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 end having to replace the prop. The causes of cavitation have always been somewhat of a mystery to us, so we asked the experts at PowerTech! to give us the technical explanation for this phenomenon.
With your prop turning 4000 rpms how are you to know if you have power-robbing and prop-damaging 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.
In my manner of thinking, cavitation could be defined as the phenomenon of the formation, rapid collapse, and subsequent implosion of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. As these bubbles collapse, energy is released in the form of a shock wave that can, and often does, damage the surface of the material in question (Stretching the analogy to a screw, you might say the water thread 'strips'.)
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.. That said, they can, also, be caused by improper geometry, or upstream imperfections related to the hull, such as, hull design/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.
This drawing show where cavitation can occur. 1. Generated cavitations 2. Cavitations coved backside of the propeller 3. Cavitations vanish away from propeller. 4. Turn 5. Ship's driving direction 6. Water stream based on a propeller.
Cavitation wastes energy, 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.
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 wa...