Actually, something some do with boat motors to break them in is to run them on the bench without cooling, other than an oversized aircraft prop, and use the needle to actually regulate the engine's temperature. By running the engine rich, you actually cool the engine. As you continue to break it in, you lean it out a click or two every tank or two until it's close to where you want to run it. At that point, they install it in the boat and head to the pond. One thing to be aware of, however, is that a boat engine has to be set richer to get it to run properly. We run smaller props with a much higher pitch and at a much higher RPM so the load is greater than it would be with a comparable aircraft engine. Just for example, I have an OS 25FP that turns around 17K with a 9X6 prop. In one of my boats, I have a Nova Rossi 21 that turns up to 35K with a 40X56mm prop while in another, it's a CMB 67 running at up to 28K while spinning a 60X90