Thanks for the info, most of it over my head (just a plumber, you see). Can't find wattage specs on the drill motor. And the available room in my hull won't allow a heatsink, however I can fabricate a water jacket to cool the motor if I find it necessary.
The motor is out of an 18v cordless drill, and I have used them to drill 1" holes through 4x4 lumber and the motor never gets hot, a little warm maybe, but never blistering hot. I'm hoping that by asking it to push 8 lbs of batteries and hull, that it won't be reaching it's limits.
Thanks again for your response, and if I can find the wattage, I'll use your formula to figure it out. Meantime, i'll be launching tonight, hopefully to the tune of 'No Magic Smoke from the Boat Tonight",