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Thread: Prismatic Coefficient and getting wrapped around a wheel

  1. #21
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by GDFL View Post
    I'm happy to have finally found this forum and especially this thread. I have been slowly working my way through an aluminum power cat design for the past many months. One of my key focuses is ease of building. I have some questions about cp in relation to my design. On a 45ft LOA, the hulls are constant beam (BWL 40 inches) except for the forward 8ft and there is no rocker (pretty much constant draft). The bow and stern are a bit different, but the hull below the waterline isn't too different from a regular pontoon boat. The cp as it is currently, is .83, which is way higher than anyting I've seen. Does anyone know how this design might perform? My speed goals are a fuel efficient cruise slightly below hull speed on up to a top speed of maybe 14kts. Will the high cp make it slow relative to other hull shapes? Is it more prone to hobbyhorsing or other negative tendancies? Thanks for reading.
    This is pretty much a multihull sailboat forum. I'd post your question here:

    http://www.boatdesign.net/forums/multihulls/

    The Tennant type of design seems to be state of the art for displacement power catamarans:

    http://www.catamarans.com/news/2006/...Comparison.asp

    Here's a Tennant design in your size range: http://www.pedigreecats.com/tennant/tenn44.htm

    Really, I suspect that the Korean design has been optimized for ease of building or interior room rather than efficiency or seakindliness. One would have to see the lines to say much about them, of course.
    Last edited by BigCat; 27th September 2009 at 06:46 PM.
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  2. #22
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Thanks for the reply. I hadn't read that Tennant article in quite awhile. I understand that this is primarily a sailing cat forum. That's the problem with most information sources. There is virtually no good information on the cp of power catamarans. I was hoping that with so many people in this thread with such a high understanding of cp and performance, that maybe someone could tell me just how high a cp is too high. I frequent the boatdesign forum quite a bit. The problem with power catamaran info is that most is geared towards small planing cats and there is little info for a DIY builder and/or designer unlike the abundance of info for sailing cats.


    Quote Originally Posted by BigCat View Post
    This is pretty much a multihull sailboat forum. I'd post your question here:

    http://www.boatdesign.net/forums/multihulls/

    The Tennant type of design seems to be state of the art for displacement power catamarans:

    http://www.catamarans.com/news/2006/...Comparison.asp

    Here's a Tennant design in your size range: http://www.pedigreecats.com/tennant/tenn44.htm

    Really, I suspect that the Korean design has been optimized for ease of building or interior room rather than efficiency or seakindliness. One would have to see the lines to say much about them, of course.

  3. #23
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by GDFL View Post
    Thanks for the reply. I hadn't read that Tennant article in quite awhile. I understand that this is primarily a sailing cat forum. That's the problem with most information sources. There is virtually no good information on the cp of power catamarans. I was hoping that with so many people in this thread with such a high understanding of cp and performance, that maybe someone could tell me just how high a cp is too high. I frequent the boatdesign forum quite a bit. The problem with power catamaran info is that most is geared towards small planing cats and there is little info for a DIY builder and/or designer unlike the abundance of info for sailing cats.
    http://www.tedbrewer.com/yachtdesign.html discusses this. Your cruising range pf 9 knots is about 1.43 x the sq. root, assuming a 40' DWL. So, .65 or .66 would be optimum. You really need to pick a specific speed to optimize for, not just a range.
    Currently concentrating on http://earthnurture.com .

  4. #24
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by GDFL View Post
    I'm happy to have finally found this forum and especially this thread. I have been slowly working my way through an aluminum power cat design for the past many months. One of my key focuses is ease of building. I have some questions about cp in relation to my design. On a 45ft LOA, the hulls are constant beam (BWL 40 inches) except for the forward 8ft and there is no rocker (pretty much constant draft). The bow and stern are a bit different, but the hull below the waterline isn't too different from a regular pontoon boat. The cp as it is currently, is .83, which is way higher than anyting I've seen. Does anyone know how this design might perform? My speed goals are a fuel efficient cruise slightly below hull speed on up to a top speed of maybe 14kts. Will the high cp make it slow relative to other hull shapes? Is it more prone to hobbyhorsing or other negative tendancies? Thanks for reading.
    High PC's have less hobbyhorsing and squatting tendencies, as RW noted above
    Currently concentrating on http://earthnurture.com .

  5. #25
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by Woods Designs View Post
    But no multihull can be really considered a "planing" hull. The aspect ratio is wrong. A planing speedboat has an almost square waterplane area when planing, a multihull is like a pair of water skis. Planing boats must generate lift, same as airplane wings etc so need a wide WL beam.www.sailingcatamarans.com
    I tried to explain this to Nordic Cat, but he was sure that I was wrong. A lot of people carry over information from monohull design to multihull design, without understanding that the very narrow hulls of typical displacement multihulls change a lot of factors.
    Currently concentrating on http://earthnurture.com .

  6. #26

    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Waterskis don't plane?

  7. #27
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by 44C View Post
    Waterskis don't plane?
    Good point, RWs analogy, not mine! Water skis probably wouldn't plane if they were shaped like sailing hulls under water, however - pc of .65 or less, rocker, narrow bows, etc. Also, think of the power to wetted surface ratio of a water ski, and compare it to a cruising yacht.
    Currently concentrating on http://earthnurture.com .

  8. #28
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    The Tennant 44 is a nice one. My design was beginning to look a bit more like the new Mainecat 47, although a bit more salty. Whatever I end up with MUST be capable of a tradewind circumnavigation. I really like the Tennant concept of the Globetrotter 53; long hulls with rather simple accomodations. Thanks to all for helping me get a better grasp on prismatic coefficient. My troubles seem to be that even with all the designs and pieces of designs that I like, no one seems to design for modest engines and a focus on passagemaking cruising speeds. I couldn't afford fuel to run at 20kts so the way I see it, I'm designing for about 60hp twin diesels, which is tiny compared to almost every other power cat. I'm going to hit the drawing board for some serious design changes and maybe post the results for comment.

  9. #29
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Things are finally headed in the right direction. I've done about a dozen design variations trying to get a lower cp. I finally took a closer look at the concept sketches for the Tennant Globetrotter 53 and I decided to completely change my design. I wish he had turned that into a design. It should allow for the proper cp, which seems to be around .63-.65 for a targeted cruising speed of 8-9kts. I still don't have the hull design quite where it needs to be, but I'm on the right track. Hopefully I can post a picture of the 3d model sometime this week. Thanks for the patience and guidance.

  10. #30
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    I have just deleted threads above from #29 t0 #40 and moved them to a new thread (by request) in the general sailing forum (More Efficient running with a Kite) as it would be of more interest to others and may get a greater input.
    Ian

  11. #31
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Sorry to say I sometimes (always?) write replies too fast without fully explaining what I mean.

    So when I wrote

    "A planing speedboat has an almost square waterplane area when planing, a multihull is like a pair of water skis. Planing boats must generate lift, same as airplane wings etc so need a wide WL beam"

    I didn't mean waterskis don't plane, rather that they are very inefficient planing "hulls".

    For example, you can plane in an inflatable dinghy with a 4hp outboard. But you need at least 15 knots boat speed and probably at least a 20hp outboard to get waterskis to plane.

    Interesting comment about turning radii for different beam powercats. Seems when comparing the turning radius to the beam the ratio is roughly constant?? But I'm curious why the wide beam cat turning speed was lower. You'd think that making a smaller turn would result in a lower speed drop??

    Richard Woods of Woods Designs

    www.sailingcatamarans.com

  12. #32

    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by Woods Designs View Post

    Interesting comment about turning radii for different beam powercats. Seems when comparing the turning radius to the beam the ratio is roughly constant??
    Looks like this, but such conclusion is difficult to make based only on two boats tested.

    But I'm curious why the wide beam cat turning speed was lower. You'd think that making a smaller turn would result in a lower speed drop??
    Speed before circulation was same (on straight track), sterndrive angle to engage turning motion were same. Drop of speed during circulation (i.e drop of speed due to turning motion) was different.

  13. #33
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by 44C View Post
    ????? 200/24 = 8.33*

    Also: http://www.youtube.com/watch?v=XBcDXns1W28

    Don't think they went below 8 knots very often...
    On the other hand: http://www.youtube.com/watch?v=v52LkfYNYu4&NR=1
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  14. #34
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by BigCat View Post
    Another day at the office, I guess, somewhere in his dialog, did I hear, "C'est la vie"... tranlate... "Shit happens!"

  15. #35
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by BigCat View Post
    Skene's Elements of Yacht Design, edited by Francis Kinney, is THE primer of yacht design. My copy is quite old, so I'm not sure what recent editions look like. This is the place to start. It doesn't say anything about multihulls, really.
    I received my "Skene's" book last week and have been trying to digest it. I have some questions about it that I'd like to discuss in the context of this thread (Prismatic Coefficient). Would anyone know if its OK and/or legal to scan a page to post on this thread so we all can discuss it? My first thought was that its out of print... so its ok. But then, I'm not a lawyer so I don't really know? I even tried to surf for a statue of limitations... but the legaleze and counter arguments nearly froze my brain!

  16. #36
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    Quote Originally Posted by DoubleDuty View Post
    I received my "Skene's" book last week and have been trying to digest it. I have some questions about it that I'd like to discuss in the context of this thread (Prismatic Coefficient). Would anyone know if its OK and/or legal to scan a page to post on this thread so we all can discuss it? My first thought was that its out of print... so its ok. But then, I'm not a lawyer so I don't really know? I even tried to surf for a statue of limitations... but the legaleze and counter arguments nearly froze my brain!
    I don't think anyone calculates it by hand anymore, even the most rudimentary hull design programs calculate all of these statistics.
    Currently concentrating on http://earthnurture.com .

  17. #37
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    Default Skene's Graph of Optimum Prismatic Coefficient

    Well here goes... from: Chapter XIII Resistance, p. 117, "Skene's Elements of Yacht Design" 8th Edition updated by F.S.Kinney. 1973

    Wave-making Resistance is the energy lost in the creation of waves. It is the principal resistance for displacement-type craft trying to go too fast, as well as for planning-type craft going too slowly. Mathematically, this would give us a boat which performed between the speed-length ratios of 1 and 2. To pin this down even more closely mathematically – the hump of the greatest resistance caused by the creation of waves lies between the speed of the wave, 1.34*sqrt(length between crests), or a speed-length ratio of 1.34 as the lower limit, and 1.65 as the upper limit.
    The accompanying chart (Figure 5) shows the influence of the prismatic coefficient on wave-making resistance at various speed-length ratios. The definition of prismatic coefficient is explained in Chapter XXIII, A Manual of Calculations. Its importance in yacht design cannot be emphasized too much, because there is an optimum prismatic coefficient for every speed.
    This is exactly what I was hoping existed when I commented in message 3. The little curve at the top giving an optimum Cp between 0.5 and 0.7 depending on some ratio (X axis) between 0.8 and 2.0



    My questions...
    1. Is this graph considered "current" or has it been superceded by better understanding or been totally disproved as "having made the wrong assumptions?"
    2. Between the writing and graph, I can't really determine what the ratio range represents. I think it is either (a) the boat speed divided by 1.34*sqrt(length between crests) or (b) boat speed divided by sqrt(length between crests). Which is it?
    3. I'm also under the assumption that I replace length between crests with the LWL... because we are talking about the wave crests created by the bow and stern waves. Is this correct?
    Attached Images Attached Images
    Last edited by DoubleDuty; 14th October 2009 at 01:32 PM. Reason: Apparently bmp files don't display, while jpg files do.

  18. #38
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    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by DoubleDuty View Post
    Well here goes... from: Chapter XIII Resistance, p. 117, "Skene's Elements of Yacht Design" 8th Edition updated by F.S.Kinney. 1973


    This is exactly what I was hoping existed when I commented in message 3. The little curve at the top giving an optimum Cp between 0.5 and 0.7 depending on some ratio (X axis) between 0.8 and 2.0



    My questions...
    1. Is this graph considered "current" or has it been superceded by better understanding or been totally disproved as "having made the wrong assumptions?"
    2. Between the writing and graph, I can't really determine what the ratio range represents. I think it is either (a) the boat speed divided by 1.34*sqrt(length between crests) or (b) boat speed divided by sqrt(length between crests). Which is it?
    3. I'm also under the assumption that I replace length between crests with the LWL... because we are talking about the wave crests created by the bow and stern waves. Is this correct?
    This is the square root of the waterline length versus resistance at various speeds, with the speed in relative terms, ie. relative to the square root of the waterline as opposed to in knots. Bear in mind what you have already heard, which is that at low speeds (ie. speed as a multiple of the square root of the waterline,) low PCs have less resistance, and at higher relative speeds, higher PCs are better. This is that information in more detailed and graphic form.

    The three solid lines are three different boats, one with a PC of .50, one of .60, and one of .70. The left side of the graph is units of resistance. You don't really have to understand more than that, to get the drift. The numbers on the bottom are speed, expressed in terms of the square root of the waterline x knots. If you have a 36 foot DWL, 1 - 6 knots, 2 = 12 knots. If you have a 64'waterline, 1.0 = 8 knots, 2 = 16 knots.

    The upper portion of the chart is basically a different chart, and combining them was unnecessarily confusing. The numbers on the upper right are PCs, and they relate to the optimum prismatic line at the top. As I say, this should have been two charts.

    In fact, it should have been 3 charts, because I think the frictional resistance, which is a function of the amount of wetted surface, is meant to relate to speed and resistance, but not to PC at all. You're confused because it was confusing to put the 3 charts together. It would be OK in a lecture, where a lecturer could explain it, but in a book, not so much.
    Last edited by BigCat; 14th October 2009 at 06:23 PM.
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  19. #39
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    Default Re: Prismatic Coefficient and getting wrapped around a wheel

    I agree with Big Cat.

    But also remember that finer hulls can use lower Cp's. Having said that I would still stay in the 0.6 - 0.7 range. The relative shape of the curves are still the same though.

    Another book that used to be the bible of naval architect students (and may still be) is Barnaby's "Basic Naval Architecture"

    Richard Woods of Woods Designs

    www.sailingcatamarans.com

  20. #40

    Default Re: Skene's Graph of Optimum Prismatic Coefficient

    Quote Originally Posted by DoubleDuty View Post
    Well here goes... from: Chapter XIII Resistance, p. 117, "Skene's Elements of Yacht Design" 8th Edition updated by F.S.Kinney. 1973


    This is exactly what I was hoping existed when I commented in message 3. The little curve at the top giving an optimum Cp between 0.5 and 0.7 depending on some ratio (X axis) between 0.8 and 2.0




    My questions...
    1. Is this graph considered "current" or has it been superceded by better understanding or been totally disproved as "having made the wrong assumptions?"
    2. Between the writing and graph, I can't really determine what the ratio range represents. I think it is either (a) the boat speed divided by 1.34*sqrt(length between crests) or (b) boat speed divided by sqrt(length between crests). Which is it?
    3. I'm also under the assumption that I replace length between crests with the LWL... because we are talking about the wave crests created by the bow and stern waves. Is this correct?
    The theoretical hull speed of 1.34 x sq root waterline length has been found only to apply to hulls with a LWL/BWL ratio of around 8:1 or less.

    Most multi's are better than that.

    http://www.multihulldynamics.com/new...?articleID=174

    "Hull Beam and Hull Length to Beam Ratio
    A very significant factor in boat performance is the ratio of waterline length
    (Lwl) to hull beam (Bh) (not overall beam), Lwl/Bh vs. Length, plotted in Fig. 1
    below. In Bruce and Morris work Design for Fast Sailing, it was shown that a
    good performing cruising catamaran or trimaran (main hull) should have a ratio
    of at least 8. In displacement hulls, the bow wave includes a trailing trough
    that a boat stern tends to set down into, limiting the speed of the boat. This
    is called hull speed. Slender hulls, those with Lwl/Bh ratios greater than 8, do
    not create a large enough bow wave for this to be an appreciable effect[3]."

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