> One sees this regularly on commercial
> airliners where the flaps are seperated by several inches from the surface
> of the wing when deployed. However in looking at the water flow patterns
> behind many kayaks, it seems pretty clear to me that the rudders are well
> abaft of this, working in the turbulance instead. His design is inherently
> avoiding this problem.

Actually, alot of the benefits of multi-segment airfoils is due to the slot between the two segments. The higher pressure on the lower surface forces air through the slot. This air is then redirected onto the top surface of the next portion of the wing (the flap, if you're already that far back, or the main surface if you're still forward at the leading edge slats) thus increasing the speed of the flow on the top of the wing, giving a boost to the bernoulli principle. Remember, alot of the lift is generated by the speed difference between the upper and lower surface flow.

To see this, try this: hold a piece of typing paper by two corners of its 8.5 inch edge. Hold it sow that if the paper were rigid it would be in a flat, level plane. The edge you are holding should be closest to your face, about 1 or 2 inches below your mouth. The paper forms what looks a little like an airfoil so that if you blow, a person looking from the side will see an aerfoil shape with a flow of air (from your mouth) blowing over the top of it. Now blow down onto the top surface of the paper at a slight downward (20-30 deg) angle. the paper will rise. Don't blow on the bottom, only the top, and blow downward. This is the bernoulli principle.

Most of what Bill Nye says at the link below is accurate, except for the thickness of the boundary layer. Turbulent boundary layers are thicker, and stay attached longer, which allows the flow to go smoothly over the surface. Laminar flow boundary layers are thinner, produce even less drag, but detach more easily. A turbulent boundary layer is better than none (detached flow), but a laminar boundary layer, if you can keep it attached, gives significantly lower drag. This is why "Laminar Flow Airfoils" are so popular in high performance situations like sailplanes. By reducing the pressure gradient's abruptness as one moves from leading to trailing edge (pressure also varies fore and aft) we can keep the laminar flow better attached.

A spherical golfball is a shape which does not lend itself to support of a laminar boundary layer, so we force it to go turbulent right away with the dimples.

> Is that drag symetrical above and below the surfaces of the wing? I tend
> to suspect not, but I don't know. If not, that effect could be significant
> in the case of his kayak. Since a kayak is very light, and is presenting a
> large aerofoil to the water. Which could result in some very quick turning
> if this is a significant force.