Ships' engines are amazing, but heavy fuel oil is really really gross stuff. It's basically all the bits of the barrel that are left after distillation. Ie wh

Ships' engines are amazing, but heavy fuel oil is really really gross stuff. It's basically all the bits of the barrel that are left after distillation. Ie what they've dug out of the ground but can't sell off for more money as aviation fuel, gasoline, diesel etc. Therefore it has really high sulphur content and very long/complex hydrocarbon chains that burn much less efficiently than gasoline or diesel.