The title says it all what in the heck is ventilation and cavitation? Like many before you, you probably are wondering what these words often used interchangeably are. You may say “I’ve heard these before” but what exactly do they mean. I thought they were the same. The truth is they are both a distinctly different physical phenomena. Both affect a vessels performance occasionally for the better but usually for the worse. So it is important to know what they are, how to recognize them and how they may affect your boat.
Cavitation by definition happens when low ambient pressure and/or high temperature conspire to turn liquid water into a gaseous vapor, forming a cavity within the surrounding water. This physically looks like bubbles forming around a props rotation through the water (see picture). Cavitation can be caused by a undersized propeller. Quite simply that is why bigger engines need bigger props both in diameter and total blade area. The creation of cavities reduces prop efficiency. The forceful collapse of the cavities quickly after creation results in tiny implosions that can cause physical damage, particularly pitting, to surrounding structures such as propeller blades, struts and rudders.
Ventilation by definition happens when air is drawn from the waters surface, forming a void around a prop or rudder. Low pressure is the culprit here again just as it was during cavitation. When a prop or rudder is near the surface or close to the transom is how this low pressure can be formed. This is why outboard motors usually have a molded plated attached to the lower united above the prop. The plate is often called a anti-cavitation plate but it is correctly called a anti-ventilation plate. The side affects of ventilation are decreased performance and decreased lift. An air pocket forms in the wake and slows the vessel down by decreasing needed lift to get on plane. To combat this simply slow down and allow water back into the air-filled void.
Ventilation on the other hand is used for performance vessels that thrive off this phenomena. Very light fast boats such as racing boats and certain military craft use a smaller prop to deliberately induce a void. By operating props in water vapor or air, friction is lowered and performance is improved. Ventilation is also a necessity for vessels using stepped planed hulls to operate correctly.
Recap. Sub par performance, noise at the stern, pitted props, and rudders with missing paint in distinct patterns are all signs of possible cavitation. Props or rudders “blowing out” in turns when forward thrust decreases, causing speed loss and lack of control or during rapid starts is a sign of possible ventilation. If you can recognize these signs and understand the problem, you are on your way to the solution.
Travis Palmer, SAMS® AMS®, , AIMU, ABYC