I know that the conventional wisdom is no stainless below the waterline. Our steel boat has had stainless prop shaft and rudder
pintles for 25 yrs no problems.
A few years ago I had a machine shop turn up a thick walled stainless stub threaded to take a stainless seacock. This stub is welded to the hull
I have expected to see pitting, and inspect it at each slipping, but so far no problems with stub or seacock. Must be around five years now.
outlet is done differently. Four stainless bolts through the hull and with their heads welded. A marelon (sp?)flanged stub is retained against the inside of the hull by a suitably drilled stainless plate which is held down by the four studs. There is a gasket
between the flat head
of the stub and the hull. There is also a gasket
between the plate and the hull. I have checked it once in 25 years and it was as good as the day it was installed.
These are the only two through hulls in the boat. The cockpit
drains above waterline, and the sink runs into the cockpit
drains. The engine
and toilet share a seacock with a valve at the toilet so that the engine
cannot suck air back on a long run.