Ventilation and Cavitation are two completely different phenomena that are observed, under specific conditions, during the rotation of the propeller and their understanding is often confused. Both the cavitation and ventilation of the propeller (except in certain special cases) have a negative impact on the performance of our boat.
Ventilation is the result of the entrance of air or exhaust gases from the exhaust outlet (they are drawn into propeller blades) between and around the propeller blades which lead to a sudden reduce of water load on the blades and thus to increase the engine’s rpm and the loss of its grip on the water.
Propeller ventilation can be caused in:
- very high engine mountings
- excessive engine trim
- sharp turns
- lower-unit imperfections (knicks or scratches) which prevent the water flow to the propeller
- bad position of the trim tabs or depth-finder transducers
- hull imperfections that disturb the water flow to the propeller
In most cases, propeller’s ventilation should be avoided because it leads to loss of thrust.
In order to reduce the effect of ventilation we should keep the lower-unit housing and our hull in very good condition, not mount our engine too high and use propellers with relatively large rake angles.
Cavitation, unlike Ventilation, is a more complex phenomenon due to low pressure and can cause severe damage.
It is known that water boils at 100 °C. However, what is less known is that the water may also begin to boil, even at room temperature, when the atmospheric pressure decreases significantly.
During the rotation of the propeller at high speed, the pressure on the back side of its blades is reduced. If the pressure falls below a certain value, the water begins to boil, resulting in formation of vapour cavities (bubbles) near the leading edge of the blades.
As the bubbles are migrating towards the centre of the blades, where the pressure is higher, they start to implode and ultimately cause repeated explosions, sometimes so violent that it is likely to lead to fatigue of the metal surface and finally to considerable wear of the blades.
Cavitation leads to:
- loss of thrust and speed
- noise and vibration
- erosion of the blades
The maintenance of our propeller in excellent condition, among other things, will limit the cavitation phenomena.
We need to know that one of the most common causes leading to cavitation is the propellers that have their blade edges bent or broken off.