A tractor motor
would be a better option. Designed for more torque at lower speeds--but any diesel engine can be converted for marine
You will need, water-cooled exhaust
piping, or a saltwater cooled heat exchanger
cooling system, can be in-sump or separate. Can be air or water
seawater through heat exchanger
water- cooled exhaust manifold and exiting through exhaust elbow
with nozzle baffles to prevent ejected warm sea water
getting back to the cylinders when engine is idle..
This water also cools the exhaust stream, making it possible to use special plastic and rubber exhaust components.
Plastic or metal exhaust silencer and water trap.
Plastic riser pipe with siphpn breaker valve to stop seawater surging back to the engine through the silencer, because engine exhaust outlets are at waterline or just above and are frequently submerged when engine is not running.
Of course this can be simplified by using a dry exhaust system--in which you need two concentric pipes and a fan to assist in oil
cooling and blowing the hot air out of the outer pipe. Most commercial
trawlers in Australia
use directly keel
cooled diesel engines and dry exhausts. Just be sure not to take the ground on the heel cooling pipes. Think in terms of them being part of the keel itself, or being shielded by lying close to the keel.