Powerboat shaped canoe body vs Mac Gregor vs Sailing

Discussion in 'Boat Design' started by vv3k4i, Mar 10, 2008.

  1. vv3k4i
    Joined: Mar 2008
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    vv3k4i Junior Member

    Hi,

    I've been reading boatdesign forum for some time, but never felt a need to write about any problems. But now I'm little confused. I see boats like Mac Gregor or Odin, and they're bottoms seems to be good for planning, but software and theoretical calculations says slightly different. What I've found and dont know if it's correct, is that Mac gregor - shaped bottom is about 15-20% less efficient than bottom of typical sailing yacht, and about 30-35% less than powerboat shaped bottom. Assuming the same LWL, BWL, Disp and velocity around 25kts. Any thoughts?
     
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    When you say "what i have found" - is that your calcs? As an ex owner of a MacGregor I would have said 10% off the hull speed sailing. I never got to compare 26ft power boats with 50hp motors, but 18 knots seemed pretty good for the weight and the mast and rigging.
    I am in the design stages of a similar boat, and i told the designer to ramp up the powerboat hull efficiency, at the expense of sailing.
    My logic is that a percent of 6-10 knots is hardly worth bothering about, but 10% of 20 knots is more noticable.
    I wonder how the really flat bottom boats like Norwalk island sharpies and Bolger designs work out?
     
  3. terhohalme
    Joined: Jun 2003
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    terhohalme BEng Boat Technology

    To my eye, the bottom shape of MacGregor 26 is more or less optimal for speeds 10-20 knots. At sailing speeds there is lot of viscous pressure resistance caused by the transom separation, approx 15-20% more total resistance at 5 knots compared to optimized sailboat form.

    The numbers are from semiplaning test project, which end up very similar hull shape.
     
  4. vv3k4i
    Joined: Mar 2008
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    vv3k4i Junior Member

    Well Yes, I have counted it one more time, there is realy less resistance on the MC hull at 10+ knots. I've just used wrong methode.
    So another question. What do You think about designing hull for velocities exeeding 25 knots? I dont know much about yacht bechaviour at this speed. Is it still safe?
     

  5. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I know 20 knots under power is great in smooth seas, but with enough wind to sail a boat at 20+ knots you are usually going to get very rough seas as well.
    Powering at 12+ knots through very rough seas is veeeery uncomfortable in a monohull.
    I would prefer something long and narrow in heavy seas like a tri or cat, but rolling and tumbling in 25+ knots in a monohull is hell, as any sydney to hobart yachtie will tell you.
     
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