High-Speed Pentamaran Yacht

Discussion in 'Boat Design' started by High Life, Jan 23, 2013.

  1. daiquiri
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    daiquiri Engineering and Design

    Without any intention of taking anyone's part in this discussion, I have to say that Ad Hoc's explanation regarding parametric roll and it's influence on the choice of hull configuration is very clear and understandable to me. And it is also a rather logical one. It sheds another very interesting light on the process which led to the choice of this configuration of the pentamaran in question.

    Whether it is the actual reason for the hull form we have seen here, that's something only the designer of this ship would be able to explain. Perhaps we could have him involved in the discussion?

    Cheers guys
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Never assume. If your assumption of an inferred definition is incorrect, then everything that follows from your assumption is also. If a paper does not define “something” yet refers to it…you cannot use that which is mentioned with any degree of confidence, period. One is not supposed to guess, it must be stated clearly and concisely, no ambiguity.

    Well, that’s easy., since you don’t need to accept my opinion on this matter. The paper you cited above even states:

    “..Screening of parametric roll resonance shows that dynamic stability can be critical for a trimaran despite its great static stability characteristics…”

    And this is one of endless research papers all with the same opinion/facts. It appears you think differently?
     
  3. groper
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    groper Senior Member

    Oh comon, more semantics... ive not stated the definition of a stabilized monohull or a trimaran either - yet you know what im talking about when i make this distinction. Where are we going to draw the line?

    Do you think a boat like the following has a significant parametric roll problem, it is a trimaran afterall?

    [​IMG]
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Speaking for myself, hull form selection is almost the last thing I worry about. Since it is just a 3D shape, dictated by the length-displacement ratio. Anything else is just "background noise".

    I suspect the same is true here, but probably influenced by the marketing arm of BMT to use their heavily paid research from one project to inject it into another possible market. Everyone does it and there is no magic to it.
     
  5. daiquiri
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    daiquiri Engineering and Design

    This is not a good example for the comparison. A sailboat's motions are heavily dampened by the aerodynamics forces acting on the sail, and by the hydrodynamic forces acting on the keel/fin. Hence, a sailboat is generally much more stable when under way, than a motorboat of a similar hull form.

    Motorboats like the one in the OP, or like cargo ships in the example which has triggered this dispute, generally have no similar aerodynamic devices to dampen their motions in heavy seas.

    The dampening effect of sails is so high that some types of motorboats purposely use the so-called "steadying sails" to stabilize their roll motions in a seaway.
    Few examples:

    [​IMG]

    [​IMG]
     
  6. groper
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    groper Senior Member

    I was being semantic daquiri, of course there is no comparison but Adhoc likes to get all littoral and was seeking proof that backed his notion suggesting all trimarans have a parametric roll problem. He should have been more particular with his wording and specified a very particular type of stabilized monohull, with a series of parameters that makes them predisposed to the problem of parametric roll and then ask if id read a paper that suggested otherwise...

    Needless to say, any trimaran with enough ama transverse separation, enough ama length and displacement, and a low center of gravity will never yeild a significant parametric roll effect for that hull - like the sailboat i pictured above - even without the sail or rig it still cant do it thanks to its extreme dimensions which prevent its significance.

    Now we design a slender monohull, load it up to the eyeballs top heavy, add some tiny stabilizers and hey presto its all good - except for the parametric roll problem.... Ah thats easy fix, just add another set of sponsons and were good to go...

    As for a SOR, i dont think there is one... has anyone ordered one of these things and thus provided a SOR, or has BMT just "put it out there" and hope someone bites...?
     

  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Very long thin amas might be subject to whipping loads in seas. Also, very long
    amas might "trap" some wave frequencies, especially in quartering seas. I'm happy to be over-ruled by the NA's on these guesses :)
     
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