Page 255 - Introduction to Naval Architecture

240 PROPULSION

The term controllable pitch propeller should not be confused with a
variable pitch propeller. The latter term is applied to propellers in which
pitch varies with radius, the blades themselves being fixed,

Self pitching propellers
A propeller which has found favour for auxiliary yachts and motorsail-
8
ers in recent years is the self pitching propeller. The blades are free to
rotate through 360° about an axis approximately at right angles to the
drive shaft. The angle the blades take up, and therefore their pitch, is
dictated solely by the hydrodynamic and centrifugal forces acting.

Shrouded or ducted propellers
9
The propeller is surrounded by a shroud or duct as depicted in Figure
9.20. The objects are to improve efficiency, avoid erosion of banks in
confined waterways and shield noise generated on the blades.

Figure 9.20 Shrouded propeller

The duct can be designed so that it contributes to ahead thrust so
offsetting the drag of the shroud and its supports. Most early
applications were to ships with heavily loaded propellers like tugs. Its
use is now being extended and it is considered suitable for large
tankers.

Pump jets
10
This is an advanced variant of the ducted propeller for use in
warships, particularly submarines, where noise reduction is important.
A rotor with a large number of blades operates between sets of stator
blades the whole being surrounded by a specially shaped duct. The
rotational losses in the wake are eliminated, cavitation is avoided and
there is no resultant heeling torque acting on the ship. The last point
is of significance for single screw submarines.

Contra-rotating propellers
Another way of eliminating the net heeling torque is to use two
propellers on the one shaft line rotating in opposite directions. It has