Page 268 - Practical Ship Design

226 Chapter 7

Assuming that reasonable attention has been paid to these design features and that
modern anti-fouling paint has been used a figure of between 10 and 15% for the
weather allowance would seem to represent general modern practice, although a
smaller figure could be used for some routes whilst other routes and relatively
small ships may still demand a larger allowance.

7.8 DEVICES TO IMPROVE PROPULSIVE EFFICIENCY

The fact that even a well designed slow revving propeller is unlikely to have a QPC
greater than about 0.75 and that the QPC of the higher revving propellers which
must necessarily be used on many ships can drop to about 0.50 or lower, has caused
a lot of attention to be devoted to examining where the lost energy goes and trying
to avoid this loss or reclaim it.
Much of it appears to go into rotational energy and two different approaches
have been adopted to avoiding/reclaiming this.

7.8. I Pre-rotating

A number of devices try to avoid the loss by pre-rotating the water flowing into the
propeller in the opposite direction to the “twist” given by the propeller so that the
ultimate wake is partially straightened.
Possibly the most radical way of doing this is to change the whole aft end lines
of the ship, making these asymmetrical as shown in Fig. 7.16. Apart from some
extra loft work this appears to involve minimal extra cost and no extra resistance.

Fig. 7.16. Asymmetrical lines.