Hi James321;
I also used to be annoyed with cooling a power source of RC tank.
For instance,in my case the tank was 1/10 scale(actually 67cm long)0.7mm steel made one weighing about 25kgs.
This was powered by OS 2 stroke 45 class nitro engine which was,OF COURSE,enclosed mostly by the steel made hull that
became toooo hot naturally under the violent sunshine in the mid-summer in Tokyo where it became normally around 35degreees C.
The engine was running in the very narrow and abnormally hot conditions.!!
I finally came to succeed in running the tank for continuous 30 minutes under such circumstances by water cooling system,
when the fuel tank had been exhausted completely.
The tank was equipped with a water tank of 400cc ,a simple handmade radiator with a primitive fan and a micro pump.
Instead,the brass water tank became hot with a touchable level like a bath water after the running.
I added a certain level of a coolant for car at that time.
I thought in those days about 30years ago that water cooling be so effective for severe circumstances such as the above.
A running tank in the pic below was taken in the winter.!
Even I once thought to challenge building the Azimut Atlantic Challenger.....
Wishing you a successful work of the superb boat.!!!