Hello Tim, long time no see. I didn't know you were involved with the Ellerman's vessels but I only made a flying visit for the sea trials on the "Plymouth". I remember you from the early days of the Appledore-built UMD dredgers "City Of London" and "City Of Westminster" and their 6MB275 main engines, although my Mirrlees colleague Paul Beetham was more deeply involved than I was.I was Ellerman's Project Engineer for that class of ship built at Appledore. Great ships - shame about the 58JS3s! I had wanted the engine to drive a CP propeller at constant speed but the boss wouldn't have it. I think it was killed by the yard wanting a £45k extra which is a shame as it would have transformed the engine by doing away with the starting assister and probably considerably reduced the piston ring and liner wear problems encountered in service. Interestingly we found that the dead band was not as big as the theory suggested and it was further improved on the second shop when we changed the phasing of the propeller with the crankshaft .
At the beginning of the contract we were badly deceived by British Shipbuilders and Doxfords who told us the 58JS3 was a slower speed(220 rpm) version of the "fully developed"Seahorse. Unfortunately that was totally untrue but it was too late before we found out the full truth. The first engine was over 6 months late and development had to continue with the ships in service .
Perhaps a TM410 or K Major wouldn't have been so bad after all !!
Developing engines at sea seems to be a constant theme, I spent a lot of time on Appledore's Rowbotham vessels "Echoman" and "Tankerman" trying to get them to run reliably on heavy fuel, I suspect Rockies were told they were "fully developed" as well.