I looked at van Nierop et al. and there seems to be something of a tradeoff here. They compared foils with smooth leading edges to foils with varying sizes of tubercles, but otherwise similar shapes. They found that the smooth foils had the highest coefficient of lift (Cl), but that the relationship between Cl and angle of attack had an abrupt inflection point at about 12 degrees (i.e., the foil stalled very suddenly at 12 degrees). In contrast, if tubercles were added to the leading edge the foil, the peak lift coefficient was reduced somewhat, but higher angles of attack could be used without a resultant stall. They show that a smooth foil had a peak Cl of about 1.2 at about 12 degrees, with the aforementioned precipitous drop-off in lift above 12 degrees. Their "tubercled" foils had a peak Cl as low as about 0.9, but a much flatter Cl/angle of attack curve at stall. (Note: The foils used in van Nierop et al. have tubercles along the entire leading edge, rather than just near the root of the foil, as in the pic on Bob's blog. So I suspect that the relationship between angle of attack and Cl would be a bit different for the actual boat rudder in the pic.)
Correct me if I'm misinterpreting things here, but it seems to me that a smooth foil should in theory give one the best performance, but the tubercles give the helmsman some feedback before the stall actually occurs. This would allow the helmsman better control, at the sacrifice of a bit of rudder performance.
EDIT: Changed "Cf" to "Cl" (my bad; I just reviewed a paper earlier today that was full of references to coefficient of friction, Cf; in other words, Duh)
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Last edited by SlowButSteady; 10-11-2011 at 09:14 PM.
Reason: Duh....typing faster than thinking