two boats one 2004 42 fountain top speed normally is 84 with 525 with the heat and humidity flat water the boat ran 75 at 5100 rpm. the second boat 35 ex with 496's normally 78 to 80
same conditions the boat ran 72 mph.whats up with that?

I prop down in heat of summer.

high heat high humidity = less air
Cold air less humidity = more air denser


Now if a engine is a airpump what scenario will produce more power?


if I set my Idle at these temps. When the cool air comes back
it dont idle for SH@T.

Modern EFI with proper sensors will adjust the boost or
Mixture or both to compensate.

Rule Of Thumb. If It's Hard For You To Breath It's Hard For The Engine To Breath.

are we the only ones with this issue or are there more out there.

That seems like quite a bit of speed loss unless it was really humid. I usually only see a few mphs. How many rpms did you lose?

Yep hot moist air is less dense therefore less oxygen.

Colder air is more dense with more oxygen. Airplanes fly much better in cold air than in humid air as well. Your top end max will come in the early spring, and late fall timeframe.

I was thinking his first boat mentioned was excessive.
However up here in Mass. last few days have been 106 temp
with 98% humidity.

Kind of like standing in a glass of warm water

Means your on earth like all of us.

Air... Humidity... as they mentioned above.

Also the water is more than likely warmer making the mills run hotter and less efficient