Most of the watercooled vee type engines in World War II aircraft, such as the Allison V-1710, had a reduction box that reduced the output RPM, the ratio is usually around 2:1. So, at a maximum engine RPM of 3000, the prop only turned about 1500. The larger the prop diameter, the lower the ratio had to be in order to keep the prop RPM down below the point where the tips of the props exceeded the speed of sound. It is also possible to cavitate a prop in air, just like you can in water, since air becomes a fluid with the same characteristics of water at a certain speed.