Page 197 - Practical Ship Design

Powering I I63

(6.13)

or as
1000. R, . v,
P,(kW) = 0.7355 . (6.14)
75
in which

R,=C; 1/2.p.S.V:
Combining these gives

1 0.7355 x 1000 x (0.5 144)3
P,(kW)=C, .-.p.S.V; x
2 75~9.81

which reduces in salt water to
P,(kW) = 0.0697 C, .S .V; (6.15)

In the above equations (!I is one of R.E. Froude’s “circular” notations which are
discussed in 86.3 (xi), and C, has already been defined in $6.2.
The denominator in eq. (6.13) allows for A in tonnes and P, in metric horse-
power before the conversion to kilowatts.
In traditional British units of tons and imperial horsepower the denominator
used to be 427.1.
A = displacement in tonnes (s.w.)
V, = speed in international knots
V, = speed in m/s
S = wetted surface in m3
r =density = 1.025 tonnes/m3 for salt water
= 1 .OOO tonnes/m3 for fresh water
p = mass density = r/g
1 metric tonne = 0.984 British tons of 2240 lbs
I metric horsepower = 75 kg mass x metres per second
= 1000 Newton metres per second
= 0.7355 kilowatts
= 0.986 British horsepower
1 British horsepower = 550 lbs mass x feet per second
= 0.746 kilowatts
The figures normally used as the equivalent of speeds in knots are:
International 1852 m/h; 30.867 m/min; 0.5 144 m/s
Imperial 6080 far; 101.33 ft/min; 1.69 ft/s