Page 200 - Practical Ship Design

166 Chapter 6

(1 + x) = ship-model correlation factor, if not already included in
Pe(naked) (see 56.2)
A= additional resistance of appendages expressed as a fraction of
the naked resistance (bilge keels, stabilisers, bow thrust tunnels,
twin rudders, shaft brackets, bossings, etc.; see also 57.3).

(ii) Quasi-propulsive eficiency qd
This is made up of the open water propeller efficiency qo, the relative rotative
efficiency qr, and the hull efficiency qh.
qd = qo qr qh (6.21)

qh in turn is made up from the thrust deduction and wake factors as follows:
1-t
qh =- (6.22)
1 - w,

where
T-R
t = thrust deduction factor = - (6.23)
T
and
v -v,
w, = Taylor wake fraction = - (6.23a)
V
T = thrust and R = resistance
V = ship speed and V, = speed of advance.

An approximate value of qd, derived by Emerson is
-
NJL
qd =K-- (6.24)
10000

where N is propeller RPM and K is a constant, originally given as 0.83, but which
should probably be increased to 0.84 for single-screw ships with today’s more
efficient propellers. (See 57.5 for a discussion about this formula.)

(iii) Delivered horsepower Pd

This is the power at the propeller and is given by the formula

‘e(]>
Pd =- (6.25)
qd