Bob mentioned this earlier. I'll paraphrase in my own words.
For you "putterers" who enjoy long lazy days in protected waters at hull speed
while enjoying the local scenery:
Extended running at low rpms is NG (read not good) for your engine.*
If I enjoyed such, I would opt for having an undersized engine that I could run
at operating speed (well above idle but somewhere below 80% of max rpm).
This could be the "kicker" on single main engine vessels. NG for a twin engine
guy.
On twin engine vessels, it's more of a problem unless you can alternate engines
(one in, one out) with the "in" engine running above 3,000 rpm. This is not a
great solution and may cause other problems since the working engine will be
over propped. It all goes back to the bold face above.
And, there goes all the fuel savings many seem so concerned about.
Aye
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*2014 Honda Owner's Manual BF90, Pg. 80
"Running the engine below 3,000 rpm for more
than 30% of the time so the engine does not
warm up."
Results in
"Water condenses in the engine and mixes
with the oil, resulting in a milky appearance."
and
"The engine oil deteriorates, becomes less
efficient as a lubricant, and causes engine
deterioration."