Page 226 - Introduction to Naval Architecture

PROPULSION 211

It remains to establish how the hull, propulsor and relative rotative
efficiencies can be determined. This is dealt with later in this
chapter.

Propulsors

Propulsion devices can take many forms. They all rely upon imparting
momentum to a mass of fluid which causes a force to act on the ship.
In the case of air cushion vehicles the fluid is air but usually it is water.
By far and away the most common device is the propeller. This may take
various forms but attention in this chapter is focused on the fixed pitch
propeller. Before defining such a propeller it is instructive to consider
the general case of a simple actuator disc imparting momentum to
water.

Momentum theory

In this theory the propeller is replaced by an actuator disc, area A,
which is assumed to be working in an ideal fluid. The actuator disc
imparts an axial acceleration to the water which, in accordance with
Bernoulli's principle, requires a change in pressure at the disc, Figure
9.1.

Figure 9.1 (a) Pressure; (b) Absolute velocity; (c) Velocity of water relative to screw