Like the petrol-powered Sabre aero engine, the Napier Deltic was a remarkably high power-to-weight ratio engine with highly original features and layout to work in mainline express rail locomotives, minesweepers, and high output fire pumps.
A valveless 2-stroke diesel engine with pistons in opposed cylinders, the 3000+ horsepower arrangement found within the Class 55 locomotive with a total of 36 cylinders and 72 pistons across 6 crankshafts is remarkable – see more information in this video:
Within the marine arena, the Deltic marine diesel engines were historically used in Royal Navy minesweepers like HMS Ledbury, these were replaced by CAT C32 4 stroke diesel engine, which, to be fair, being a conventional 32 litre V12 4-stroke engine has likely been a lot more reliable!
Historically a Deltic engine was installed in a captured German E-Boat to replace the Mercedes Benz diesel unit and provided much more power at a fifth of the weight.
A technical marvel, the Deltic marine diesel engine also saw service in the US Navy during the Vietnam conflict, but these days is only found in heritage railway Class 55 examples.