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Join Date: Jun 2011
Location: West Vancouver B.C.
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Some of the comments on this thread linking props to engine operating temp. are a bit off IMHO. Engine operating temp should be regulated SOLELY by the thermostat - that's what it's there for.
A marine engine should be able to reach maximum recommended (operating) RPM, or near it, at hull speed. More specifically, it should reach its peak TORQUE rpm. Prop dimensions control THIS and should not control engine temp.
If you have a functioning thermostat of the correct temp (per the engine manufacturer) and are having cooling or heating problems, the solution is elsewhere. If you overheat you either have circulation problems of one sort or another, you have an inadequate heat exchanger (unlikely if it is OEM) or you are over-propped, in which case your max RPM should be too low.
Over cooling usually just means a thermostat problem (too low a setting or stuck open) or running the engine too slow (not an inability to run it faster).
Many people seem to think an engine is better off running cooler - 150 or even less - but this is not true. Take a look at contemporary cars - they run close to 220 before the fans even come on, racers run way over 200 as well - it gives you more power. Those temps don't leave much margin for problems though.
Engine oil needs to be hot to work properly so I like to see an engine run 180 - 190 in my boats.
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.