Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Chapter 1: Propellers and Shafts

Cavitation

Propellers drive a boat forward because their blades act as hydrofoils as they are rotated
through the water by the torque and power of the shaft. Like all foils (aerofoil or hydrofoil) the
blades generate a negative pressure—called lift—on their forward faces (termed the blade
backs) and a positive pressure on the blades’ rear-facing surfaces (termed the blade faces).
The resulting force pushes the propeller forward, which in turn pushes on the shaft, which then
pushes on the thrust bearing in the gearbox, which then transmits this thrust to the engine beds
to drive the boat forward.
Cavitation results when the pressure around the blades is reduced below the local vapor
pressure. Bubbles of vapor implode against the blades with enough impact to make considerable
noise and vibration and to erode the blade surfaces. You can get a more complete explanation of
cavitation from the relevant chapters in my books The Nature of BoatsandPropeller Handbook.
A full understanding of cavitation still eludes science. It is a fascinating area of inquiry
and connected with such truly odd phenomena as sonoluminescence. In theory, the force of the
implosion of a cavitation bubble is nearly infinite at the point of impact (though, of course, in
reality, the force is not infinite). It was this property of cavitation that led to the fantastic “cold-
fusion” claims of professors Pons and Fleischmann a few years back.
In any case, even if we don’t fully understand cavitation, we do largely know how to
predict its onset and to minimize it.

Figure 1-3. This

segment of an SAE

illustration clearly

shows the smaller

nut innermost

Figure 1-4.

A standard

3-bladed prop and

wide blade

5-bladed prop

(Michigan Wheel;

Paul Bremer)