I finally retired the Otter's engine a few years ago after 20 years of steaming; the main bearings and seals were worn to the point of leaking and knocking. Considering the compressor cost about $15 at a scrap yard in the 80s, I felt I certainly got my money's worth
. Some things I learned along the years:
* Buy some good cast iron & bronze (or cast iron again) for the slide valve; this gets some serious abuse in marine engines - especially when throttling down after a hard run - the superheat from the throttling makes things squeak.
* Avoid superheat with slide valves. My first boiler (a Len Williams special) had quite a bit of superheater surface area, and I ended up resurfacing the slide valve every other year or so.
* If you use the existing piston(s), run mostly water w/ a couple of table spoons of thick heavy oil (600W steam oil) in the crankcase. If you're condensing, I hear Mobile 1 140W gear oil works well and is much easier to remove from the hotwell.
* Use a feed water heater in the exhaust - there's a lot of waste energy there on a small single.
The Otter now has a engine from 1895 or so; I had to make a new spool valve, valve rod, piston, piston rod and misc. valve gear parts. It's about 40% larger displacement and with the switch from 13x17 to 16x20 prop, the Otter is quite lively.