Have you ever run your finger along the face, or edge, of a prop and felt a pitted area? If you have, then you know what cavitation can do to a prop. Given enough use, that pitting will only get deeper and you will have to replace the prop.
With your prop turning 4000 RPMs how do you know if you have power-robbing and prop-damaging cavitation?
What is Prop Cavitation?
Cavitation very commonly occurs on damaged, or imperfect, propellers and can occur if an attempt is made to transmit too much power through the prop. At high rotating speeds or under heavy load (with high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapor pressure of the water, resulting in the formation of a pocket of vapor, which can no longer effectively transfer force to the water.
What Causes Cavitation?
The true cause of cavitation can arise from several factors, but we see them occur most often as a result of leading-edge imperfections, such as nicks, dings, scratches, bends, etc. They can also be caused by improper geometry or upstream imperfections related to the hull, such as hull design and configuration, hull imperfection, incorrectly installed accessories/appendages, etc. Since there are so many possible causes, diagnosing the problem should really be done on a per case basis.
Cavitation wastes energy and makes the propeller “noisy” as the vapor bubbles collapse, and most seriously, erodes the prop’s surface due to localized shock waves against the blade surface.
What is Ventilation?
A similar, but quite separate issue is ventilation, which takes place when air is introduced to water around, or to the water flow fed to a propeller. This can be caused by a number of different sources, such as porting, over-and-thru hub propellers, tunnel hulls, extreme motor elevation, excessively high trim angles, stepped hulls, or even hull appendages (transducers, pick-ups, and the like). As the propeller meets the aerated water, the water’s flow attachment to the propeller’s respective working surfaces is detached. As the water detaches, the propeller’s loading is reduced, and the torque applied to the propeller through the shaft can more easily spin the propeller to a much higher rpm even at the same given power input. This not only causes the motor to rev easily, but it reduces the thrust provided by the propeller, slowing the boat’s speed and/or acceleration, as well as reducing the propeller’s control over the boat (blow out on turning, on hole-shot, etc.).
Cavitation and ventilation can significantly damage moving parts and are usually undesirable phenomena. It is specifically avoided in the correct design of props and that is why prop fabrication and prop repair must be relegated to the experts.