HomeCalculatorsSpeed–Power Cubic Fit

Speed–Power Cubic Fit

Project delivered shaft power at any speed from a single reference speed–power point using the Rayleigh cube law, corrected with an adjustable exponent (typically 2.8 – 3.3).

VoyageSpeed–power

Reference point

Reference speedknUsually the design service speed or a recent sea-trial speed.Reference delivered powerkW

Target point

Target speedknSpeed exponent n–Cube law n = 3. Tankers 2.8 – 3.0, container ships 3.1 – 3.3 depending on hull form and resistance regime.

Power at target speed

5,120kW

Fuel ratio

0.512× ref

Saves 48.8 % vs ref at same hours.

Formula, assumptions, and limits

The Rayleigh cube law assumes wave-making resistance scales with V³. Real ships deviate: at low Froude the exponent drops toward 2.5, at high Froude it rises to 4+. Fit n from two sea-trial speeds, or use the type-specific defaults above.