Symmetrical v's asymmetrical

Discussion in 'Multihulls' started by Sailcy, Nov 7, 2016.

  1. Sailcy
    Joined: Feb 2016
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    Sailcy Junior Member

    Speaking about the displacement powercats.
    Could anyone help me to understand the principles please.
    Symmetrical hulls have the ability to carry more weight and obviously required more HP. At the same time asymmetrical are more economical to operate, but limiting themselves with the payload. Am I correct?
     
  2. mydauphin
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    mydauphin Senior Member

    I believe the design is supposed to help in reducing heel, or heeled performance too. But this a great question for some of our experts to discuss.
     
  3. brian eiland
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    brian eiland Senior Member

    Sneezing

    Note that he said powercats rather than sailing cats.

    In general there are not to many pluses for recommending asymmetric hulls as they definitely can limit the load carrying aspects of already challenged long slender hulls of catamarans.

    But one factor can recommend them,...the flow between the two hulls of powercats can experience what has been termed 'sneezing'. if the two hulls are too closely spaced, or too 'full' in their bows the waves/water forced down the restriction between the two hulls wants to 'sneeze' back forward, and likely up over the bows,...can make for a wet ride.

    You might also have a look here:
    http://www.boatdesign.net/forums/multihulls/center-nacelles-wave-pounding-cats-19139-2.html
     
  4. patzefran
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    patzefran patzefran

    Richard Woods uses asymmetrical hulls for small dayboats motor cats to decrease hull interaction drag and symmetrical for higher length live aboard to increase load carrying capacity. He explain this in his blog. It seems small racer motor cats uses assymetrical too.
     
  5. Sailcy
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    Sailcy Junior Member

    Thanks to all to making it all more clear to me!
    Actually, what I am trying to figure out is advantages in symmetr. vs asymmetry hull design (again speaking particularly about displacement powercats) giving the boat same length, width, weight and HP what difference in performance is expected?
     
  6. patzefran
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    patzefran patzefran

    Likely, the difference in performance should be littlle. the wave making resistance of individual hulls depends mainly on displacement length ratio and so should be the roughly the same. If the hulls center separation and the bridgedeck clearance are high enough the interference drag should be very near. Only the beam and the wetted area of each asymmetrical hull should be higher (for the same displacement) than the symmetrical one (assuming semi circular sections under the waterline level), and according the friction drag of the asymmetrical somewhat higher. So I think symmetrical is likely a little more performant, but I may be wrong !
     
  7. JSL
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    JSL Senior Member

    oops- I meant what size of cat are you looking at?
     
    Last edited: Nov 7, 2016
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The main reason I can see that might make asymmetric attractive, is if it was required to restrict the beam somewhat, and the interference drag would be too high with symmetrical demi-hulls.
     

  9. Sailcy
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    Sailcy Junior Member

    what side of cat are you looking at?
    Sorry, probably there is a typing error. If you mean SIZE, it's about 30 ft
     
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