While the primary physical attributes of a propeller — number of blades, diameter and pitch — are well-known, there are a host of less easily understood characteristics that affect the performance delivered when the thing starts to spin. These include skew, cup, camber, rake, disc-area ratio and the oft-cited but ill-understood slip, cavitation and ventilation. Couple this plethora of variables with the fact that, when a boat’s inclination, aka its running angle, changes — and inclination changes with speed variations, sea state and the captain’s input via trim tabs or trimmable drives — so does the angle of attack of the prop. So if nothing else, it’s easy to see that there’s nothing easy about understanding propeller performance. You’ve got as good a chance of bending your brain around string theory.