Britain had just converted its entire fleet from the unsteady power of wind to the constant force provided by coal. Now, Churchill declared, Britain had to transform its navy a second time. Burning a pound of fuel oil produces about twice as much energy as burning a pound of coal. An oil-fueled ship could thus travel roughly twice as far as a coal-fueled ship of similar size. Oil's greater energy density meant that it, rather than coal, was the fossil fuel of choice.