Page 246 - Introduction to Naval Architecture

PROPULSION 231

These are merely two ways of defining the same phenomenon.
Generally the wake fraction has been found to be little affected by ship
speed although for ships where the wave-making component of the
wake is large there will be some speed effect due to the changing wave
pattern with speed. The full-scale towing trials of HMS Penelope
indicated no significant scale effect on the wake. 6
The wake will vary with the after end shape and the relative propeller
position. The wake fraction can be expected to be higher for a single
screw ship than for twin screws. In the former the Taylor wake fraction
may be as high as 0.25 to 0.30.

Relative rotative efficiency
The wake fraction was based on the average wake velocity across the
propeller disc. As has been explained, the flow varies over the disc and
in general will be at an angle to the shaft line. The propeller operating
in these flow conditions will have a different efficiency to that it would
have if operating in uniform flow. The ratio of the two efficiencies is
called the relative rotative efficiency. This ratio is usually close to unity and
is often taken as such in design calculations.

Augment of resistance, thrust deduction
In the simple momentum theory of propeller action it was seen that the
water velocity builds up ahead of the propeller disc. This causes a
change in velocity of flow past the hull. The action of the propeller also
modifies the pressure field at the stern. If a model is towed in a tank
and a propeller is run behind it in the correct relative position, but run
independently of the model, the resistance of the model is greater than
that measured without the propeller. The propeller causes an augment
in the resistance. The thrust, T, required from a propeller will be
greater than the towrope resistance, K The propeller-hull interaction
effect can be regarded as an augment of resistance or a reduction in
thrust. This leads to two expressions of the same phenomenon.