Join Date: Dec 2004
Location: Portugal, West Coast
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Re: Advanced keels
My conclusions from the three Chalmers investigations are:
The first conclusion is that maximum thickness of the bulb must be placed well back from maximum thickness of the fin. Otherwise an extended lowpressure area is generated on the suction side (with contributions from both the bulb and the fin) that causes flow separation. Loss of lift is obtained and a really bad keel from the hydrodynamic point of view is obtained. Two T-keel designs were investigated, one with coinciding maximum thicknesses of the bulb and fin and one with maximum bulb thickness behind the rear end of the fin. The results clearly showed that the second one was best.
The second conclusion is the a flat bottom with sharp edges (chines) of the bulb is not a competitive design. A optimization was peformed where the shape of the bulb was allowed to vary from flat bottom (with sharp edges) to rounded. Rounded showed higher performance.
The Chalmers investigations were performed about 2 years ago, the results are public and should be known by serious designers (especially as Lars Larsson was the supervisor). Fixed keel design for recent custom yachts do not show an impact of this and my personal conclusion is that keel design is an ad hoc thing.
I am not sure If I understand what you want to say. There were 4 keels on that study :
and the conclusions are:
As can be seen in the graph, of the bulb keels, keel 4 has lowest drag at zero angle of attack. It increases faster than keel 3, which means that at 4° angle of attack keel 3 and 4 have equally large drag forces. Keel 1 was as expected best performing, but it was only evaluated as reference.
Figure 50 describes the most important behaviour and performance of the keels: the relationship between lift and drag forces. From the graph any given lift can be set, and a corresponding drag can be read. It can clearly be found that keel 4 is the better performing one of the bulb keels; especially at
small angles of attack. At larger angles of attack the trends from this curve, and more specifically from .. keel 3 is the best performing keel. This however only for very large angles of attack, for more reasonable angles of attack keel 4 is the best performing bulb keel. It is quite clear that keel 2 is inferior to the others at all lift coefficients measured.
On the evaluation of performance even if the differences are very small the results confirm the drag and lift conclusions. They have considered 16K of wind but I believe that if they had considered a much smaller wind speed the differences would be bigger.
As can be seen in Table 7 the results between the keels are very even. Keel 1 has no bulb and that makes it not comparable to the keels with bulbs. During the VPP keel 1 has only been used as a reference. Between the bulb keels, keel 4 is the winner; it has a higher velocity at all wind directions than both keel 2 and keel 3. The Keel with the worst performance is keel 2; but it is very close between the last two bulb keels.
The keels were also evaluated against each other on a race....
The times for completing the race are extremely similar. Keel 4 has the lowest lap time for the different true wind speeds. What’s most interesting is that at low true wind speeds keel 4 is even faster that keel 1.
Those are the conclusions of the study and those are common knowledge.
What he has done regarding CFD in the keels is what all NA, at least the ones that design race or performance sailboats do when they design the boats. They do that for the hull, for the keel and for the rudder(s).
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