Generally the old trans comes out with the dead diesel, but maybe you can figure out how to re-use it if the reduction ratio is about what you need. I used an enclosed gearbox that bolts directly to the motor and to the mount plate, with a stub shaft that is coupled to the prop shaft. Had to use a journal bearing as a bushing to make up the difference in diameter. Output shaft is 7/8" and prop shaft is 1".
You will be pulling a lot of old wires and hoses, too. And your fuel tank. You will need to build a motor mount too.
You will need an appropriate controller for the motor. This is a little magic box that sends current in the right pulses at the right timing to the three phases in the motor windings. The throttle can be a fancy pants overthought sort of thing or it can be as simple as a 10k potentiometer. You may need a reversing switch depending on the controller firmware. I don't. My throttle is bidirectional.
There are quite a few 5kw motors that would work fine for your boat, I think. No need to go bigger, An ME0907 would work fine. Or what I used, a ME0201014201. These are brushless DC motors which are actually permnent magnet AC 3 phase motors. You dont want a brushed DC motor if you will have propane or gasoline accessories aboard, of course.
I would recommend going with a 48v system. Line losses are terrible with a low voltage setup. 24v you might use for a canoe or something. You want 48v. Cheap way to go for a fair size battery bank is GC-2 6v golf cart batteries, 8 in series. They are cheap at Sams club. For about $700 you get a pretty decent bank that will give you some nice range in such a small boat. Yeah GC-2 are about 85lbs each, but remember you are getting rid of a bunch of iron and a fuel tank so the weight hit isn't too bad. Also you are no longer going to need your old 12v bank so more weight saved.
You need an appropriate charger, of course. And a dc/dc converter and I suggest two, so you have redundancy. This will efficiently convert your 48v power to 12v for your nav lights and stuff. And an inverter. With that much juice stored, you might want to run very small household appliances or electronics occasionally.
$2k would be pretty hard to do. Maybe with a motor and controller salvaged from a forklift or golf cart you can save a couple hundred. Downsizing the bank to 4 group 29 12v will save you a little bit. But figuring on staying under $3k instead of under $2k is more realistic and you will make fewer compromises.
Lots of good info already posted on this board, so do your homework and soak up some background knowledge.
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