Interesting question. I was having a discussion on this yesterday. Air temp is easy to figure using standard dyno correction formulas you can determine pretty accurately the power loss between a 60 degree and 80 degree day. You also need to know barometric pressure and humidity on these same days. You're probably looking at 2-3% power loss from temperature alone(60 to 80 degrees). I'm curios to see what other's thoughts are on water temperature. I think it has a bigger impact than most realize. I've been building engines and testing boats for many years. I always test at the same stretch of water. Almost without exception boats run faster in the spring than in the fall, even though the weather conditions are virtually identical. The big difference is the water temperature is around 60 degrees in the spring and 85 degrees in the fall. Obviously an intercooled engine would benefit tremendously from cool water. But a naturally aspirated engine running a thermostat should not be affected by water temperature. My thoughts are that the cooler water is more dense resulting slightly less prop slip and a little better hull lift. Kind of hard to really document though.
Bob Lloyd
Full Throttle Marine
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