Page 388 - Introduction to Marine Engineering

358 Appendix

propel a ship of a known displacement at some speed can be
approximately determined using the Admiralty coefficient method.
The total resistance of a ship, R t can be expressed as follows;
Total resistance R t = pS V"
3
where p is density (kg/m )
2
S is wetted surface area (m )
V is speed (knots)
2
now Wetted surface area <* (Length)
3
Displacement, A « (Length)
thus Wetted surface area x (Displacement, A) 2/3
Most merchant ships will be slow or medium speed and the index 'n'
may therefore be taken as 2. The density, p, is considered as a constant
term since all ships will be in sea water.
2/3
Total resistance, R t = 4 V 2
Propeller power oc R t x V
OC ^2/3y2 y

or Constant =
p

This constant is known as the 'Admiralty coefficient'.

Example

A ship of 15000 tonnes displacement has a speed of 14 knots. If the
Admiralty Coefficient is 410, calculate the power developed by the
machinery.

Admiralty coefficient =
P

Power developed, P =
c
2/3
= (15000) (14) 3
410
= 4070 kW

Fuel estimation

The fuel consumption of an engine depends upon the power developed.
The power estimation method described previously can therefore be