I became involved with jet boats a long time ago, and I have seen some really goofy setups guys have made for their cooling systems. I have seen engines where the frost plugs got pushed out of the block because they had so much water pressure. I had a customer at one time , and he had this fluctuating water temperature, didn't run a thermostat, had all kinds of manual valves for this and that, and the only way it would even begin to remain somewhat constant, was when it was wide open. It was a race boat, but still this was ridiculous. I thought about how to make this somewhat simpler and less hands-on, and more automatic. I was told thermostats don't work. I thought, they work in everything else, why wouldn't they work in a boat engine. So, first of all, I installed a pressure gauge, and went for a ride. At speed, we had 120 P.S.I. No wonder the thermostat wouldn't work. I drilled the back of the intake manifold, and put two 1/8" lines out the back to remove any possible air pockets from the block, (like an outboard). I then added a bypass line to the line coming straight from the pump that just dumped straight overboard. This reduced the block pressure to around 40 P.S.I at speed. I added a second bypass line with a ball valve to bring the water pressure down to 15- 20 P.S.I. (with the thermostat closed). There is also a third line that goes to the headers which is also regulated. There is a single line that comes out of the thermostat housing (-8) which goes to the oil cooler and then dumps overboard. This has worked perfectly for me. I have done it many times, and it always works the same. There are no wild temperature fluctuations, the oil warms up quickly, no water in the engine, and on step, it maintains the temperature which the thermostat is calibrated for. It has never given a problem. There is an unbelievable amount of water available from the pump. When I first put it together, I didn't know if it would work, but it does. One thing to remember, is to put a pressure gauge on, just to monitor what is happening. Once you know, you can take it off. The key is to keep the block pressure where the thermostat can do it's job. The water pressure will drop when the thermostat opens, but it will come back as soon as it closes again. I have never had an overheating situation, or running too cold. Just like a car. There is not so much pressure at idle (around 5 P.S.I), but it doesn't matter, the engine stays the same temperature.
