Hypothetical Question - can the point where wakes cross between hulls be moved by hull shape?

Discussion in 'Multihulls' started by ElGringo, Feb 12, 2019.

  1. ElGringo
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    ElGringo Senior Member

    It has been a long time since I last visited this website due to a severe medical problem. I have sold all the materials I was gathering and have accepted the fact that I will never get to build the small catamaran that I asked a lot of design questions about. These days all I can do is wonder, and I come up with some crazy questions.
    The question that is bothering me at the moment is... can the point where the wakes cross between the hulls be moved by hull shape? If you had a common looking hull could you cut it straight down the center line for a distance and then make the transition to the wider width of the aft part of the hull and shift the crossing of the wakes to the aft?
    As you may remember, once I get past the basic 6+4=9 I get lost in a hurry so a yes or no answer would be appreciated, or maybe a short explanation.
     
  2. ElGringo
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    ElGringo Senior Member

    OK, 92 people have read this and I have no answer so I must not have made myself very clear so I'll try again. If you had a catamaran hull that was symmetrical and shaped like a pencil, and you split it down the middle and made the insides straight and flat. Using half the original hull on each side, would you still have waves crossing each other between the hulls?
     
  3. TANSL
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    TANSL Senior Member

    Of course. What has changed?. Wave will be,probably, smaller but is still there.
     
  4. ElGringo
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    ElGringo Senior Member

    TANSL, is there a way to figure where the waves will cross?
     
  5. TANSL
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    TANSL Senior Member

  6. Richard Woods
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    Richard Woods Woods Designs

    Michelet?? of course the point where the bow waves cross each other depends on the hull spacing. Make it wider and the crossing point is aft the hull, narrow and its under the bridgedeck

    Photo, not very clear I'm afraid, but it was taken over 40 years ago, shows me testing a catamaran hull in the Southampton test tank to see how the drag varied with variations in hull spacing

    Richard Woods of Woods Designs
    Richard tank test 2.jpg
    www.sailingcatamarans.com
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    As I understand the query, he wants to know if the spacing can be narrowed, by having less hull on the inside, and even none, with a box tunnel, with slab sides, like a monohull boat split down the centre.
     
  8. ElGringo
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    ElGringo Senior Member

    I am really wondering if the drag from the wave crossing between the hulls can be reduced or eliminated by making the inside of the hulls a straight line.
     
  9. TANSL
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    TANSL Senior Member

    Yes, drag from the wave crossing between the hulls can be reduced.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Actually, I think one of Richard Woods' Skoota cats has asymmetric hulls, with less "shape" on the inside face, but he'd be the man to explain the rationale with that.
     
  11. BlueBell
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    BlueBell Ahhhhh...

    So, you mean the bow waves.
    I'm pretty sure the answer is no but a lot can be done to reduce the size of that wave.
    But it really depends on so many factors: weight, length, beam, shape, power vs sail vs human power, etc, etc, etc...
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess the real question is whether the interference drag can be reduced, but without then increasing resistance in other ways, and the overall drag equation.
     
  13. redreuben
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    redreuben redreuben

    It can, that’s what bow bulbs on bulk carriers are all about but they are designed for a fixed speed since sailboats change speed all over the place no one shape is going to cover that.
    I thought what is now the pretty much standardised hull spacing was the best one size fits all solution.
    Investigating hull spacing might provide you a better answer.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Limited at best, unless it is a very bluff bow.

    Yes. These waves are divergent and transverse waves.
    But i suspect your real questions, do they add any additional drag?

    Yes, again simple hydrodynamic theory will tell you that:
    upload_2019-2-17_13-42-13.png

    But, again, I suspect your question is more about the spacing apart of the 2 hulls, and their interference.
     

  15. Manfred.pech
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    Manfred.pech Senior Member

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