Page 262 - Introduction to Naval Architecture

PROPULSION 247

speed. The point has been reached in the analysis where the last aim
can be met.
The general principles involved were outlined at the beginning of
this chapter. In Chapter 8 an example was given illustrating the
calculation of a hull's effective power. This same ship can be used to
calculate the machinery power needed to propel it at the 15 knots for
which the effective power was 2502 kW allowing for roughness.
Continuing:

If the hull efficiency elements and the quasi-propulsive coefficient
determined from experiment were Taylor wake fraction = 0.27, hull
efficiency = 1.15, QPC = 0.75 and relative rotative efficiency = 1.00,
then:

This is the power for calm conditions. If 20 per cent is allowed for
average service conditions the installed power to maintain 15 knots in
these conditions is 4288 kW.
The actual power to be fitted will depend upon the powers of the
machinery sets available. For the present example it is assumed that
the closest power available is 4275 kW and that the slight difference
is accepted by the designer. It follows that:

The choice of propeller revolutions is generally a compromise
between propeller performance and machinery characteristics. Pro-
pellers are more efficient at low revolutions and machinery is lighter,
for a given power, at high revolutions. Reduction gear can be fitted
to bridge the gap but the cost and weight must be set against the
advantages gained. It is assumed initially that propeller revolutions
are to be 100.