Although cruise ships are already capable of being powered by alternative fuels like biomethane/RNG, there remain a number of barriers to overcome before we will actually see them in use. The main problem is the low quantity of biomethane available, which does not reach the amounts that these giants of the sea require. Just one cruise ship requires around 30,000 tonnes of biofuel a year, a quantity that far exceeds the 3 or 4 tonnes currently in daily production.