Page 229 - Introduction to Naval Architecture

214 PROPULSION

A helicoidal surface
Consider a line AB, perpendicular to line AA', rotating at uniform
angular velocity about AA' and moving along AA' at uniform velocity.
Figure 9.2. AB sweeps out a helicoidal surface. The pitch of the surface
is the distance travelled along AA' in making one complete revolution.
A propeller with a flat face and constant pitch could be regarded as
having its face trace out the helicoidal surface. If AB rotates at N
revolutions per unit time, the circumferential velocity of a point, distant
rfrom AA', is 2jrA?rand the axial velocity is NP. The point travels in a
direction inclined at B to AA' such that:

If the path is unwrapped and laid out flat the point will move along a
straight line as in Figure 9.3.

Figure 9.3

Propellers can have any number of blades but three, four and five are
most common in marine propellers. Reduced noise designs often have
more blades. Each blade can be regarded as part of a different
helicoidal surface. In modern propellers the pitch of the blade varies
with radius so that sections at different radii are not on the same
helicoidal surface.

Propeller features
The diameter of a propeller is the diameter of a circle which passes
tangentially through the tips of the blades. At their inner ends the
blades are attached to a boss, the diameter of which is kept as small as
possible consistent with strength. Blades and boss are often one casting
for fixed pitch propellers. The boss diameter is usually expressed as a
fraction of the propeller diameter.