Salt water in oil?

[dmksails]

I found some grey foaming stuff in the gearbox on my Yanmar 3GM30F. I also found some of the same grey foam on the underside of the oil fill cap.

The oil on the dipstick - looks just fine. A pressure test of the cooling system did not reveal any problems.

The mechanic I had look at the engine suggests that it is condensation.

How can I determine is this is fresh or salt water?

[Ded reckoner]

A chemical analysis could detect the presence of sodium (i.e. for sodium chloride in seawater). If you did the analysis on clean transmission fluid and clean oil, then you could have a baseline. Such analyses are available and done not uncommonly on engine oil to gauge wear.

Are you using heat exchangers for your oil, transmission, engine raw water? Certainly, the latter. If so, how old are they and how is their condition?

[FPNC]

It could very well be condensation.

A lot of sail boat engines are not run often enough. You should run that engine hard every couple of weeks and get the temp up hot enough and long enough to evaporate condensation in the block. Operation for only short periods just gets steam up into the top of the engine.

This is particularly important when you live in a moist, and humid environment with wide temperature changes.