It's normal. Sucks, but it's normal.

"physics" is what's up with that

Boat is propped for the cool weather. When the temp & humidity go up the same prop will make a big difference since you are now down on H.P. & will be down on speed since now the prop is too big for the HP killing even more speed. If you were to prop down to get max rpm, you will gain some of the speed that you lost. You gave the RPM of the engine with the slower speed, what was RPM on the faster runs?

rpm 5100

Something does not compute. If you ran 84 @ 5100 rpm & you ran 75 @ 5100 rpm either you have messed up a prop, boat has gained a lot more weight, tach is wrong, or ??? Same prop, same boat, same weight the RPM's should be down on the 75 mph run compared to the 84 mph run.

I'm down about the same #'s you say.

I put most of the blame on humidity.


One thing I like about the fall.......those #'s will come back

I pulley down in the heat of summer.
Meaning, I add boost to compensate...

does this condition show up more on carbs than efi.i have seemed to loose some top end on my 454 mag carb but my buddies efi seems to run the same.

Exactly MM. When we were drag racing (cars) in super stock, carbs had to be rejetted not only daily but possibly multiple times in one day to keep up with the changing weather. You must have a weather station at your trailer now or your pizzing in the wind. If we were running a race that went from daytime heat on into the night and the heat and humdity dropped significantly, we had better fatten the old girl up a bit or....

I thought the speed losses listed above seemed a little excessive but, depends on what the weather was when the baseline runs were taken. Might have been the 180 degree opposite extreme (60 degrees and 25% humidity).