USCA hull form design

Discussion in 'Boat Design' started by John Hazel, May 29, 2006.

  1. John Hazel
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    John Hazel Junior Member

    I have been studying the United States Canoe Assosiation "pro" class canoes.

    http://www.uscanoe.com/publications/USCACompetitionRules2005.pdf

    These boats have a requirement for a mid section of 27"beam at the 3" waterline and 33" beam at gunnel with min height of 10" from the zero waterline. No concave surfaces below the 3" waterline are allowed and no concave more than 1/4inch in the cross section below the 8" waterline.

    They are 18' 6" (5.64m) long and travel at 9min/mile (3m/s) on flat water conditions. This gives Froude number of 0.4. Their displacement varies from 300lbs to 450lbs (130-200kg)

    The typical prismatic coeficient is hard to estimate but what I have been able to measure appears to be around 0.53 and the max beam is always at the most rearward allowed location (14" from midship).

    I am working hard to bring up my understanding of wave drag in hopes of designing a lower drag hull for the 3.0 - 3.2 m/s racing speed. However I'd also like to avoid "re-inventing the wheel" so I'm posting here.

    The current hulls seem to have a rectangular profile. This makes turning very slow. Directional stability is poor but the divergence happens so slowly that it's not much of a problem.

    So here are my questions:

    1. Is it reasonable to say the low prismatic coeficcient of current boats indicates that there is a performance improvement available by fattening the overly sharp ends of the hull? What shape would be best at the forward and aft 25%?

    2. Would an elliptical or circular arc draft profile on the front half coupled with a rectangular profile on the back allow the boat to align itself more closely to the direction of travel and thereby reduce the real world drag?
    (lots of polar moment of inertia here...)

    3. Would a transom stern reduce the wave drag by riding in the stern wave?
     
  2. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Fn = 0.4, is that a speed/length ratio close to 1.4?
    (Yes, lazy late at night :)

    What do you think would be the ideal Cp?

    I am no expert paddler, but I have seen that canoes and kayaks when driven hard is rotating quite a bit for each stroke, if you see what I mean..
    Wouldn't this result in a lot of turbulence with hard chines in the after body?

    Have you had a look at this kayak page?
    http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm#coafanch
     
  3. John Hazel
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    John Hazel Junior Member

    v=3m/s, L = 5.64m, g=9.8m/s^2
    Fn = 3/ SQRT(5.64*9.8) = 0.4

    Displacement = 0.2m^3

    Fnv = 3/(9.8^0.5 * 0.2^(1/6)) = 1.25

    From what I could find in the library and on the net: Cp of about 0.6 is better for Fn=0.4.

    The better paddlers at the races are pretty smooth.

    No. Thanks for the url.
     
  4. frosh
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    frosh Senior Member

    A better C2?

    At the outset, I feel strongly that you are trying to re-invent the wheel.
    As with any sporting equipment used at elite level in the 21st. century, an incredible amount of testing and computer modelling has been done in many countries in an endeavor to extract an additional one-quarter of a percent of performance, so to give that country a small advantage in world or olympic competition.
    To get to specifics with the C2 canoe; you say that turning is very slow, and directional stability is poor. This seems to me a contradiction, but however you conclude that it is not very important and I agree with your conclusion.

    Prismatic coefficient as one determinant of wave drag is important, but why do you want to increase Cp?

    At canoe speeds frictional hull resistance is a larger determinant of overall drag, and the least wetted surface consistent with reasonable stability and maximum waterline length is an excellent feature to aim at initially.
    The main reason that max. beam is aft of centre is so that the parting of the water by the bows is as gradual as possible reducing the induced wave.

    The use of an elliptical draft profile in the bow implies significant rocker in the keel line. This is not consistent with maximum speed in flat water.

    A transom stern in a racing canoe will drag considerable turbulent water with it as the canoe moves forward, and will be disastrous! :eek:
     
  5. John Hazel
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    John Hazel Junior Member

    What you say makes sense. However, would you also suggest that America's Cup engineering is re-inventing the wheel?

    Directional stabilty and angular momentum-damping are two different things. A moving canoe will turn off it's initial course all by itself. That's directional instability. The racing canoe is difficult to turn quickly though. The large anount of lateral resistance from the straight keel resists turning. So that is how you have sluggish turning and poor stability on the same boat.

    As I mentioned the racing canoes seem to have Cp near 0.5 and at Froude numbers typical for racing canoes v/sqrt(g*L)=0.4 there are many references that indicate Cp nearer 0.6 is better.

    This is the approach of current designs. However, the Froude number of 0.4 is where wave drag gets important.
     
  6. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think you should try to build a canoe with Cp=0.58 or so, and keep the rest of the design close to the existing "standard". Then you can compare.
    If you change everything, it's hard to know what went wrong :)
     
  7. Paul Scott
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    Paul Scott Senior Member

    John-

    When I doodle double ended shapes on my Vacanti software, I've found the wave hump at, say, ~ sub .5 cp goes away to 0 before max speed (disp. mode) is achieved, where with higher cp's, say .57, the wave drag steadily continues going up to much more than the friction drag (disp. mode). If you add everything together (wave & fricton drag), you get a much higher result (like twice) in total drag for the .57 cp than for the sub .5cp. And the total hump of the sub .5cp cp hull never reaches the total drag of the .57cp hull at any corresponding point in the total resistance curve, everything else being equal. And it seems you can change the point of the max. hump in the curve depending on cp. If the stronger paddlers can routinely get over the lesser sub .5cp hump, and therefore routinely enter a regime of lower resistance, perhaps this is why the lower cp. dominates. (?) It would explain the motion of the hulls and paddlers, esp. if correlated to the mechanics and dynamics of the paddle stroke. And would seem to imply that you can tailor cp (and therefore the resistance curve) to the individual paddler's technique and strength.

    Paul
     
  8. John Hazel
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    John Hazel Junior Member

    Hey Paul, how are you keeping the displacement constant in this? Do you increase the draft?

    What was the trim and sinkage for each hull?
     
  9. Paul Scott
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    Paul Scott Senior Member

    John-

    Deeper or Boxier. I'm not getting direct trim or sinkage data from my software, so I'll have to look and see if I can infer it from something.... I am working on a sailing canoe, not a paddle canoe. I just looked. It's all static numbers outside of friction and wave. Sorry.

    Paul
     
  10. John Hazel
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    John Hazel Junior Member

    This might be why you are getting the odd results. If the model does not allow the modeled hull to change its sinkage and trim in response to the bow and stern waves, the full-ended hull would not show its advantage at higher Fn. The pointy hull woulde have less effect as the bow and stern were immersed in their waves.
     
  11. frosh
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    frosh Senior Member

    Quickest canoe in the computer!

    Hi John and Paul,
    How do you know that the answers and graphs given by your software would translate to a faster canoe than the existing benchmark canoe?
    Paul has already admitted to using a sailing canoe program when we are trying to optimise a C2. That can't provide reliable information when the power is provided by a paddle.
    What ever happened to understanding the basic hydrodynamic principles and working from there?
    If some software shows that higher Cp values would be quicker thru the water, maybe you should build the canoe and really find out.
    Also any comparison to (America's Cup) yachting is not valid as canoeing is subject to a limited number of factors, which are already almost completely understood. Therefore I believe that any improvements still to be made in the boat will be minute.
    Yachting is much more complex and hull design is only one half of the problem. Rig aerodynamics are still a long way behind the efficiency of state of the art aircraft technology, therefore there is significant potential for improvement. Also the understanding of all the dynamic forces acting on the rig and exerted by the sails is less well understood than the resistance forces acting on a canoe hull.
    I am not saying that computer simulation and computation is unhelpfull; only that we should not make definite conclusions based on such data, without real world (in the water) testing. :)
     
  12. Paul Scott
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    Paul Scott Senior Member

    frosh-

    I was only making some general observations based on some tendencies I have seen working on about 50 double ended hull iterations on my Vacanti software, which is not a dedicated sailing canoe program. I realize there are limitations to the mathematical model and functional abilities of this software, but it's what I've got. My explorations with it have been intentionally limited in scope, and I've learned some things I find valuable. (frosh, as a former Professor of Music, I appreciate a good lecture, but I know that learning can be a messy business, and I've got to start somewhere, even if the inquiry doesn't seem all that elegant!) My interest in cp was brought about by initially by 1) the secrecy surounding the cp's for TP52 hulls specifically designed for the chop of the Med, and 2) the effects of changing cp in the design of slalom boards for the Gorge.

    I have no idea whether anything I could come up with would be faster than existing hulls- in fact, I thought I was making an arguement in favor of existing cp practice! My experience is in windsurfer world, where most experimentation takes place full size, so I've learned a little in practice. I was involved in one project where we went through 20 rocker iterations on a slalom board before we found one that worked nicely. So I'm familiar with the benefits of actually building something. But it's fun to work out ideas in computerland, and I'll probably build something 'normal' when I do build, and I'll know why I'm building something normal. I have access to naval architects to bounce stuff off of- designers I've used for a boat designed and built for myself incorporating some ideas I've gleaned both practically and on my limited software, so I have some idea of the design spiral.

    It's relaxing being an amateur in boatieland, and I guard my status jealously.

    Maybe a quick retreat back to SA is the descrete thing?

    Paul
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    There are some papers at my WWW site that might be helpful.
    See:
    http://www.cyberiad.net/kayak.htm

    Also look at Michlet:
    http://www.cyberiad.net/michlet.htm

    Michlet was recently used to design some small kayaks. These hulls filled 7 of the top 12 places (including the winner) at a recent world championship. See:

    http://www.epickayaks.com/home/default.aspx

    One important consideration is the dynamic sinkage and trim of the hull during the stroke. You won't be able to estimate this using Michlet, but I have other software if you are really, really, keen.

    Good luck,
    Leo.
     
  14. frosh
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    frosh Senior Member

    Paul, thanks for your response. If you are are sailboarder primarily then you must be a good guy.
    Back in the 80's, when my friends and I frequently when shopping for new windsurfing gear, we often used to say that Robbie Naish would probably beat us sailing a door; such was the respect we had for his abilities. Also sailboarding seems more about the talent of the sailor rather than the speed of the gear when compared to canoes and kayaks.
    Best wishes, and keep doodling on the computer and most important of all, keep posting on this forum.
    Sam :)
     

  15. Paul Scott
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    Paul Scott Senior Member

    John- for what it's worth, the total drag in lbs. for the hulls in Leo's first paper is very close to what the Vacanti was giving me for similar hull forms. I guess I'd have to as Dave V. if sink is included in his Wave/Drag Calcs to know. And .52 cp is where the two curves get interesting.

    frosh- there was the infamous R.N. with a 5 meter RAF on the ironing board in 30K experiment......

    Paul
     
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