Why do multihulls have pointy bows?

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

  1. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante

    I don't agree w. you.
    The first frames are above WL always V-shaped. Together with the " modern" vertikal stem ( for a longer WL) this results in a bigger radius at deck level.
    Simple 3d geometrie.
    Please compare with older yachts with forward raked stem, whole stem has one radius.
    http://www.google.de/search?sclient.......0...1.1.64.tablet-gws..14.0.0.Kk0OC9cUA7k

    Why do mono - and multihulls with vertical stems , x - bows , inverse bows have no bigger radius at deck level ?
    Tumblehome hulls , at least the first frames !

    http://www.google.de/search?sclient.......0...1.1.64.tablet-gws..14.0.0.Kk0OC9cUA7k

    See also the latest " Hugo Boss" and wave-piercers.


    What do we learn ?
    There is a trend towards sharp stems.
    Together with the set back masts the seemingly more little volumed sharp bowsections they have a long enough leverage arm to compensate negative trim-angles ( bow bourying ). " Seemingly" , ' cause the volume is hidden in the length.
    Those bowsections don' t loose as much speed as a conventional v- shaped bow section when buried.
    The v- shaped bow sections answers to a negative trim- angle with progressive increase of volume , of static lift --for the cost of speed - thus more torque for pitchpoling---a brake.
    These bows can be seen as " non- sharp bows " .
    When waterline comes to deck level , the now negative angled deck ( (flat deck ! ) generates negative lift. one can imagine what happens --see Hobie 16. Brutal pitchpoling.

    The other world, the very long tumblehome bow sections , very sharp, nearly wave pearcers. The most drastic boats are " Sodebo" and her sistership "IDEC" , irens design.
    Even Under negative trim-angle these boats are going " thru".

    .....and IDEC spoke to the ocean:
    " Me, I am the sharp , hot knife, you are the butter ."

    There is a famous vid of Sodebo nearly pitchpoling after her start to single handed rw record attempt.
    Can somebody post it please ?


    Note that these radical racer solutions are not for cruising boats . They are impractical , you have a much longer and very narrow bow without a deck.

    But, the trend for bows is : be sharp and long.




    pogo



    P.S
    "Sharp" .
    I 've an old Joe Jackson LP , 1979 british New Wave --- "Look sharp" . I gonna hear it now.
     
  2. UpOnStands
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    UpOnStands Senior Member

    sorry but you failed to read the thread carefully. OP wanted to know why stems were not deliberately designed to prevent separation/drag, i.e. why aren't they rounded rather than sharp.
    Your #1 is contradicted by your #2. Simply, if the designer wants a traditional style bow he uses V frames otherwise not. OP has extensive sailing experience so the rest 3/4/5/6 are true but not answering the OP's question.
     
  3. BobBill
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    BobBill Senior Member

    Read thread.
    Daresay, if dodgers who ventured forth on multis, went through all these contortions to get from A to B. Never would have got going...whales been around a bit, and looke at WW subs...took some common sense.
     
  4. rturbett
    Joined: Aug 2005
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    rturbett Senior Member

    on a racing cat, there is such a thing as too sharp a bow..... When I re-faired my bows on my shark, I brought them to a relatively sharp point (still class legal by the drawings). When turning, the boat slowed dramatically. Rounding them slightly resulted in much better overall performance.
     
  5. fallguy
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    fallguy Senior Member

    Any thoughts as to why? My hulls are rounded about 3/8" radius and I have been wondering how they will do (a year to launch)
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The boat slowed dramatically, compared to previously ?
     
  7. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    Whale and Submarine mainly travel underwater, no surface wave drag ! it is why they have blunted stem !
     
  8. rturbett
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    rturbett Senior Member

    Sharp bows worked great in light air and while going straight. When tacking or jibing, it felt like is was being dragged to a stop- I was guessing it had to do with "stalling" the bows- water separation versus smooth flow
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    That is what I was thinking, a sharp edge interrupts flow more, but a bit surprising it would have that degree of drag involved
     
  10. BobBill
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    BobBill Senior Member

    As things bow to Diminishing Utility, "moderate" will eventually rule...tortioses do not fly--for long!
     

  11. Iridian
    Joined: Jan 2020
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    Iridian Junior Member

    I don't think anyone has brought up bulbous bows.

    This gives you that same rounded shape but it's used more for wave cancelling rather than flow separation. It's also completely below the waterline, solving the water/air boundary issue.
     
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