Page 245 - Introduction to Naval Architecture

230 PROPULSION

THE PROPELLER BEHIND THE SHIP

So far the resistance of the ship and the propeller performance have
been treated in isolation. When the two are brought together there will
be interaction effects,

Wake
The presence of the ship modifies the flow conditions in which the
propeller works. The water locally will have a velocity relative to the ship
and due to this wake, as it is called, the average speed of advance of the
propeller through the local water will differ from the ship speed. The
wake comprises three main elements:

(1) The velocity of the water as it passes round the hull varies, being
less than average at the ends.
(2) Due to viscous effects the hull drags a volume of water along with
it creating a boundary layer.
(3) The water particles in the waves created by the passage of the
ship move in circular orbits.

The first two of these will reduce the velocity of flow into the propeller.
The last will reduce or increase the velocity depending upon whether
there is a crest or trough at the propeller position. If the net result is
that the water is moving in the same direction as the ship the wake is
said to be positive. This is the case for most ships but for high speed
ships, with a large wave-making component in the wake, it can become
negative. The wake will vary across the propeller disc area, being higher
close to the hull or behind a structural element such as a shaft bracket
arm. Thus the blades operate in a changing velocity field as the
propeller rotates leading to a variable angle of incidence. The pitch
cannot be constantly varied to optimize the angle and an average value
has to be chosen. That is the design of each blade section is based on
the mean wake at any radius.
Model tests in a towing tank can be used to study the wake but it must
be remembered that the boundary layer thickness will be less relatively
in the ship. Model data has to be modified to take account of full-scale
measurements as discussed later.
In preliminary propeller design, before the detailed wake pattern
is known, an average speed of flow over the whole disc is taken. This
is usually expressed as a fraction of the speed of advance of the
propeller or the ship speed. It is termed the wake fraction or the wake
factor. Froude used the speed of advance and Taylor the ship speed
in deriving the wake fraction, so that if the difference in ship and
local water speed is V w: