# fin foil question...

If a fat leading edge is foiled sharply to a thin middle and trailing edge - a cross section would actually show some concavity to the sides - how would that affect performance? What about a “stepped” foil - with a fat leading edge that steps down to a thinner middle and trailing edge?

If a fat leading edge is foiled sharply to a thin middle and trailing edge > - a cross section would actually show some concavity to the sides - how > would that affect performance? What about a “stepped” foil - > with a fat leading edge that steps down to a thinner middle and trailing > edge? John, Your question relates to Laminar Flow & edge turbulence. Depending upon the rate of change in concavity and the rate of water flowing across your you surface will determine how much turbulence this shape would create. At low flow rates and gradual degrees of concavity there would be minimal turbulence. The faster the flow and the greater the curvature in concavity the more turbulence it would create. As for your step question, every time fluid flows across a “step” it creates little circular eddies in it’s flow. These eddies are the turbulence I refered to above. Turbulence = Drag. Typically, surfers don’t relate good things with drag. But, drag can be loose in low flow situations. So, it could have it’s nitch. But, it would be a hard sell.

John,>>> Your question relates to Laminar Flow & edge turbulence. Depending > upon the rate of change in concavity and the rate of water flowing across > your you surface will determine how much turbulence this shape would > create. At low flow rates and gradual degrees of concavity there would be > minimal turbulence. The faster the flow and the greater the curvature in > concavity the more turbulence it would create.>>> As for your step question, every time fluid flows across a > “step” it creates little circular eddies in it’s flow. These > eddies are the turbulence I refered to above. Turbulence = Drag. > Typically, surfers don’t relate good things with drag. But, drag can be > loose in low flow situations. So, it could have it’s nitch. But, it would > be a hard sell. Tom and John, Seems to me that anytime you depart from a convex foil surface you will intruduce any unnecessary drag factor. The reports I get from the surfer ridding the fins I’m made, so far it’s just a few) are as follows. Having a thick leading edge will make the fin more sensitive and offers a more sudden driving affect when going rail to rail much more noticable on a single fin set up. A more traditional fish body crossed foil a little on the full side will be rather neutral and probably the fastest as it’s what nature has come up with after millenia of trial and error. finnally a very shallow and narrow foil will give more drive and drag and tends to be stiffer. Correct me if I’m wrong gentlemen. Research and Development continues, Mahalo, Rich

The curve in a foil cross section is called chord (I think) and I never understood why people think the a large chord (thick fin) is faster…I have a large chord old Dolfin from FU (the organic fin of choice during the late 70’s early 80’s) and a small chord Liddle flex. I’ve stood in the swimming pool and dragged both through the water a various speeds. The Dolfin had several times the resistance of the Liddle. Maybe they were more “sensitive” or held better (my “testing tank” experiment couldn’t test those variables) but fatties are definately not “faster”. Don’t count on nature being the most efficient “foiler” either. Large chord stabilizers on cetaceans are as much a function of feeding cartilage with blood and keeping it warm with blubber as it is efficient laminar flow…Tuna fins (Greenough’s inspiration) are as thin a knife blades (no need for blubber and they are supported by bone). Newbs