What is the cube rule for Marine slow speed engine fuel consumption?

Answer 1

In operation, the ship’s fuel consumption depends on its hull condition and the speed at which it is operated. When a ship is designed, naval architects optimize the hull and power plant to a prescribed design speed which may be, for example, 15 knots for a bulk carrier or 18 knots for a small container ship. Operation of the vessel at lower speeds results in fuel savings because of the reduced water resistance, which, according to the ‘cube rule’, will be approximately proportional to the cube of the proportional reduction in speed:

F = F*(S/S*)^a

where

F

is the actual fuel consumption (tons/day),

S

the actual speed,

F*

the design fuel consumption, and

S*

the design speed. The exponent

a

has a value of about 3 for diesel engines and about 2 for steam turbines. It follows from the cube rule that the level of fuel consumption is very sensitive to speed.