2 stroke and 4 stroke outboards can work at idle without any problem.
Good. I was worried it might cause uneven wear, or overheating, or such.
The alternator will not produce its rated output (18 A in your case) at idle.
That's what I suspected. I wonder if anyone knows how much a drop there would be. I.e. are we talking ~15A at idle or something like ~8A at idle?
If the boat is in a barn now why not charge your batteries with 110V source?
I'm only thinking about the future, on the hook. Right night the AGM house battery bank is fully charged and sitting out the winter in the boat. The starting battery is at home in a warm place connected to an intelligent trickle/conditioning charger.
If the boat is in water you can run the engine at idle. If you need higher revolutions you can do so with the gear in forward or reverse.
Well, forward would make for some exciting fun when at anchor ;-)
But I guess reverse would work OK, and it certainly couldn't hurt the set of the anchor...
Would it then be bad to run it in neutral at a higher speed than idle? I.e. why have it in gear at all? Does it require a load at higher RPMs then at idle?