[nativepaul]

If you have coolant flowing internally through the motor your windage losses (or whatever you call them when its not wind turbulence creating drag but liquid) would be huge and the IO would rise astronmically water for example is around 840 times thicker than air, and oil the traditional electrical liquid coolant is even thicker and more draggy.

Most folks stopped using cooling coils in boats when we escaped from brushed motors that had holes in the cans, coils can work if wound with the right pressure to flatten the bottoms nicely and the gaps are filled with copious amounts of heat sink grease but a typical shop bought coil fitted dry is of very little use compared to a water jacket that has water in direct contact with the can. If the fins go across the motor like castles making a water jacket is a sinch slide an "O"ring between the first and second fins and another between the penultimate and last fins then slide a tube from the plumbing store over the "O"rings and there you have a jacket, glue in a couple of bits of tube for inlet and outlet and your done.
or you can get them ready made for Nue, Castle, Leopard and Feiago from offshore electrics.