Looking at a Powerquest 280. It has a Precision Marine motor that has 540 hp at 5100 rpm. Boat comes with a 5-blade Hydromotive prop. Owner says he can turn 5500 rpm on a good day. He needs more prop correct? To get into the 5100 rpm sweet spot?

when he say's, on a good day, how is the boat loaded? trimed? you should run the boat with your load, it may only run 5100 rpm, your only 400 rpm off, if you go with a larger prop, it will affect the bottom end, (getting on plane) I wouldn't be to quick to change the prop.
Larry

The boat is in New Orleans, LA. It's pretty hot and humid there. In the summer, it turns right at 5100rpm with a top speed of 76 with 2 adults. In the fall, when it's in the 50s, he says it'll turn almost 5500rpm.

What I'm concerned about is that I plan to run the boat in Kentucky where the the air temps are a bit cooler than Louisiana. But then again I'm higher (about 700 ft above sea level) if that makes any difference.

How would you correct for a 400rmp difference?

As noted in your other post, its a custom engine. It could easily be designed to run 5500rpms at WOT. I ran 5400rpms all the time in a 454 mag with a cam upgrade. 5500rpms is no big deal with good internals.

I'm with griff, 540 should have good internals, it should take the rpm with no problem, go to kentucky and run it, It may work well.
Larry

It's not a 540 as in CI. It's still a 502 with 540hp. From what he's telling me, it still has all stock internals. Just the heads, cam, intake, exhaust, and ECM have been modded.

In any event 400 rpm won't be a deal breaker.

Aftermarket Cam would be considered a internal mod..

take it to kentucky and run it, yes 700 ft will make a difference.
and that bottom end should handle 5500 just fine. but I think you will find it won't turn that when you get it home.