Why does multihulls have pointy bows?

Discussion in 'Multihulls' started by bjn, Jan 22, 2017.

  1. Manfred.pech
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    Manfred.pech Senior Member

    Don`t despair of round bows. Bjn does not need to build the hulls. A simple model can help: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/two-hulls-has-least-drag-45172.html or a CFD simulation http://www.panix.com/~brosen/catamarans.html.

    On the other hand round bows can be dangerous in waves. My first catamaran (21ft) was able to plane and was fast when it was blowing. I remenber a strong north eastern and waves about 2m. The cat planed over the crest of a wave and got stuck into the next wave. The boat stopped at once and nearly lost the nine meter mast. I`ve never tried it again.
    If I had a round bow the boat would have been destroyed.
     
  2. bjn
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    bjn Junior Member

    I'm not interested in arguing. I'm only interested in science. I was hoping we could reach a conclusion based on science.

    Looking at other boats may provide clues. But I would never copy a design if I don't understand or agree with the science the design is based on.
     
  3. pogo
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    pogo ingenious dilletante

    So you believe that all hull designs are based on science and you wanna have confirmed your own visions by such a science ?

    pogo
     
  4. valery gaulin
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    valery gaulin Senior Member

    The only thing I can say is that for the mini Transat 650 they just discovered a couple years ago that blunt bow perform better in general than pointy bow!!!! It took a while to discover tjis!!!!!
     
  5. UpOnStands
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    UpOnStands Senior Member

    you, sir, are a very lucky man. Not sure how you managed to avoid injury.
     
  6. UpOnStands
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    UpOnStands Senior Member

    I thought we went through this elsewhere. The mini Transat 650 hull shape is in no way applicable to catamarans or indeed any heavy displacement keel boat. The mini Transat 650 surfs on its side so what is up front is not "round for being round" but volume to prevent bow bury. The full volumes port and starboard yield the bluff bows.
     
  7. semelis
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    semelis Junior Member

    Yes, we have been here.
    It has been useless, it seems.
     
  8. UpOnStands
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    UpOnStands Senior Member

    This is from rastapop "bjn, the primary function of an airfoil is to produce lift (usually while also trying not to increase drag too much).
    The primary function of a bow is not to produce lift, it's to minimize drag (among some other things not relevant to the discussion).
    The differing functions require different shapes. "
    This is science, & engineering & a lot of experience.
    The primary function of a wing mast is to produce lift and minimize drag so it has a rounded leading edge.
    The primary function of a rudder is to produce lift and minimize drag so it has a rounded leading edge.
    The primary function of a dagger board/centerboard is to produce lift and minimize drag so it has a rounded leading edge.
    The primary function of a LAR keel is to produce lift and minimize drag so it has a rounded leading edge.
     
  9. bjn
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    bjn Junior Member

    I think every good design is based on science.
    science:
    "the intellectual and practical activity encompassing the systematic study of the structure and behaviour of the physical and natural world through observation and experiment."

    With this thread, I am hoping to find out if a too sharp bow is bad for performance on a sailboat.

    The consensus seems to be that the sharper the bow, the better. But my theory is that a too small radius/too sharp bow might be bad. So to me, it looks like there should be some kind of tradeoff between factors. I was hoping that someone on the internet had knowledge about this. So that is why I started this thread.
     
  10. bjn
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    bjn Junior Member

    But if the sharp bow creates more drag while sailing, than a, for example, slighty more rounded, it doesn't matter what the primary function is.
     
  11. bjn
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    bjn Junior Member

    Thanks for the tips. Yes it would be interesting to study a model with different radiuses of the bow, with different leeway angles and different speeds. Don't think I will spend the time or the resources for a study, unfortunately. I was hoping someone had already done the job! =)

    Nice software, I will see if I can find the time to play with it.
     
  12. semelis
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    semelis Junior Member

    There it is. You found out. But ....

    Yes, and they have answered. But you have not liked their answers because they contradict your theory.
     
  13. bjn
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    bjn Junior Member

    I'm sorry, but I must have misunderstood those answers in that case. Could you point me to one or two?
     
  14. semelis
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    semelis Junior Member

    No, if you have not seen them, I can't think of any way to make you look at them.
    I'm sorry for my incompetence.
     

  15. bjn
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    bjn Junior Member

    I think there is some confusion going on here though. One thread is about full blunt bows. There is a fusion between this and that thread. Which I can understand. Because I was originally thinking a large radius of the bow would be the best, since it's the best for foils. Then I could see that I was wrong. So now, when I read replies about fully blunt bows, I don't think it's related to what I'm talking about. Might be that I've missed something because of that.

    My view on the subject is I think the sharp bow may not be a big issue. A few cm of an edge in the water probably can not create much drag. But I still think it's interesting to understand more about this.

    I'm sorry if I make you feel incompetent, that is not my intention. I feel that there is a lot of hostility aimed at me. I'm trying my best to ignore that and to keep explaining myself. But I think I have failed.
     
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