I understood continuous duty to mean not only an ability of the engine to
run reliably for extended periods at full load but also that all the
metalwork, fittings etc are of a superior standard. Hence the cost.

I may be wrong here but I seem to recall that running engines off load or on
reduced loads for extended periods is not good for them. Leads to glazed
barrels or something like that???.

Referring to air cooled, I rather like the smell of engine rooms but I can
see on a boat you would have to build in a heat exchanger to isolate the
cabin from the "raw" heated air, I just think that a heat exchanger for air
is easier to build that one for a sea/fresh water combination. Also no holes
in the hull. However on the down side, don't water cooled engines run
quieter than air cooled. So many points to consider!.

Having said all of the above however I believe that the engineering required
is beyond me and by the time I gather the experience and tools to do the
work too much time will have gone by!. I will just have to save a bit harder
and buy a marine engine.

Steve