Page 262 - Practical Ship Design

220 Chapter 7

15

Percentage
increase in lo
QPC reducing
revs from a
base of
N=110 5

50 60 70 00 90 100 110
R.P.M.
Fig. 7.15. Increase in QPC obtained by reducing rpm.

As well as the problem of accommodating the propeller within the load draft,
which has already been mentioned, a larger propeller may make a deeper ballast
draft desirable. This may mean an increase in the power required in ballast
increasing the fuel consumption in the ballast voyage and cutting into the gain
made in the load voyage.
The other factor which governs the use of lower rev/min is, of course the type of
machinery used. If this involves the use of gearing, then the rev/min can be chosen
to suit the propeller. On the other hand, the propellers of ships fitted with slow
speed diesels must be tailored to the engine rev/min.

7.5.2 Slow revving propellers on twin screw ships
The advantage gained by adopting as low revs/min as possible applies equally to
twin screw ships, although here the reduction may be from 300 revs/min to 250; or
from 250 to 200. Either of these changes gives a very worth while gain in propulsive
efficiency.
Again using Emerson’s formula, the gain in the first case would be about 8.5%
and in the second close to 10%. Once again this is a major extrapolation in the use
of Emerson’s formula that Emerson himself would almost certainly find quite
unscientific, but the author is quite unrepentant in this extension to its use having
frequently found tank tests giving better confirmation of the approximate values
obtained by this method than of those calculated by more detailed methods.

7.5.3 Optimising open water eficiency
Having obtained the potential for a high propeller efficiency by choosing an
advantageous rev/min, the next step is to achieve the best open water efficiency for
the propeller’s operating regime. The factors which influence this include: