Hull Drag of Small Dingies/Skiffs

Discussion in 'Sailboats' started by Matth, Apr 23, 2007.

  1. Tim B
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    Tim B Senior Member

    Classical theory suggests that there are several. In reality only one (possibly two) are ever seen because it gets difficult to separate real resistance from electrical noise when you're tank-testing. CFD promises to show more about what's going on, but as I've been finding, it's only more accurate if you go to large numbers of cells, and then it takes a long time to solve. Especially when you think that there are 2 degrees of freedom to consider as well.

    We need some good tank-test evidence for these hulls. Particularly with respect to pitch angles. Personally, I think that you'll always have a resistance hump. Whether it is noticable or not is another matter.

    Tim B.
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  2. frosh
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    frosh Senior Member

    Sailboard speed

    Hi CT, you asked Phil Stevo about formula board speeds, planing and non-planing and how they relate to a really fast craft such as a foiler Moth. Sorry I am not Phil but I will give you my 2 cents worth anyway.
    I will describe briefly a race I entered (I wasn't taking it seriously) that was a funboard race which contained almost entirely formula boards. It was a light wind day say 5-10 knots and variable and I was on a longboard, (F2 Phoenix 340) and I thought that on the day the FW's might have difficulty planing. There were some good sailors on the water that day, including ex world funboard champ Robbie Radis. (He might be current WC, I don't really know).
    My Phoenix is lots of fun to sail (but dated), and goes quite well in sub-planing conditions, being long and narrow with centreboard, and also planes quicker than most racing dinghys, but by todays standards is dead slow for a sailboard. The other thing was that I had my biggest sail up 7.5 sq.m
    Radis and many other FW's had about 11 or 12 sq. m sails. There were periods when I was going quicker than Radis, obviously he wasn't planing.
    However a great sailor can pump up onto a plane in marginal conditions and create enough apparent wind to keep going. When Radis got up on the plane he moved away from me at probably 3 times my boat speed. It was truly amazing to watch. He won that day from another FW by more than one leg. I have watched foiler Moths get up and go, but not Rohan's boat.
    I can't say definitely, as these events happened on different days, but without much doubt, once the FW is planing no Moth is going to be even very close around a race course.
    As far as the RSX is concerned, IMHO, they are flawed design and doubt that they will see more than one Olympics. It is still hard to go past the Starboard wood composite 1 metre beam formula board, and this is now a fairly old design. In sailboard racing I believe the sailor's ability counts for such a large percentage of the speed achieved, much more so than in most high performance dinghy classes, possibly with one exception; the foiler Moth.
  3. Phil Stevo
    Joined: Feb 2004
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    Phil Stevo Junior Member

    I agree with Frosh, The formula boards are faster than the foiling moths once they are up and planing hard.
    We start togther every saturday but the boards and moths do different courses.
    Up to 10kts the moths sail away, after that the top boards and foilers are close upwind.
    But downwind the boards have about a 2 kt advantage.
    It is pretty weird on the foil moth when you think you are going fast, everything is quiet, then you hear a board coming up from behind, and passing you with lots of spray and noise.
    The two groups often compare GPS speeds in the bar afterwards and the boards always have a couple of extra numbers. Both boards and moths are from top boats in the country.

    They make the boards look easy, but I am sure they have as much difficulty as we do in rough water.
  4. frosh
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    frosh Senior Member

    In the late 70's an unusual catamaran was constructed with 2 hulls that were only slightly modified sailboards. The rig was a wishbone rigged primitive windsurfer sail stayed with wires like a regular dinghy. It was called "Itza Cat". It planed in all winds and considering the pathetic rig and small size of the sail went very impressively. (Probably got to around 20 knots). Just goes to show that it is not a "given" that the hulls of a cat or trimaran in the smaller boat sizes need to be skinny wavepiercing displacement type hulls. There is a video of the Itza Cat sailing and it looks like a huge amount of fun. Best of all is that this fairly crude design effort could be easily improved performance wise, possibly 30% at relatively low cost using todays materials and a far more efficient and larger sail.

    " Frosh, there have over the years been may attempts at making planing cat hulls. The best was probably the 12ft Kitty cat. All the others from Cunningham's 16ft XY, 20ft Attunga, right up to the recent stephull French record chaser were failures relative to their competition. So you are taking on a brave challenge".
    QUOTE- Phil Stevo

    The Kitty cat is very familiar to me and I was involved with sailing it on one day in Victoria (Port Phillip Bay) It was a 12ft. rocket ship for its time and even today I wonder if the best 12ft. monohull of current design would go quicker than a 60's Kitty cat, if it was in pristine condition. I won't comment on the other ones that you mention as I have no first hand experience of any of them although I have seen photos of them all. This is only anecdotal of course as I wouldn't know if a 12ft. skiff has ever sailed head to head with a Kitty cat.

    As far as taking on a brave challenge, I don't really agree. Tell me, did you suspect that the DC you designed and built might possibly go slower than a Nethercott IC in a straight line on any point of sail? I very much doubt it.
    Your gut feeling paid off handsomely, and similarly if my tri cannot stay with a similar sized racing cat in a straight line then I believe it will be almost certainly be the fault of the rig which has a very radical mainsail. (Soft high aspect gaff rigged wingsail) If this is the case I will put something like an A cat mainsail on it, but I wanted to save a few Kilos and a quite a few bucks until I find out if the planing vaka and amas are a success. I am thinking of building a 1.5mm ply carbon sheathed rigid wing, without using the jib as my next project anyway, providing the hulls handle satisfactorily. how do you think that a 15 sq. m rigid 2 part articulated rigid wing might go with an assy kite as well for downwind work? Can't see myself being able to resist this challenge in about one years time. Wish me luck!
    This is a link to the Itza Cat video:

  5. Doug Lord

    Doug Lord Guest

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