Sorry…I forgot to quote the rule-of-thumb caveat. “A motor that’s correctly loaded shouldn’t get hot”.
I would follow that by saying at its maximum rated power any motor is likeLy to get warm or even very warm of course.
Ventilation in a boat can also be overlooked. A closed box is basically an oven, and if there’s no ventilation, a moderately warm motor will soon heat up the inside of the boat and it will get warm, making the motor even warmer and so on.
Interestingly my sub, which uses a low power 540 on only a 7.2v stick, gets REALLY warm inside despite using no more than about 1/3 throttle most of the time, and the motor gets into the very warm” category.
The 385’s on say my Devastation, on 7.2vor 8.4v driving three blade brass 40mm props rarely get even just warm, as although undoubtably overloaded, just don’t pull the current to heat things up. Great little motors.
Computer fans that run off a usb outlet (so that’s 5v, same as a spare channel on your receiver) are cheap and move air about nicely.
Ashley