Cavitation

Cavitation is the formation of water vapour at ambient temperatures in water. This is done by reducing the pressure to the point where the boiling point of water is the same as the ambient temperature. When cavitation occurs, the small bubbles of steam will collapse when they reach a point where the pressure is higher. This tends to happen very rapidly, with the result that a small shock wave is set up. Where this happens frequently, as in a propeller operating outside its design conditions, these small shock waves can cause considerable damage over time. Propellers and structures adjacent to them will show serious pitting on their surface.

Some paddlers think that cavitation causes the bubbles they see coming from its blade at times. This is simply not true - that is ventilation.

Consider the situation - in order to produce cavitation, the pressure must drop considerably. If the bubbles move, they will collapse almost immediately. The persistance of the bubbles coming off a blade shows that they are not low pressure steam. Furthermore, if the bubbles were forming and collapsing, the paddle blade would quickly show damage. As a final argument, consider the pressures required to produce these steam bubbles. The pressure at the surface of the blade would have to be roughly 15 psi less than the water around it. Uniformly distributed over a typical paddle blade of 100 sq. in. would result in a force of 1500lb! Even if there was a gradual increase from zero to 15 psi (average 7.5 psi) that would mean a 750lb force! Typical paddling forces are about 15 lb for a sea kayak at 6 kt (that's fast). For cavitation to occur, that would require a distribution of pressure of virtually zero over 99% of the blade and the full 15psi over 1%! Since this is equivalent to water velocity over the blade, there has to be a mechanism to accelerate the water in that manner. That isn't going to happen.