Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't buy the 2.5% though, hump101. It has to be more than that. Why would anyone bother with the slender hulled catamaran hullform with all its complications to get 2.5% ? You would have rocks in your head to try.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    1% or less has a major effect for my clients. But this is where one is bringing in "external" influences to make statements of 'very significant', plays a role.

    Most of my clients like everyone, want the fastest design and with the least amount of power and fuel consumption. Even had one where an increase of fuel consumption over the contact value results in a liquidated damages of €10,000 per extra litre/hour....everything plays a part.

    But when an SOR states that the hull cannot exceed a draft of 1.0m, yet requires a long list of wants to also satisfy. Stating that a narrow hull is always best, but ignoring the deeper draft required to gain back the buoyancy to fit all their wants in...tends to fall on deaf ears and misses the whole point of what "design" is and how the SOR influences hull design far beyond binary issues. Thus comparing apples with apples.

    Hydrodynamics is just one process in a long chain of many that must be satisfied to fulfill the SOR. Design is more than the sum of its individual parts. Those that design understand this, those that don't harp on about absolutes.
     
  3. hump101
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    hump101 Senior Member

    B is beam, T is draft, so for my example, B/T changes from 0.4 to 10, whilst L/B correspondingly changes from 10 to 2, so you can see that a B/T change from 1.5 to 2.5 is a relatively small change on a hull.

    It is worth noting that on Patrick's graph presented above, at Fn=0.45 the difference between B/T=1.5 and B/T=2 is more than 14%, somewhat more than the 2.5% quoted.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    And at a Fn of 0.2, the difference is almost 0%...your point being?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Ad Hoc, stick to the original question, that of resistance being almost entirely dictated by the length-displacement ratio, we don't need to hear about the insistent demands of your customers. By your reckoning, you can double the beam of your cat hulls, halve the draft, and still have much the same resistance, yes or no ?
     
  6. powerabout
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    powerabout Senior Member

    I guess you need to add the dynamic factors as well, talking to beach cat designer builders over the years, a rockerless hull is the fastest...in flat water so everything outside of that its slower.
    Thats got to be a factor as important as L-B and the plumb or reverse bows also add to that variable.
    If you include powered cats then you need to think about the speed as they will get aero lift at a certain point, all food for thought.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    This is a dead give-away. I do hope it is the only pretence to non-existent knowledge you have meandered into in this thread. Show me the "endless factual evidence" that "proves" that "there is no God". :eek: As a matter of fact, I'll even take anecdotal evidence, though I'd draw the line at "my nanna said so". And don't tell me to prove there is, you are the one making the claims, not me. :D
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Well, you can't prove a negative :D

    But failing that you can always start with Richard Dawkins :p
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Richard Dawkins is very successful at seeing publishing opportunities, but throws no light on the subject at all. Stephen Hawking likewise, there is a "quid" to be made peddling atheism. Hawking says God is totally unnecessary, 'cos the laws of physics allow the cosmos to arise spontaneously. That may well be so, but omits to explain how there are "laws of physics" to enable that. I just take it all as a signal that rational thinking is not unlimited in its scope, and cannot penetrate the "riddle of existence". But, Hawking has an answer to that, he says it is a case of us not having noticed that nothing does actually exist. There is an equivalence of matter and anti-matter, he says, and when the two finally come together, phooof ! Nothingness. What a tease, all our trials and tribulations for nought. I don't know why he worries his head about it, if he sees that as the truth, but stuck in that wheelchair and diseased body, what else is there to do.
     
  10. redreuben
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    redreuben redreuben

    Mr E,
    There maybe a "quid" to be made peddling atheism.

    But religion generates massive fortunes !
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course, but I can't understand what they talk about at atheist's meetings ? They just agree there is nothing there to talk about ? :rolleyes: The atheism "bible" of Dawkins is a "book about nothing" ? Seinfeldianism at it's best. Crazy, the real atheists, are like those who don't believe in the tooth fairy, they don't think or talk about it, or buy books denying the poor fairy's existence. :D To run around spouting atheism is to me a sign that you are really not an atheist. Or you see a marketing opportunity ! :eek:
     
  12. redreuben
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    redreuben redreuben

    Reminds me of a joke; How do you tell if someone is Vegan ?

    Don't worry they will tell you !

    OP, sorry for the drift, normal (?) programming will resume shortly.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Yep, I think Atheism and Veganism have something in common, it gives you something to distinguish yourself from the "sheeple". And yes, atheist vegans probably are hell-on-wheels. :p
     
  14. redreuben
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    redreuben redreuben

    :idea: The perfect storm for parties and bbq's,

    The atheist vegan multihuller ! :D
     

  15. oldsailor7
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    oldsailor7 Senior Member

    For Gods sake. :p -----What has this got to with the price of Yabbies. :p
    Lets get back on Topic.
    There's a great deal of muddled thinking going on, on this thread at present. Lets just try and separate the wood from the trees.
    At any length, width or depth of the hull, the displacement can only equal the weight of the boat. I am not an NA, but I am struggling to see the relationship of Displacement to the Length.
    Waterline Width to Length I can understand because it involves detrimental wave making.
    Can any one please enlighten me. (and it's got nothing to do with God.)
     
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