I saw the thread get a little technical below so I'm going to bring it back to basics. If you really know your stuff you can take your own chances but I tend to stick relatively close to the briefing instructions (and usually a binder that's provided with the boat).

1) your charter company will give a briefing and it should include how to use the genset on THAT PARTICULAR boat. With so many different boats they have to vary.
2) we've chartered 3 boats with gensets. A Moorings 54 mono 6 years ago and 2 different older 44' cats (Lagoon and Voyage) within the last year and ALL of them told us to NOT RUN the generator while underway. I don't remember why but I do remember the specific instruction.

The weather was so perfect it wouldn't have been needed underway anyway and the only time we needed to ramp it up fairly quickly was during a full day of rain when we were stuck inside with the hatches closed. Otherwise they would cool down in plenty of time for sleeping.

S/V Tortuga
Lagoon 46
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