Cat's Hulls resistance to leeway

Discussion in 'Hydrodynamics and Aerodynamics' started by Erwan, Jan 24, 2012.

  1. Erwan
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    Erwan Senior Member

    Hi Everybody and Happy New Year if not too late.

    After basic centerboards and rudders lift calculations, it appears that for light cat like F18 or A-Cat, at low to moderate speeds, the leeward hull should provide a significant part of the lift, in order to balance rig's lateral force (sailing windward of course).
    I would like to know where to look and what to search, in order to have a better picture of hulls'lift and its related "induced drag".

    Not sure I am very clear

    Thanks in advance and best regards

    EK
     
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  2. Doug Lord
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    Doug Lord Flight Ready

    Erwan, as two 18 footers-F18 is relatively heavy and A Cat relatively light-right?
    I can't imagine using cathulls on these two designs to develop any lateral resistance when it can be so much more efficiently done with the lee hull boards. I guess if the boards are symmetrical and the trunks aren't toed in then the hull will develop some lateral resistance. Seems like the ideal solution would be toed in trunks so the hull is not going sideways at all.... Do the rules prevent toed in trunks/boards on the F18? I know they don't on the A cat.
     
  3. Erwan
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    Erwan Senior Member

    Dough,

    You are right A-Cats are much lighter than F18 for the same lenght, but both hull shapes are flattish, compared to older design: Wild Cat vs Tiger, and lateral force is proportional to righting moment, so it is not so different.

    Toe in is a good solution but cannot handle all and it is difficult to put more than 1°, but I did that a long time ago on an old A-Cat, it was very effective, especially in the breeze windward.

    Regards

    EK
     
  4. Doug Lord
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    Doug Lord Flight Ready

    -----------------------
    Erwan, I'm just curious why the boards can't handle all the lateral resistance?
    Talked to Dave Carlson and he said he was using .75 degree of toe-in-seemed small to me.
     
  5. Petros
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    Petros Senior Member

    the hulls provide a small amount of lateral resistance, but compared to a keel or dagger board it would be very small. Consider it would be a very low aspect ratio surface, and with a round bottom, not much way to keep the flow from rolling under the hull.

    Flat bottomed with perhaps some concave surface on the sides, with some strakes at the chines might give you enough lateral resistance to eliminate the dagger boards and still have control, but it would not be as efficient as a round bottom hull with dagger boards.
     
  6. Collin
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    Collin Senior Member

    [​IMG]

    Maybe if the cat hull looks like that :p

    You need deep hulls to give good lateral resistance. A round hull can give lateral resistance-but it would need to be skinny and deep.
     
  7. Perm Stress
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    Perm Stress Senior Member

    Hobby cats do not have boards and rely on hulls + rudders for leeway prevention.
    They have quite deep hull shapes.
     
  8. Erwan
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    Erwan Senior Member

    Dough,

    You are right, if you consider an A-Cat sailing
    @ 9knts (4;5 m/s)
    with a 0.7m x 0.15 m daggerboard so Aspect Ratio around 9
    If only the centerboard provides lift implied Lift Coef must be around 0.66 for 700 Newtons lateral force.
    According to the aspect ratio AoA should be around 7° to 8°.
    And that is the point, I dont feel sailing winward @ 9knts the boat drifting by 7° to 8°

    The rudder can probably handle only a small part as its aspect ration is much smaller But, overall, I must admit we are not far.

    but if the hull should handle some part, how much and at which cost, that is the point I investigate to see if something could be optimized. Because @ 4° to 8° the hull'sinduced drag is probably not negligible.

    Petros,
    You are right too, a rought computation using classic formulas lead me to find 5 Newtons lateral force with around 0,5 sqm lateral area for 1° AoA.

    You read in my mind regarding your suggestion with flat bottom + concave sides and starke on the chine.

    In addition such a hull section could be interesting to minimize parasitic or spray drag with a smaller beam at the water line and above, while maintening sufficient area under waterline to dampen pitching.

    Also by minimizing main section area with concave sides in the middle, you increase mecanically the prismatic, as long as bow and stern do not change so much.

    As far as I know, I don't think MICHLET can manage concave sides, I think the software makes a vertical projection of the hull section above waterline. As I don't know how to guess the impact on drag in the chop for such concave sides, it is still an idea, not yet a hull.

    Thanks for taking time to post, hope your week end was nice, have a good week

    Regards to all

    EK
     
  9. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    Erwan, seems to me the daggerboard is too small in that case. Going to the hull for lateral resistance seems like it could be a very draggy mistake since,as a foil, it is very, very poor.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Michlet doesn't necessarily use a projection above the waterplane. You
    can use the sinkage option to raise or lower the hull with respect to the water.

    That said, however, Michlet is not an appropriate tool for this sort of work.
     
  11. Erwan
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    Erwan Senior Member

    Doug,

    The daggerboard is A-Cat size, just like the 700 Newtons lateral force, Today curved board's projected area is not very different.

    Leo,

    Thank you for your comment, if Michlet is not appropriate, do you know which kind of software would be, or a good Excel spreadsheet with appropriate equations could address separatly the spray drag problem, for such hull shape ?

    In fact the point would be to compare vertical sides vs concaves sides everything else equal, that is why I mention Excel spreadsheet

    Good week Everybody

    EK
     
  12. tspeer
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    tspeer Senior Member

    Unfortunately, this is a problem that involves the free surface as well as board/hull interaction. There's not going to be a simple (or cheap) tool that can really do the job.

    You might get some idea of how much lift you want the hull to produce by using lifting line theory, but to analyze the foil/hull interaction means you need something like a panel code. If you assume very low speed (zero Froude number) or very high speed (infinite Froude number), the free surface can be represented by a solid surface or a linearized free surface. Either can be handled with a panel code like CMARC. To handle realistic Froude numbers, you're talking something like SPLASH.

    After that, I'm afraid you're into the world of supercomputers and RANS codes. Or tank testing.

    That's why, for small boats, it's about as cheap to experiment at full scale as it is to use sophisticated analysis tools.
     
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  13. Erwan
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    Erwan Senior Member

    Thank you Mr Speer for your comments and I apologize for late answer, I was not at home these last 2 weeks.

    The software you mention are probably too much for me.

    I think Hobie Cat is in the best position to make some testing, If they use 2 similar rigs, one on a Wildcat, and the other on a Tiger, as long as they use also the same centerboards and rudders.

    I will be possible to see if one hull need more or less centerboard down to achieve the same performance windward.

    Thank again and good wind everybody

    Erwan
     
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