Page 261 - Practical Ship Design

Powering II 219

rpm, typical of diesel engines of some 15 years ago, to 80 rpm, typical of today’s
large ships is 7.6%.
An ability to use a lower rev/min depends on whether a larger propeller, which
for a given power, must accompany a reduction in revlmin, can be accommodated
within the stern aperture, which in turn depends on the load draft.
A relationship which can be used for quick exploration of propeller diameter
and rev/min is:

Pbs0.2
d = 16.2- metres (7.22)
N 0.6

Phs = service power in kilowatts. A plot of this is given in Fig. 7.14.
This relationship was derived from the general equation

Pd/pN3 D5 = w (J ,F,, O, R,)
so D5 = P,/K,iV3
orD=cpf2 NO.^

The constants were derived for a medium range of rev/min, power and diameter
and caution is advised in extrapolating the use of this formula to very high or very
low rev/min values.

Propelle
diameter
metres

SHP in Kilowatts

Fig. 7.14. Propeller diameter versus SHP and rpm.