Alternatively, pick a sensible master voltage for the boat, and use step-up/down converters to provide auxiliaryvoltages for other purposes? They provide efficiency in the 90% range, and power of about 5A for a couple of quid. I have been doing this to get radar dishes to rotate at precisely the speeds I want....
if your intending to use a car ESC have a good read before you buy some of them require you to select reverse then go back to neutral before selecting reverse again to actually reverse the motor. The first reverse acts as a brake. Just to satisfy my curiosity why did you want brushed rather than brushless? There are a lot of brushless motors used in the model car sport and generally have advantages over brushed motors.
"that's not a bug its just an undocumented creature."
Sir Terence David John "Terry" Pratchett, OBE (28 April 1948 - 12 March 2015)
Hi Dick, I have a few tugs and the largest at 49" uses a geared 540 running from a 12v 7ah sla battery giving a good 90mins running time. This model weighs in at 15 kilos ready to sail. I also have a caldercraftmodel tug Joffre. Which is about 30" long and runs using a 6v monoperm motor and gearbox. 6v 4.5ah sla battery giving about 1 hour run time. Weight is 5kilos ready to sail. Hope this helps. Cheers Colin.
Dick, as far as I know model boat/car style ESCs have a reverse function. I would think a geared 540 motor would be quite sufficient. People tend to overpower most model boats, especially those not required to take off like ducks on acid! Like tugs.