The relatively low technology readiness level of onboard storage (3) and propulsion (3)* make AMMONIA the least advanced sustainable alternative fuel. There are no vessels yet in service or on order with ammonia as a main fuel engine type but some press releases indicate that ammonia-ready vessels are being ordered. These vessels are built in a way that once the fuel is available, the vessels can be retrofitted relatively easy.
According to the Clarkson’s database there are currently no vessels on order using ammonia as a fuel but a few shipowners announced that they are ordering "ammonia-ready" vessels. Even though there are no ammonia-ready vessels yet in the orderbook, both MAN ES and Wärtsilä are developing ammonia engines. But both engines are still in the test phase. As with the other hydrogen-based fuels, fuels cells are being looked into as well.