Following the theoretical analysis of the boundary layer and the roughness needed to stimulate BL flow transition, I've experimented with a simple turbulator (turbulence stimulator) on my Ikon fin. Until recently, I used a SAILSetc fin with 7% thickness ratio, and tried turbulators before without really noticing much difference. I've now fitted a new fin with 6% thickness ratio, and wondered if the thinner fin might be somewhat more prone to early stall while beating. So I tried a turbulator again.
Turbulator on fin
The first photo shows the general position of the turbulator strip at 70% of chord, the position Graham Bantock recommended as being appropriate for the foil section he uses. The second photo shows a detail view of the strip, cut with pinking scissors to give a zig-zag about 1.5 mm wide. The strip illustrated is about .010" thick, approximately .25 mm. This is pretty much the minimum thickness needed towards the fin tip according to the roughness spreadsheet, and may be a little too thin for the chord at the fin root. I have plans to experiment with a slightly thicker turbulator on the upper part of the fin. The line drawn on the fin marks the 70% chord position, so the turbulator starts its stimulation with its peaks placed along this line. In an earlier trial I had the turbulator at 50% of chord, and this seemed to give rather more of a drag penalty without a noticeable improvement in lift or postponement of stall.
I've found the results to be quite pleasing. The turbulator doesn't seem to have hurt performance very much on the run; that is, the drag penalty seems acceptable. But I feel that the boat is now more effective on the wind, more willing to point. So the fin seems able to either generate a little more lift than before, or to do it more efficiently. Also, the boat doesn't seem quite so prone to stalling during low speed manoeuvres, like creeping towards the starting line with sails sheeted out and not too much way.
©2008 Lester Gilbert