designer/Builder

    Maybe it would be more helpful if we had more information about the design k-skit.

    Other then some very general observations, such as you're not really up on plane at 1.4 S/L or desirable buttock angles, I'm still at a lose as to what you're interested in.

    Some images of your design may shed some light on her potential.
     

  8. k_skit
    Joined: Aug 2006
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    k_skit Junior Member

    Dear Mr. Sponberg
    Dear PAR,

    Sorry for the late answer - I was on the business trip and have had no time to think it over.

    Actually I have modeled approx 30 different hulls while I was reading Larsson/Eliasson book. But the actual hull is not designed yet.

    I will do as Mr. Sponberg told in his post. After all this is to be an amateur boat intended for cruising, not for races.

    Thank you and
    best regartds,
     
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