The cube law is a rule of thumb that indicates that the power required to propel a ship is proportional to the
ship speed cubed. It is more accurate for ship speeds of about twenty knots or less, but nevertheless, it
provides a useful indication of the high cost, in terms of power and therefore of fuel consumption, of high ship
speeds. For example, if a merchant ship were to serve a trade at 19 knots instead of 15 knots, it would
require a power plant about twice as powerful, and would consume twice as much fuel. As another example,
the warship capable of a maximum speed of 30 knots at full power would require only an eighth of its power
to cruise at 15 knots.