Paul Schilter wrote:

Since external water used in cooling engines causes a lot of problems, why
hasn't a car type radiator system with fan ever been used?
Paul




Probably because it is difficult to get enough airflow to a radiator.
There's also the problem of the exhaust. Dry stacks are noisy, smelly,
and throw off a lot of heat in the boat's interior.

But there are closed cooling systems...look up "keel cooling."