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Thread: Prow shape

  1. #1
    Join Date
    May 2007
    Location
    Mahwah, NJ USA
    Posts
    252

    Default Prow shape

    A question for the cognoscenti:

    Is it better to have a sharp prow on a Footy, or is a rounded prow better? I have noticed that very few boats have a really sharp prow.

    Because of the very low Reynolds numbers where Footies operate, I am guessing that the sharp prow has less drag as long as the boat is going straight. But if it is bouncing around or turning, the sharp prow might cause early flow separation and high drag. The rounded prow also adds a litle more flotation up forward, to counteract nose-diving. A sharp prow is also more susceptible to damage.
    Footy #95 - Pepsi Torpedo (returned to township re-cycling bin)
    Footy #140 - 3-Liter Torpedo (root beer bottle) - still competitive, partially retired
    Footy #340 - Modified Bob-About 2 - actively competing
    Footy #395 - Keel-Forward Footy with 14" water line

  2. #2
    Join Date
    Dec 2006
    Location
    London, Ontario, Canada
    Posts
    297

    Default Re: Prow shape

    I'm trying for a compromise with a sharp brow, but swelling to a fatter bow further up to counteract nose-diving.Both are still works in process although both have sailed, but not competitively. With hulls this small, you can really only assess the benefits of anything by comparison with another boat. GPI sailed well but is a wet boat, while GPII seems slow. Still trying to learn!!!
    I've been a bit stalled on Footys due to servo problems with HS-55 Hitecs. I have two which got immersed and one wont operate at all, while the other will only return to center if it is manually turned to one side, but not when turned to the other. That is the only function it will carry out. Both have been taken apart, cleaned and dried and sprayed with contact cleaner--but no joy. All info or suggestions welcome.
    Attached Thumbnails Attached Thumbnails Click image for larger version

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  3. #3

    Default Re: Prow shape

    Walt,
    My Cobra 3 (which you have seen) has a fairly blunt bow, while the new Razor 3 has a very sharp bow. Both go to windward very well. Right now, using identical sails, the Razor has a slight edge going to windward, while the Cobra has a slight edge downwind - all as you might expect.

    My next boat will be a Cobra 3 varient where Bill Hagerup has extended the bow to give a sharper prow and a slightly longer waterline. One is currently being built on Long Island and i will get to start mine in a month or so. It will be interesting to see how they compare.
    Scott

  4. #4
    Join Date
    Aug 2006
    Location
    Colwyn Bay, England
    Posts
    1,608
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    Default Re: Prow shape

    I was one of the first people to use what I call 'bullet bows' in Footys. I do bot say that the logic is correct or that the theory is anything but witchcraft. For better or worse, here it is.

    1. The effect of widening the bow right forward on the moment to bury the bow by x mm is surprisingly great. The volime is small but the lever arm about the centre of action is large. This is pure Archimedeann mechanics. Since it has been uded in genral engineering practice for about 2500 years, it is pretty likely right.

    2. Much nearer to witchcraft. In Smooth(ish) water I think that the globular/bullet stem generates an area of dead water ahead of it that becomes part of the effective wave-making length - i.e. we get a slightly longer boat free.

    3. Quasi-witchcraft. There is published research on mooring supertankers that suggest that they will 'fly' upstream in a tidal stream. Most of the research applies to vessels with bulb bows and globular stems, but thre is evidene to suggest that the bulb bow is not fundamental.

    Hope this helps,

    A.
    A.

    He who dares sometimes wins.
    He who doesn't always looses

    Footys GBR 26 Little Gull
    GBR 28 Dingo
    GBR 61 Merlin
    GBR 71 Akela
    GBR 77 Sun Dog
    GBR 171 Mandeln i gröten

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