Catamaran dinghy 10-11' long

Discussion in 'Multihulls' started by Boatguy30, May 9, 2015.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Easy things are always more difficult to explain to those without a background in engineering!

    Just because i can talk about the left and right ventricles, doesn't make me a heart surgeon nor qualified to give advice to others too!!
     
  2. buzzman
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    buzzman Senior Member

    Nah, you're still not getting it. ;) ;)

    I *know* that from a naval architect textbook position a 'planing hull' is identified by hydrodynamic lift rather than hydrostatic lift.....

    I'm not completely stupid. Well, not completely.... :)

    The point I'm attempting to make is that only naval architects call it that.

    The vast majority of the boating public recognise a 'planing hull' as one that sits up on its tail and skims the water with only its aftermost third immersed.

    I *know* that we're all completely wrong - from a strictly 'naval architect hydrodynamic perspective' - to oversimplify it this way, but still, crap happens.

    Technically, whether a cat hulled boat is doing 3 kts or 30kts, it's *planing*, as lift is hydrodynamic, not hydrostatic, but to say so, especially on a public forum where not everyone is as qualified as A-H, generally confuses the f*** out of people who don't actually understand the strict dictionary definition of a planing hull design.

    So I know that you're right. From the strictest naval architect, dictionary definition, point of view.

    What you seem unable or unwilling to accept is that *MOST* people don't understand this, and so don't refer to planing hulls "correctly".

    To most people, and most 'generalist' texts, I might add, a catamaran does not have a planing hull but is also a displacement hull, like most 'traditional' sailboats and cargo vessels.

    So *generally speaking* the vast majority of people understand a 'planing' boat (as opposed to a planing hull) to be one that skims the surface as described.

    As cats patently *don't* do this until they approach really fast speeds, it just confuses the f*** out of people to say that a cat doing 3kts is a 'planing hull' as, to a layperson observer, it isn't obviously 'planing'.

    Hopefully that's clarified what was clearly a misunderstanding of my position on your part.

    I knew what I was saying was not 'technically correct', but was aimed at a lay audience.

    But if you can do a better job of explaining this, mate, knock yourself out.

    But remember you can't use the term 'hydrodynamic lift' because if you do, people's eyes will begin to glaze over.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Hardly, you can virtually interpose the two terms, planing and hydro-dynamic lift, for the purposes of this discussion.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I said no such thing. If someone wants to ask a question and learn that’s fine, I or others can answer their question. But you don't wish too..that's fine and YOUR prerogative.

    And therein lies YOUR problem.

    Using “big” words go over your head and you are assuming it shall others. Just because you cannot understand correct terms and their true meaning or even ask for someone to explain them in order to understand, do not assume others fall into your ignorance. You are doing a disservice to others with little knowledge but come onto the website and do wish to learn.

    You are digging yourself a deeper and deeper hole I'm afraid. :eek:
     
  5. buzzman
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    buzzman Senior Member

    So, explain it, in words of one syllable, that even someone as thick as me can understand, so I can then explain it to others who also don't understand.

    And I think it's clear that I *do*n in fact understadn the correct terms, so your continuing to assert that I don't is either deliberately provocative, or just simply lazy.

    I don't intend to feed the trolls any longer.

    Either explain the difference, in layman's terms, or go back to the bridge from under which you came.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Well, if that is your attitude to learning....then continue trolling, as that is all you're capable of doing.

    Those with an open and enquiring mind have no such chips on their shoulders.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    buzzman, if you accept that a cat has less waterline plane area, and dynamic lift is roughly proportional to the bottom area, you realise that the planing action is not going to be as overt as a monohull of similar weight and length, at the same speed. There is no mystery about it. Some people even look at chineless planing cats and assume they must be displacement craft for that reason, but not so, they can still be planing hulls, albeit not terribly efficient at it.
     
  8. buzzman
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    buzzman Senior Member

    I don't have a 'chip on my shoulder', but neither do I look down my nose from the vantage point of your exalted height, like a seagull dropping piles of verbal excrement on those you see as beneath you...

    Either respond as requested, or your inability to do so proves either:

    a) you don't in fact know what you're talking about, nor how to explain it in layman's terms
    or
    b) you are in fact a troll who simply enjoys insulting or putting down those who are less knowledgable or less articulate

    Either way, none of us who *need* to be educated will gain anything from the exerience.

    Essentially, put up or shut up.
     
  9. buzzman
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    buzzman Senior Member

    Mr E, our posts crossed in the ether.

    As A-H clearly prefers wisecracks to explanations that anyone could understand, and you seem to have at least a similar grasp on the subject, perhaps you could attempt to answer the question?

    Assume you're standing in the bar of a hotel, and some no-nothing layman, overhearing you comment that a catamaran hull is of course a planing hull, says: "How come? It doesn't get up on the plane?"

    How would you respond, in words of less than five syllables and preferably in one paragraph....
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    You've got your work cut out Mr.E..he's spat the dummy already and name calling, like a child in the school play ground trying to elicit a response as he's floundering now. sigh...good luck.

    Sure, i haven't a clue, no idea what dynamic lift is or boundary layer or residuary resistance and oh gosh..those darn big words again..what do they mean???!!
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    A 6-metre planing cat will have under 1.2 metres of effective bottom width. A 6-metre planing monohull, 2-2.2 metres, typically. That is a big difference, and usually the cat will be significantly heavier, there obviously being a lot more material in the making of it. Consequently there is a lot more "loading" on the lifting surfaces, a concept that also applies to aeroplanes. You can't expect it to spring out of the water like a jack-in-a-box, there is insufficient lift on the reduced surface area, at equivalent speeds. But the lift is of the same kind, just less.
     
  12. buzzman
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    buzzman Senior Member

    Now you're simply being childish. You do yourself no favours, and your attitude and repsonse says far more about you than it does about me.

    You are the one who is demonstrating 'troll-like" behaviour.

    Comments like:
    "and oh, gosh...those darn big words again...what do they mean???!!

    ....are clearly not intended to convey anything other your contempt, and to re-affirm your own sense of your own importance based on an arcane set of knowledge that you choose neither to impart nor to share in a format any non-specialist can understand.

    I, on the other hand, a "mere" layperson, was attempting to do just that.

    Badly, I grant you.

    I have not "spat the dummy" - I merely object to being treated like a *****.

    If you're not willing to share your knowledge, and make it accessible even for those of us with the meanest of understanding, then what are you doing here?

    If you can't be bothered being nice, then just don't bother.

    Really, life's too short.

    :)
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I have heard it a hundred times, from those that say these boats don't plane at all, they just "go through it faster". It is a matter of degree, not a black and white situation that some insist you must come down on one side or the other. It turns out the lack of strong planing action gives a nice pay-off with a more comfortable ride on most headings. People want the soft ride without the fuel bill, which they blame on the refusal of the boat to "get up" and plane, the way a planing boat "should".
     
  14. Will Fraser
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    Will Fraser Senior Member

    I created a model of your 7.7' model in the Michlet program, calculated the drag at the speeds that you achieved in your tests and used it together with the various motors' rated power to arrive at an overall efficiency value of 17%. I know it looks really low but it does include all the real losses such as prop efficiency, lost ponies due the the age of the motors as well as some 'calibration' constant to account for inaccuracies in my model and/or any discrepancy in the calculation. At least it was consistent between different motors.

    Using the scaled up version (500lb, 11.5') and applying the same efficiency to a 6hp motor, I calculated a top speed of just under 8kts. I would be interested to see how the real performance compares to this estimate.

    There seems to be some interference drag between the two hulls starting at just under 4kts and peaking at 5kts (20% of total drag) and then tapers off as speed increases. At top speed the program shows that there might even be some wave drag cancellation. This interference effect (both constructive and destructive) appears to decrease as hull spacing increases.

    The program also shows a 10% reduction in drag by rather using deeper, narrower hulls (1:14) and another 5% by keeping the bottom panel level from midships all the way to the bow, i.e. little or no rocker in the forward half of the hull.

    I am no expert and would really like to know if these drag reductions are consistent with real examples, notwithstanding secondary effects on handling etc.
     
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  15. Manfred.pech
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    Manfred.pech Senior Member

    Thank you, Will. Great.
     
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