Work out fuel consumption at sea instantly with clear inputs, formula shown and shareable results.
Ship fuel consumption follows an approximately cubic law with speed, because resistance rises with the square of speed and power is resistance times speed. Dropping from 14 to 12 knots cuts daily burn by roughly 37%, which is the arithmetic behind slow steaming.
Cubic fuel law
burn = reference burn x (actual speed / reference speed)^3
Marine results are planning estimates only. Verify against the vessel's approved stability booklet, loading manual, class society rules and applicable maritime regulations before loading or sailing.
It is a good approximation in the displacement regime. Near hull speed or in heavy weather the exponent rises above three.
Fuel falls sharply but the voyage takes longer, adding charter hire, crew and cargo financing cost, so there is an economic optimum speed.