Return of the Opposed Piston Engine


Up to that time (early '50s). Today its efficiency (~40%) is far exceeded by the low-speed diesels from Wartsila, Wartsila-Sulzer, and MAN (>50%)


I think we are talking about different things here. Weight vs. HP is different from Fuel efficiency.
The Napier Deltic was definitely not fuel efficient, but on fast crafts weight is king.