Hello!
Is it possible that my 2009 heat 496 HO runs faster in the cool spring then in the hot summer?
The difference is 2 - 3 miles.
Thanks.

It is possible......but if your in an area that has "summer" gas with ethanol but uses the regular stuff in the winter then the ethanol fuel is more likely the culprit (ethanol fuel makes less HP than non-ethanol).

Cool air is more dense, more o2, more hp.

Hello Casper,
I remember seeing your EU certified Heat. Nice boat.
Short answer, unfortunately yes.
Longer answer, the climatic effects on horsepower are:
Atmospheric pressure, mostly determined by altitude, but also by high and low pressure fronts (higher pressure is better, lower altitude better). I think you have the low thing covered in Holland ....High pressure acts like a mild form of supercharging.
Temperature, since air expands as it increases in temp, it disperses oxygen molecues. There are less per square foot of air. Conversely cold air is denser and therefore has more oxygen molecules per sq'.
Humidity, humid air has more water molecules, which displace some oxygen molecules and therefore makes less power.
So ideally, a cold, dry, high pressure, sea level environment would be perfect for making horsepower.
Since we usually use our boats in the summer (hot/humid), you can see why we lose horsepower and speed/rpm. If it's drastic (your's is not) you can get back on the RPM powerband by using a lower pitch propeller, as some of our mountain friends do.
Or, you can install a supercharger on your motor and make your own high pressure ...............
Regards,
Steve

To expand on what Steve was saying. I dyno test engines at my shop. During the summer it is quite common to see a 10% power loss due to weather conditions. On a 400 HP engine that is 40 HP which is about 3 mph on a Heat. In fact it is possible to see greater variations than that. Factory HP ratings are set using an standardized correction factor that assumes the engine is being operated at sea level, zero humidity, 66 degrees air temp and 29.92 barometric pressure. So, if you test the boat on a cool crisp spring day it's possible that it is developing more than advertised HP. If you run the boat at higher altitude you will lose a lot more power. It is common to see a 10 mph speed loss when boating at high(+5000' elevation) altitude.

I only use my boat on Lake Garda in Italy, i found the following information about the lake;

Statistics about Lake Garda
Area: 370 sq km (230 sq miles)
Maximum depth: 364 m (1,194 ft)
Altitude: the shores of Lake Garda are 65 m(213 ft) above sea level

When i reveived the boat it had a 26P prop, i allready changed it to 24P be cause the engine had to less rpm with the 26P.

I could barely get my 496 28' Heat to 62mph on a hot/humid summer day at sea level, but could get it to 67 on a cool fall day.