🚤 Cavitation
When the pressure drop on the back of a spinning propeller blade is severe enough to boil the water into vapour bubbles, costing thrust.
It is often confused with ventilation, which is air being pulled down to the blades from the surface — different cause, similar symptom of revs rising while the boat slows.
What it actually does
Cavitation is when the propeller spins in aerated water rather than solid water, so it produces far less thrust. The boat suddenly revs high and slows down — the engine or motor is working but the prop is not gripping anything.
In practice
It typically happens when the prop is running too shallow, when air is drawn down from the surface in a turn, or when the hull is trimmed nose-high. Prop depth, strut angle and hull trim are the usual adjustments.
Worth knowing
Sustained cavitation is hard on the drivetrain because the load is constantly changing. If a boat cavitates in every turn, address the setup rather than driving around it.