If capsize is a serious possibility, waterproofing would be a great idea. However, between charge controller and motor controller, you will have significant heat dissapated by your electronics. I would presume a watertight case to also be airtight. So you might consider making your case out of aluminum and bolting the controllers directly to the case, maybe with a finned heat sink also bolted to the outside.
Are you sure you want to go with 12v? Sorry... I have a thing about 12v so I just have to ask. You should be using REALLY BIG wire for 12v, and motors rated well in excess of your projected power levels. Bigger motor, bigger wire in the windings, lower copper losses. A quarter volt or so of voltage loss is peanuts in a 48v or 36v system, but it is a big thing when you are only working with 12v to start with. Also line losses will be lower anyway, with a higher voltage system. And losses within the battery pack will be substantial at 12v. I think if you went with about 30ah at 36v you would see considerable performance increase. Or 20ah at 48v. As a matter of fact, I just did an endurance run in my boat on a 20ah 48v lithium ebike battery, just to see what it would do. I got I guess 6mi in a 6700lb 27' displacement sailboat hull at 250w (5a more or less) at 1.7kt or so, average speed. The pack gave up about 750wh before it quit. So a couple of 48v or 36v ebike batteries might make a good off the shelf power source for you. They usually ship with a charger and have a BMS built in. Light and compact. You could have a very nice metal case containing both controllers and a battery or two that would weigh total around 45lbs, I am guessing. You may as well make the case big enough to float, too. Pull-free connectors to the outside world, circuit breakers too, and a capsize or outright sinking would have that much of your system floating free of the wreck, for easy recovery. Trolling motors are of course capable of surviving a dunking if you are using an external controller.
Your proa is a sailboat? I don't want to assume anything here. And if so, do you use your prop a lot, or seldom? Just for docking? Or do you sometimes go mechanical when the wind dies? If a typical day's outing sees you using the prop for 5 or 10 minutes total, you can get away with an awful lot of inefficiency and maybe 12v would not be quite as unacceptable. If you spend a lot of time motoring, you should try to optimize your system a bit so you either dont have to buy as much battery, or at least get more range for the same amount.
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