Page 201 - Practical Ship Design

Powering I 167

(iv) Shaft horsepower P,

This is the delivered horsepower increased to allow for the transmission efficiency
rlt.
Pd
p =- (6.26)
rlt
A normal value of q, for a modern ship with machinery aft is 98.5-99%. For
machinery amidships the value may reduce to about 98%.

(v) Brake horsepower P,
This is the shaft horsepower increased to allow for gearing efficiency, q, (if
gearing is fitted).
p\
P, =- (6.27)
q,

A fairly usual value for q, is 96%.

(vi) Service power Pb,

So far, the powers calculated apply to a clean ship in “trial” weather conditions. If
the power required is for a fouled ship in service conditions a percentage must be
added. Possibly the most usual addition is one of about 20%, but see 57.7.

(vii) Maximum continuous power Pbc

With diesel engines it is usual to limit the service power to a fraction of the power
the engine is capable of developing on a continuous basis in order to improve its
life and reduce maintenance. This is known as derating (d,) and the most usual
values of d, are 85 or 90%.

(6.28)

This appears a useful place to define other factors used in this chapter.

(viii) Admiralty coefficient A,
A2I3 . V3
A, = (6.29)
‘b
The Admiralty coefficient is a crude but still useful method of estimating power.
Provided its use is confined to cases where there is only a relatively small change in