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Thread: Keel Fin Chord taper

  1. #1

    Default Keel Fin Chord taper

    I've seen many keels with the chord getting smaller towards the bottom, is there a reason for this? Is there a 'magic number' or formula to taper the chord at the bottom?

    I'm making my Footy fin. It's .040 brass sheet with a strip of 1/16" ply on both sides, glued with Gorilla polyurethane glue & sealed with Polyurethane clear. It's really stiff for the thickness. I sanded the leading & trailing eldges to get a good & thin foil-shaped cross-section.

    10"x 2"

    thx
    Sail honest.
    Rock music.

  2. #2
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    Default Re: Keel Fin Chord taper

    Yes - there are theoretical reasons for the taper in the fin, to do with induced drag - but I very much doubt there would be any measurable effect for a Footy.

    Most of the really interesting reading I've done of this subject has to do with the shape of aircraft wings, particularly sailplanes/gliders - but there is a reasonable amount out there on the web with regard to full-sized sailboats. Lester has some information on his website too. Much of the stuff applicable to aircraft - particularly slow flying aircraft, is applicable at least in part to boats too. But we need to keep in mind the different range of angle of attack applicable, different Reynolds numbers at which we operate, and the effect of the lump of lead on the end - both in terms of the structural requirements and the possible end-plate effect.

    For practical purposes, I note that in the IOM class, just about any keel planform works in the hands of a capable skipper. Parallel keels may be a little out of fashion - but you only need to look at race results to see that they are competitive as part of the bigger package.

    It's all good fun.
    Muzza

  3. #3

    Default Re: Keel Fin Chord taper

    Thanks Muzza

    I'll just go with what I think looks good to me for this first one, and then refine the next ones.
    Sail honest.
    Rock music.

  4. #4
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    Default Re: Keel Fin Chord taper

    Yes Tomo, Muzza nails it.
    IMHO the Renolds number data is not aplicable to most model yachts because of the speed and chords that are involved.
    In other words if it "looks good," it most proberbly is good.
    Do it NOW before it`s too late.
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  5. #5
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    Default Re: Keel Fin Chord taper

    Mine are tapered, strictly 'cause I thot it looked better. I went from 1.25" to 1".
    Bob

  6. #6

    Default Re: Keel Fin Chord taper

    Bob says 20% taper.
    Sail honest.
    Rock music.

  7. #7

    Default

    From pragmatic thinking you'd think that a thin fin with sharper edges would help decrease drag, but also, at the 'slow' speeds of the Footy, it doesn't matter. The same thinking goes for the "Pinewood Derby" cars the cub scouts race; I've seen some "bricks with wheels" beat the streamlined models, because for something that little, there's (no) drag.

    I'm going with the sharper fin for now to give my footy the best chance it can get. Since it's the first, it'll have other problems slowing it down, until I work those out too.
    Sail honest.
    Rock music.

  8. #8
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    Default Keel structure

    Surely the structure of the Tomohawk's keel is wrong! To get rigidity we want the strong, inelastic material on the outside and the 'space filler' on the inside. Make a balsa fin and skin with kewlar, carbon or somerthing like that.

    A.

  9. #9

    Default

    Even just two plies of the same material is much stronger & stiffer than the one ply; I forget the formula for the stiffness. Plus, we are using what could be scraps for these boats. I don't have any scrap kevlar or carbon sheet around, but some thin ply, brass & Ali ali sheet.

    Having two dissimilar materials is the key, but both must be strong or stiff enough. They need to be sandable to shape. Others are using just a strip of baswood! How stiff do you need a 6 inch keel to be to support 8 oz of ballast??
    Last edited by TomoHawk; 08-07-2006 at 05:07 PM.
    Sail honest.
    Rock music.

  10. #10
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    Default Keel structure

    I take everything you say - except that he high modulus material should be as far as possible from the neutral axis.

    Surely the real point is not 'how stiff' but 'how thin'. There is little practical data yet on Footies but an IOM, for example, will typically have a fin thickness ratio of 6%. Suppose we take the root chord as 33 mm (about an inch and a half) this means a root only 1.98 mm wide. Say 2 mm or 1/12 inch. This is VERY thin.

    Note that IOMs with a displacement of 4 kg (8.8 lb) typically have pure carbon fins. Now on a poor little Footy that IS overkill.

    A.

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