Centre Keel Shape

Dear All,

I am in the throes of restoring a Tucker 24 which was originally designed as a self build. There are twin keels on either side of the main long keel (picture)whch has a welded steel box filled with something (possibly scrap iron sealed with cement).

I was about to remove the keel box and have a more rounded profile made up. A fellow inhabitant of the boat shed with more hulls than me suggested latest thinking in Wharram circles was that a square edge "gripped more" to windward.. Is this really true?

What I was actually going to do was to add some profiling to the twin keels and was considering adding a small wing to the base of the box to improve performance as well setting cast pigs (to be sure of the weight) into a more rounded keel box and replacing.

Am I wasting my time or is there some way to improve performance of the triple keeler by what I am trying to do or is there some other design modification whcih would help?

Best,


Stuart

 

Another pic

As a prompt to your creative powers here is a picture of another Tucker 24 from Boatshed with cut outs in the bilge plates where mine are unusually hollow but also flat in profile.

All suggestions as to what can be done (if anything) to improve this profile welcome.

Stuart

 

Hello Stuart,
Technically speaking, a keel of this type is an equivalent of a very-low aspect-ratio wing platform (cropped delta-wing, to be more precize) used in aeronautics. As such, it's hydrodynamics has some particularities which make it very different from modern, more slender keels.

The most important thing about this kind of keels is that their lift is in great part due to a very large vortex which is created along the leading edge when the keel is at some positive angle of attack. This vortex then travels back along the suction side (windward side of the sailboat) of the keel and creates a zone of low pressure, and thus the lateral force. The stronger the vortex, the stronger the lateral force acting on the keel. The price to pay for this is a much bigger induced drag, when compared to modern, higher aspect-ratio keels and centreboards.

A good by-product of this vortex is that it keeps the flow attached to the surface up until very high angles of attack. Delta wings will usually stall at an angle of attack of around 30°, or even more. The ultra-low aspect ratio keels, like the one shown in your photos, will practically never stall. Even if they hypothetically moved at an angle of 90° to the waterflow, there would still be a vortex attached behind, which would create the said suction.

So, knowing how this type of keel works, it is clearly advantageous to have this vortex as strong as possible, if you aim for higher lateral force for a given angle of attack (as you do). And the best way to create a vortex like that one is to make the waterflow pass around a sharp edge. In fact, talking about aircrafts again, delta-wings with round leading-edge have a less-sloped lift curve, when compared to delta wings with sharp leading edges.
Hope the message is clear, regarding your first question.

I have attached 2 pics here. In the first one you can see the keel "sliced" at various sections, and how the vortex builds up along the chord of the keel. In the pic n.2 one of these sections is shown and the position of the vortex along the keel is visible. You should visualize this as the keel laying in horizontal position, the hull is at the left side of the keel, the tip is to the right.

 

As about the question in the post #2, I must admit that I didn't understand it... Sorry - language barrier, I guess.

 

Thanks Daiquiri,

What you have demonstrated is what I understood form the cat owner that the low aspect keel should have a square edge so thanks for that.

The question of the bilge plates is that I have read that the three keels ie the long keel + the plates creates lots of turbulence and hence drag. I had considered adding some profiling to the plates to make them give more lift but the tubulence may render this pointless.

Last night someone suggested removing the bilge plates altogether. I had also thought about adding a wing to the centre keel, this would make more sense if the bilge plates were removed. I know the wing tips on the Airbus A380 are needed because the wing size is limited. There is the question of stability when the boat drys out on my mooring but this sounds like a good solution.

If I go for the wing keel addition to the square section centre keel is this going to be the magic solution I am lookng for or is this no better than just adding the weight of the bilge plates to the centre section in the conventional way?

Stuart