12 BASIC POWERBOATING. SAFETY & RESCUE FOR SAILING INSTRUCTORS


Water flowing over the surfaces of a propeller blade produces a higher pressure on one side than the other. This pressure difference generates lift, which results in thrust, as well as a sideways force (torque). If pressure on the low-pressure side of the blades gets too low, bubbles of vaporized water (low temperature steam) will form on the blades. This bubbling action disrupts the water flow, causing the blades to lose lift and thrust, and the engine to speed up.


This phenomenon is called cavitation. When it occurs, reduce throttle and allow the propeller to “re-grip” the water. Cavitation can happen when too much throttle is applied too quickly, or if the propeller is damaged or not the right size.


Vaporized water bubbles (cavitation)


Blade Q - Q + Lift Thrust


As a propeller rotates through the water, the pressure on one side of the blades is higher (+) than the other (-), which generates lift and thrust.


Turbulent water flow


Cavitation causes the blades to lose lift and thrust.


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