I've been puzzling over this problem all day so i'm hoping someone can come up with some better ideas than I have: When you put a boat that has a single propeller in reverse it will also turn. The direction in which it turn is based on the direction that the propeller rotates; if it rotates to clockwise (when viewed from the back) the boat will turn left, if it rotates anti-clockwise the boat will turn right. Basically it seems that the lower part of the propeller rotation has more 'bite' that the upper. There seem to be a few theories out there based on the effect: 1) The greater pressure on the lower part of the pressure generates more drag [as far as i can see the hydrostatic pressure shouldn't have any effect on the drag]. 2) The greater density of water generates more drag on the lower part of the propeller [would be true if water wasn't incompressible]. 3) Asymmetric thrust or the p-factor, similar to that experienced by aircraft. This creates a torque due to the difference in blade velocities caused by the propeller being mounted at an angle to the water flow. [this seems plausible when the boat is moving, but prop walk is most noticeable when the boat is stationary] My thoughts are there is some interaction between the hull and propeller wake, but I can't seem to find any solid facts to base this on and I've found no scientific papers or books even mentioning prop walk. Does anybody have the answer?