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"


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.

Thanks Muzza

I’ll just go with what I think looks good to me for this first one, and then refine the next ones.

Yes Tomo, Muzza nails it.:icon_smok
IMHO the Renolds number data is not aplicable to most model yachts because of the speed and chords that are involved.:magnify:
In other words if it “looks good,” it most proberbly is good.:spin:

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

Bob says 20% taper.

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.:cool:

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.


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??

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.


If you really want to go thin, then I would just use a single layer of carbon or steel or ali- about 2 or 3mm.

I commisioned a top Airfoil designer to develop fin and rudder sections for my new Footy design which will be avalible soon.The results are interesting to say the least.(guess this should be in cheque book racing thread:) )
These sections cost me money…so don’t expect to get anymore than that out of me for now!!

Thank you TomoHawk. That is a valuable contribution. Thank you. My full-size thinking leads me to think in terms of a box girder or monocoque with some sort of filler inside to stop the shell buckling. This is pretty impractical at this size - you could probably pultrude it in carbon but I fancy the costs of the dies would be astronomical.

So look at a fin section in which we can ‘bury’ a block of carbon sheet without compromising the shape. What about ‘bread and butter’ - thin carbon sheets glued together to occupy as much of the area of the section as possible and then cover it with microballoon filler and sand to shape?

Incidentally, this goes some way to answering your original question. Such a keel would me much easier to make (and hence be more likely to be the intended shape) if it had a constant chord.



The Footy is a whole different animal than an IOM, or anything else, so we really should throw out all the usual rules & techniques used for building bigger boats, since they would probably be “over-exaggerated” in size and strength on the Footy. I’ve got 6 hulls on the shelf (not fully finished- yet) and I’ve learned a lot to building Footies from those. Some things can easily be overbuilt, and in some places you can eliminated things like bracing, because the hull itself is probably all you need in terms on strength. An example would be in some of the photos I’ve seen where the servos were attached to ice cream sticks! I’d never do that on a 1metre.i

Please do what you like, learn what you canfrom it, and share it with the rest.

TomoHawk. You are so right as I am beginning to find out! Suppose we use a single skin of 16 g/m2 carbon. This will wet out woith epoxide at about 1:1. Because very thin laminates are awkward, let us assume 1:1 by volume rather than weight. SG of epoxide is about 1.9 - call it 2 - so we have a total layup of about 48 g/m2. But the hull area of a Footy is about 0.5 m2. Therefore we are probably talking about a bare shell weight of around 3 grams.

This will buckle easly so we will need a couple of top-hat stringers and frames (fiddly) but we are unlikely to be much above 15 g. Done well the answer should be more like 7 or 8.

I guess that a balsa hull weighs about 30-35 g. plus paint (necessary) - say 45 g. So a badly made (in terms of the internal reinforcement) carbon boat has a weight advantage of about 30 g. and a well-made one an advantage of around 37 g. Since the original Bobabout is specified as having a ballast weight of 150-200 g, this is an advantage of a ‘light’ carbon boat to a ‘heavy’ Bobabout of 25% of the ballast. That is as different as chalk and cheese.