Asymmetric hulls

Discussion in 'Multihulls' started by Trevlyns, Oct 15, 2006.

  1. Derek_9103
    Joined: Oct 2019
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    Location: Waterbury, CT

    Derek_9103 Derek

    Thank you for sharing your knowledge Tom.

    I'll do a little trial and error "test group / control group" experimentation. I'll rig two radio-controlled trimarans, one "stock", and then various versions of a "ama as a foil", then race them against each other to windward. Not looking for the "ama as a foil" version to win, just not lose too badly.

    Well said, and I get your point. I get there's enough negative impact to not race a Hobie 16 against something with a more efficient foil like a daggerboard - but if I make allowances and dial back speed expectations a little, it seems there still might be some room to play with, to also buy some simplicity and peace of mind for cruising. Even if I could shrink the centerboard by 40% that would be worth it to me. If I play around with it and it's a TOTAL flop, so be it.

    Cool, good to know!

    [Edit: I'd guess graphs I've seen showing induced drag, parasitic drag, and combined drag assume the object of discussion is an airplane. But wouldn't a couple factors have that graph look different for boat leeway control? First, a plane needs to lift its whole weight into the air, that's a lot of lift, and a boat only needs to counter leeway forces decently less than how much the whole boat weighs - and - water density is way higher than air density, so wouldn't that also increase the "relative weight" of parasitic drag compared to induced drag?

    I'm factoring in Tom's statement about hull as a canard, rudder as a wing, but MAYBE even with that bass ackwards approach there's still a potential benefit to harvest. Yes, there's enough induced drag so a Hobie 16 won't win many races, but, I want to test out the "good enough for 1970, so good enough for a cruiser" benefit implied by the fact Hobie 16's aren't considered a total joke, and they kicked off an entire industry.

    In racing, a 5% increase in induced drag is the difference between consistent wins and consistent losses, I'd like to test the theory that the Hobie idea can't get any love because they can't win races, but it's still a workable idea outside of that context of racing, because they're still fast-ISH.]

    Worst case I just start with Hobie's foil profile, adjusted for people's critiques to improve them I've seen in other places. And maybe a modified NACA 4412, since I wouldn't know where else to start.

    I know there would be a performance hit - with that as a given, any idea of what avenues to MAYBE play with? I'm willing to fail here - whatever the outcome, it will be a learning experience.
     
    Last edited: Sep 23, 2020
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    The flow around an ama has very little to nothing to do with the flow around an airfoil, even the the waterline of the ama has the same shape as an airfoil. The flow around the ama is inherently 3D. The flow around an airfoil is 2D or close to 2D.
     
  3. Derek_9103
    Joined: Oct 2019
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    Location: Waterbury, CT

    Derek_9103 Derek

    Mike Waters used the idea of sharp edges in his W17 trimaran to at least partially address this 2D/3D disparity, if I understand correctly - I think you could "stack" the waterlines of the foil with all the lower (outer) edges on top of each other to get a similar "sharp outer edge" with the same effect. That's the idea I had in mind to address what you're saying.
     

  4. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Sharp longitudinal edges may increase lift by causing longitudinal vortex formation, not be making the flow closer to 2D.
     
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