Germans had the idea of making a smaller version for long range bombers in WWII but didn't have enough time for development, After the war Napier's took over the development, with a lot of R&D and one of the major changes was giving the exhausting piston a lead of 17 deg over its opposing piston (amongst other tweaks) it became the Deltic.
Having spent the last 12 years of a 23 year stint in the RN in the engineering department on Ton Class mine sweepers/hunters Bronnington being one of them. (HMS Bronnington was brought into the lime light when Prince Charles was made the skipper) I joined some 10 years later.
We had 2 18/7a as main engines on both Sweepers and Hunters, free end of the engine was a massive blower driven from the phasing gear case on the drive end, the phasing gear case (as seen in the Bronnington photos) took the drives from all 3 crankshafts through this gear case then a final drive through an hydraulic clutch oil pressure being supplied from the engines own oil supply for ahead and astern manoeuvres, then to the prop shaft. On the Sweepers a 9 cylinder engine was installed for pulse generation in 3 phase for sweeping mines.
All boats where double skin African Mahogany on aluminium frames.
The whole boat had a very low magnetic signature.
A pair of 18/7a flat at 1478 max revs makes a hell of a racket, these engines only worked well flat out. Slower speeds caused small amounts of engine oil to be carried over in the exhaust gasses settling in the base of the funnel. After hours of slow running and towing a quick increase in speed could see the carry over of hot carbon which could cause a funnel fire, I never heard of one taking place, a slow but steady increase in engine/shaft revs would blow all that muck up and out of the funnel, at night this may cause a fire works display akin to a Roman candle. British rail also had the same problem with oil carry over but don't know of their fix. These engines where 2 stroke with 2 injectors per cylinder and started via a large cartridge (as for the old radial engine fixed/rotary wig aircraft).
Fuel consumption was 1 UK gallon a min flat out for each engine.
The engines where also installed in PT boats in the US.
Anybody interested google HMS Bronnington where she lays in a hell of a mess in Liverpool docks in a once caring maritime museum, it breaks my heart