Round Bilge vs hard chine

Discussion in 'Multihulls' started by nickvonw, Apr 11, 2011.

  1. nickvonw
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    nickvonw Junior Member

    OK, have been doing lots of research and was hoping someone could answer the following question

    apart from aesthetics and building methods what advantages /disadvantages does a round bilge hull when compared to a hull with hard chines

    cheer

    yeeeoooooowww


    nick
     
  2. Doug Lord
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    Doug Lord Flight Ready

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    If you look at a narrow hull- and you compare two equal area sections a semicircular section will have less wetted surface but be slightly wider than a hard chine rectangular section. In the Moth class known for very narrow hulls(10/1-11/1) I was told by John Ilett that the skinnier hull was faster and that was why you see most moth hulls with hard chine square sections. Apparently, the reduction in wavemaking drag by being skinny trumps the reduction in wetted surface of the semi-circular hull.
     
  3. nickvonw
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    nickvonw Junior Member

    thanks doug,

    so why does round bilge seem to be more desirable??

    is it purely asthetics??

    nick
     
  4. Doug Lord
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    Doug Lord Flight Ready

    --------------------------
    There are a lot of factors-structural, aesthetic etc. Once you get into a L/B ratio above, say, 14/1 the advantage of the hard chine hull fades away and the semicircular works best.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Are we talking about powerboats or sailboats?
     
  6. rayaldridge
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    rayaldridge Senior Member

    Round bilge is also easier to pop out of a mold, and is stiffer for the same scantlings.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    You will find I have answered several of your questions on the FAQs page of my website

    And, if you look at the Sango page (follow the link from the under 25ft designs page) you'll see how close a flat panel hull can be to a round bilge hull

    If you are building your own boat I would be wary of building an over-curvaceous design. Filling/sanding is a soul-destroying time-consuming job for little noticeable performance gain. But if you are buying a production boat then, as Ray says, round bilge hulls tend to be stiffer and lighter, (as you don't have extra glass on the chine joints).

    Whether building or buying I would avoid barrel shaped decks unless you and your crew are all very agile or you want a pure racing boat

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. Petros
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    Petros Senior Member

    this has been answered many times on this forum. Similar size displacement hulls have little performance differences (expect perhaps in the most competitive racing only designs), on light weight planing hulls, hard chines allow it come up on plan faster and offer less drag when in planing mode, when in displacement mode the round bilge hull usually has less drag.

    The choice is usually made on the basis of what is easier to build for the method used.
     
  9. redreuben
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    redreuben redreuben

    There are no chines on a fish !
    RR
     
  10. Submarine Tom

    Submarine Tom Previous Member

    ...or on an apple!

    -Tom
     
  11. oldsailor7
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    oldsailor7 Senior Member

    Round bottom hulls are obviously better because they have the lowest wetted surface and skin friction drag.
    BUT.
    The difference can be very little if the chined bottom closely resembles the round.
    As proof of the pudding I took my chine bottomed Buccaneer 28 and glued on 2" slabs of foam, which I then sanded to a circular section and sheathed with glass/epoxy.
    I was never able to discern any difference in the performance. :rolleyes:
     
  12. redreuben
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    redreuben redreuben

    Prob because the float is a sml proportion, if you had done the main hull too ? but you're right not worth the exercise if the boat is built.
    RR.
     
  13. oldsailor7
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    oldsailor7 Senior Member

    That WAS the mainhull. :rolleyes:
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Round bilges reduce wetted surface area and what is called "skin drag" which is desirable if propulsive power is limited, as in a man-powered craft or a power boat designed for minimum fuel consumption. Hard chines assist a boat to plane. A hard chine boat is usually easier and quicker to build. There are many other considerations and it is a complicated swings-and-roundabouts thing. Performance usually catches most peoples attention, but the choice between hard chine and rounded bilge is one of the least significant performance factors.
     

  15. Dean Smith

    Dean Smith Previous Member

    Chime in different point of view
    from a metal builders
    In say a 14m yacht you will add a month(one man) for a round bilge build, a mere 160 man hrs
    Not a radious chine mind you but a really nice forefoot, U shaped, and lovely lines
    For this you can get, quiet upwind sailing, lovely looks and ADDED RESALE
    Chines, yes in planing power boats
    But in yachts , sailing, the hull form is terribly limited with chines
     
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