Re: Powering a 12 ton boat; need advice on big DC motors
If you went with the outboard lower units, that would allow you to handle reverse in gearing, not in reversing the motor. The only issue I could see with that is the mechanical losses in the direction change before connecting to the prop.
How are you planning on connecting a prop to the motor? Long straight shaft similar to a mud motor or something more similar to an outboard lower unit? 12HP seems like a very small motor for a 12 ton boat but I know you are working with what you have.
I like your motor control idea using relays. I think I'd add a middle circuit of 36v for "cruising". I'm thinking once you are up to speed, you could drop down to 36v and maintain your speed but keep some heat out of the motor.
As far as circulating water, I would try a pipe pointed toward the bow as a pickup then pipe it where it needs, and dump out at the stern. Just trying to come up with a way to keep pumps, etc out of the mix if possible. I think when you are headed down the waterway, the force of water should be enough to push the water in the pipe up (depending on how far up is).