BWL and Hull resistance

Discussion in 'Multihulls' started by APP, May 12, 2011.

  1. APP
    Joined: Nov 2010
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    APP Junior Member


    Is there any formula or graph that relates BWL and Hull resistance? For various Fn ranges?
    Suppose we keep Length, Displ. and wetted surface constant and we increase the Hull BWL, how is affected the resistance?

    Thank you
    Best Regards
  2. Leo Lazauskas
    Joined: Jan 2002
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    Leo Lazauskas Senior Member

    For thin ships, wave resistance increases with the square of the beam.
    For stubbier vessels, there might also be an increase in form drag and from some other non-linear effects, such as the interference between the hullwave and the boundary layer.

    Good luck!
  3. APP
    Joined: Nov 2010
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    APP Junior Member

    OK Thanks. Thin Hulls: If I need to increase the Displacement figure (given a certain CAT length e.g. 18 to 20 m), I have to increase either BWL or Draft or both in a proportion. Is there any common practice suggested?

    Best Regards
  4. BigCat
    Joined: Jan 2008
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    BigCat Junior Member

    Catamaran hull waterline beam, depth, and length.

    Look at John Shuttleworth's website. If I recall, he said never have a hull depth greater than a hull beam, from the standpoint of resistance. Of course, in a cruising catamaran, the desire for a nice cabin in the hull usually precludes that. In practice, I think you rarely see a hull waterline beam that isn't at least half of the hull draft. In general, a waterline beam that is one tenth or more of the waterline length is pretty good, 12 is pretty sleek, and 16 or more is a pure racer. Naturally, smaller cruising catamarans are more like 8. A DWL length of less than 8 waterline beams is likely to be pretty slow. Once you have your waterline beam selected, and a displacement and prismatic coefficient, there aren't really any decisions left to make that will affect hull depth. My Bigcat 65 has a hull beam of a tad over 12. (DWL length / DWL beam > 12.) See: :eek:
  5. Adler
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    Adler Senior Member


    Dear APP,

    If you astrict your interest within the limits of Semi-displacement hull definition, the gathered data and the processed results will be strong correlated. I supposed so, that the extracted conclusion should be easier.

  6. rayaldridge
    Joined: Jun 2006
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    rayaldridge Senior Member

    Freeship gives you access to various tools you can use to assess the drag of various hulls. One that I use is the Kaper module, which was designed to measure drag in canoes and kayaks, and is therefore usable for some multi designs.

    One approach to increasing displacement that I've used in the tiny cats I've designed is to dispense with the extra chine and use dory-like hulls. For a given length, draft, and fineness, the dory hull will have more displacement than one that uses chines to approximate a semicircular section. Because I feel that the turbulence generated by this approach is less important than the hull fineness obtainable, this is the approach I prefer for very small cats, in which it is difficult to get sufficient displacement.

    As examples, my 16 foot open beachcruiser Slider has 10 to 1 hulls. My 23 foot voyaging cat will have 11 to 1 hulls. The latter displaces 2200 pounds and draws 11 inches.
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