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Thread: Prismatic Coefficient and getting wrapped around a wheel

  1. #61
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by steve sharp View Post
    So far the discussion has focussed on the Cp and LCB of the hulls when on their DWL. Since we rarely sail directly down wind I suggest that it is also important to look at the LCB when a hull is immersed 400mm or 500mm below the DWL.

    A case in point is the cat I currently own, Crowther Design #462 www.bloomfieldinnovation .com When driving hard on a spinnaker reach at speeds of 16 to 18 knots (this is a heavy cruising cat displacing 14.5 tonnes loaded so it is not as fast as lighter models) the leeward bow actually lifts out of the water 100 mmm or so...
    Steve
    This has been very interesting (starting at #51 and continuing). Thanks Steve, BigCat, et al. I glossed through it when it first arrived. But now that I've gone back through it several times, and really studied it, I'm now getting it. This sounds like an incredibly clear unveiling of one of those "secrets".

    Its a shame... that this is almost easy to accomplish in a trimaran, but brutally, black magic hard to achieve in a catamaran. Is this correct? If I wanted to get a trimaran to take advantage of this I would:

    Optimize the main hull for sub "hull-speed" speeds... say...

    Cp ~= 0.57
    Cb way aft (> 55%)
    With lee amas just kissing the water.

    And optimize the amas for high speed as when its really pressed.
    Cp ~= 0.7
    Cb way forward (< 40%) or at least in such a way that the sails trying to heel and pitch the boat forward are balanced by the then current Cb. From engineering... Sum of the moments about the center of gravity = 0. i.e. the push forward at the height (center of effort) is balanced by the Cb shift.

    If this is a correct way of describing it, then this would be very difficult to achieve with a catamaran. But it sounds like this Crowther design achieves it very well.

    Would there be any bad side effects from a catamaran hull designed to achieve this? From experience Steve? From theory BigCat?

  2. #62
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    What PC to have for a trimaran? You wouldn't want a PC of .57 unless this is such a sporty trimaran that the main hull will be lifted above the water like BMW Oracle USA, or unless it is so heavy and underpowered that it won't sail very fast.

    The main hull's PC should be .61 to .65 in a fast cruising trimaran.
    Currently concentrating on http://earthnurture.com .

  3. #63
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    With our particular displacement hull form, provided it is allowed for at the design stage, a bit extra weight usually makes very little difference to its performance. This is because most of the major parameters determining the hulls resistance:- the half entry angle, the length to beam ratio and the prismatic coefficient vary very little with increased immersion. True the wetted surface [viscous drag] will increase but this is only important at relatively low speeds. At speeds above approximately six knots to the 25 to 30 knot range where we are often operating the major component of resistance is wave making drag, rather than viscous drag, and that is largely determined by the before mentioned parameters which do not change. - Malcolm Tenant, found at http://www.catamarans.com/news/2006/...comparison.asp
    Currently concentrating on http://earthnurture.com .

  4. #64
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Every single paper I have read regarding hull drag shows DLR (weight) to be the predominant factor in drag in any reasonably designed hull. Everything else is just background noise. A child could design a 40 foot 5 ton cat hull form which would have less drag than a M&M designed 40 footer at 10 tons. See the linked paper in the other thread I started.
    Last edited by dennisail; 9th January 2015 at 06:51 AM.

  5. #65
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by dennisail View Post
    Every single paper I have read regarding hull drag shows DLR (weight) to be the predominant factor in drag in any reasonably designed hull. Everything else is just background noise. A child could design a 40 foot 5 ton cat hull form which would have less drag than a M&M designed 40 footer at 10 tons.
    The comment was entirely a quote from Malcolm Tenant. I think he might possibly have known something about the subject.

    "Over the 45 years of Malcolm’s career, a steady stream of designs have left the office, with more than 270 designs at last count—ranging over the whole multihull gamut—have issued from this office, from small racing craft and folding trimarans up to large passenger carrying ferries and ocean-crossing private motor yachts." -http://powercatamaran.typepad.com/homeport/malcolm-tennant-rest-in-peace.html
    Last edited by BigCat; 9th January 2015 at 07:16 AM.
    Currently concentrating on http://earthnurture.com .

  6. #66
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by BigCat View Post
    The comment was entirely a quote from Malcolm Tenant. I think he might possibly have known something about the subject.
    Doesn't matter who wrote it. Who knows under what context it was said and why. It was probably correct when applied to something in particular. But read like you quoted it, the quote seems to suggest weight virtually does not increase wave making drag! Do you actually believe this?

    By quoting it like that you are misrepresenting the fact that DLR is by far the most important parameter when it comes to drag. Sure a little extra weight causes a little extra drag. A lot causes a lot of drag. The relationship is so good because its the most importance factor in drag!

    Do you have any actual evidence that weight only matters because of the increases wetted surface it produces? Can you link me to any papers on the subject? I bet you cant find any.

  7. #67
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by dennisail View Post
    Doesn't matter who wrote it. Who knows under what context it was said and why. It was probably correct when applied to something in particular. But read like you quoted it, the quote seems to suggest weight virtually does not increase wave making drag! Do you actually believe this?

    By quoting it like that you are misrepresenting the fact that DLR is by far the most important parameter when it comes to drag. Sure a little extra weight causes a little extra drag. A lot causes a lot of drag. The relationship is so good because its the most importance factor in drag!

    Do you have any actual evidence that weight only matters because of the increases wetted surface it produces? Can you link me to any papers on the subject? I bet you cant find any.
    Actually, I have not made any assertions at all. I am the one who started this thread, and I started it in order to draw together the rules of thumb concerning yacht design that I have come across. You seem to ignore the fact that DLR is NOT weight. DLR is a relationship between weight and length. This is the second time you have alleged superior knowledge based on papers which you say you have read, but which you have not cited. It's quite obvious that you have never designed a yacht. If you had, you would understand what Tennant was saying. Frankly, what he said is over your head, and you are too conceited to realize it. I can't imagine being someone with no real experience of a complicated discipline, and just imagining that I know more about it than a man with a successful lifetime career in the field. You don't just pull a weight out of a hat when you design a yacht. You design a yacht to requirements, and once you change one little thing, it affects every other little thing. Google 'design spiral.'
    Currently concentrating on http://earthnurture.com .

  8. #68
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by BigCat View Post
    Actually, I have not made any assertions at all. I am the one who started this thread, and I started it in order to draw together the rules of thumb concerning yacht design that I have come across. You seem to ignore the fact that DLR is NOT weight. DLR is a relationship between weight and length. This is the second time you have alleged superior knowledge based on papers which you say you have read, but which you have not cited. It's quite obvious that you have never designed a yacht. If you had, you would understand what Tennant was saying. Frankly, what he said is over your head, and you are too conceited to realize it. I can't imagine being someone with no real experience of a complicated discipline, and just imagining that I know more about it than a man with a successful lifetime career in the field. You don't just pull a weight out of a hat when you design a yacht. You design a yacht to requirements, and once you change one little thing, it affects every other little thing. Google 'design spiral.'

    Do you prefer to argue with personal attacks rather than evidence? And throw in a bit of argumentum ab auctoritate?

    Tennant must have written thousands of paragraphs of design notes. YOU have chosen this snippet and posted it here possibly out of context. Tell me what this snippet as read when posted by you was supposed to mean in the context of hull drag and weight? What was the point YOU were trying to make when choosing it? What could you expect the layman (which I agree I am) to learn from it other that what I mentioned?

    I have not not alleged any superior knowledge. That is your assumption. I always urge people to look things up see for themselves, even if I post my own interpretations of what I see. I only made comments on a paper I read and cited regarding a subject with much hype, and urged others to read it and comment. Which I think is more relevant than your cut and pasting.

    You are also assuming I do not know about the design spiral. I have a few papers on it which I have read. So I know you don't pull the weight out of a hat, but the design changes as weight studies are done. Also your assumption regarding that I have no idea what the L in DLR stands for is incorrect. DLR changes without the L changing if you change the weight. A Seawind 24 is lighter than a winning Americas cup boat but its not faster. I wonder why?

    I am nothing but an enthusiast. I built models as a kid, I fixed up boats, I plan on modifying the hulls of my cat soon, and I find discussing these matters interesting even if there are disagreements. Disagreements allow me to learn when I am wrong and others to learn also if I post some relevant accurate info, which I aim to do without resorting to personal attacks.

    But here is a cut and past from Eric Sponburg if reading the results of tank testing is too hard.

    DLR was invented by Amiral David W.Taylor, the farther of modern model testing in the unites states and first published in 1910, his book The Speed and Power ofShips - A manual from marine propulsion. Taylor found that when towing models in a tank and following Froudes law of comparison, the resistance of the model was proportional to displacement
    http://www.sponbergyachtdesign.com/t...n%20ratios.pdf

    Also, DoubleDuty appears to have started this thread not you?
    Last edited by dennisail; 9th January 2015 at 09:45 AM.

  9. #69
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Also the quote below is from this very thread by a respected NA. Richard woods!

    I also agree that the Slenderness ratio, DLR, or weight versus length, is the most important speed factor.




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