Page 228 - Introduction to Naval Architecture

PROPULSION 213

The efficiency of the disc as a propulsor is the ratio of the useful work
to the total work. That is:

This is termed the ideal efficiency. For good efficiency a must be small.
For a given speed and thrust the propulsor disc must be large, which
also follows from general considerations. The larger the disc area the
less the velocity that has to be imparted to the water for a given thrust.
A lower race velocity means less energy in the race and more energy
usefully employed in driving the ship.
So far it has been assumed that only an axial velocity is imparted to
the water. In a real propeller, because of the rotation of the blades, the
water will also have rotational motion imparted to it. Allowing for this
2
it can be shown that the overall efficiency becomes:

where a' is the rotational inflow factor. Thus the effect of imparting
rotational velocity to the water is to reduce efficiency further.

THE SCREW PROPELLER

A screw propeller may be regarded as part of a helicoidal surface which,
when rotating, 'screws' its way through the water.

Figure 9,2