Wow, there is a blast from the past! Doxfords at one point powered a huge percentage of all ships at sea.
My only experience of Doxfords was a 3 cylinder beast on a msall container ship called the City of Manchester (ex Ellerman's), I spent a month staring at this thing wondering how the whole thing managed not only to keep running, but to stay in one piece.
As Traddles says they have two pistons which means that the forces usually absorbed by the cylinder head are actually transferred to the crankshaft, making them very fuel efficient, but mechanically very complicated. Unfortunately the company did not put the money into research and development that others did and eventually the popularity fell away and the company went out of business as other manufacturers produced better and better engines. Their hayday was the J-type which is shown in the photo in Traddles post.