Originally Posted by RickH
The 45 degree diamond configuration suggested will probably give you some sort of improvement, but here is a true "thistle" cross-section plotted from the Gerr article mentioned earlier. It's basically a modified airfoil with the fishtail on the trailing edge. Gerr also strongly recommends the use of end plates (on the top and bottom of the rudder) to help channel prop wash through the fishtail. The line through the middle on the pic is a scrap of 1/4 inch ply to represent the existing flat plate rudder (actually 5/16").
Having hundreds of hours of advanced aerodynamics I can really appreciate the thistle shape..owning a 6.5 knot trawler and I think the thistle shape for a rudder is a complete waste of energy...but I bet it looks really cool..
On most slow, highly manueverable tug boats I see all kinds of things like flanking rudders, kort nozzles, rotating drives...but never a thistle rudder....hmmmmm
Not saying they don't improve performance but I just can't believe it's even measurable...is it?
any independent written comparisons?