With the motor runny while motor sailing? First, to get your alternator to "put-out" adequate amps, you'll need need to do more than just 1800 rpm if you want to charge the batteries. (It depends on your alternator, it's voltage regulator, time you want to leave the engine running, and your "need" for recharging.) You probably aren't doing any harm by running the engine at low rpms, but keep in mind that fuel usage relates to load, not only to RPM. I believe, if you're motor sailing and the prop isn't loading-up, you're difference in fuel use is pretty low by simply running (say) 1800 rpm. The approach I'd use, just talking about it: i) if you were sailing alone and your speed was, say, 5 knots. Then engaging the engine at an RPM that didn't move the speed up very much at all really is doing not much more to fuel consumption than idling the engine. In very low speed conditions, then the engine needs to get up to 1600-1800 RPM, or your just aren't putting-out many AMPS. IF you're charging along at near "hull speed", then you'll not have to bring the engine up to 2300 RPM to get the alternator turning. You transmission should be cooled because you're engine/transmission cooling system should be doing it's job with the engine on.