Page 260 - Practical Ship Design

218 Chapter 7

7.5.1 Slow revving propellers

Whilst it is possible to design, and optimise the open water efficiency of, a propeller
for any required power output, propeller diameter and revdmin, there is no doubt
that the starting point of a search for efficiency should be the use of the largest
propeller diameter that the ship can be designed to accommodate without un-
acceptable adverse consequences in association with the lowest revs/min that suits
this propeller and can be obtained from suitable propulsion machinery.
An indication of the gains to be obtained by using low revslmin is provided by
Emerson’s approximate formula for QPC which with a slight increase in the
constant to bring it into line with modern propeller design, is:

qd = 0.84 -- N& (7.21)
10,000

where
N = revlmin
L = length BP in metres

A plot of this formula is given in Fig. 7.13. In this figure Emerson’s formula has
been extended to values of N smaller, and values of L larger than the data from
which it was originally derived. Remarkably, in spite of this extrapolation it
continues, in the author’s experience, to give reasonably accurate answers, although
caution must be advised in its use at low Nand large L values.
Figure 7.15 shows the improvement in propeller efficiency that is obtained by
today’s slower revving engines. On a 300 m ship the gain from a change from 1 10

0.80

0.75

Q. .P .c.
0.70

0.65
50 60 70 80 90 100 110
R.P.M.

Fig. 7.13. QPC versus rprn and ship length.