One more thing, if you're going to an aluminum prop,(they're easier to spin) IMO, you might need a 21 pitch prop. Or you might overrev! Judging by my old 20' SeaRay bowrider 5.0 with a 19" pitch Black Max, I could get close to 5000rpm with that, and you have a much more powerful later model 5.7
I just don't want you to have go thru a prop exchange?
???? An aluminium prop of x pitch will create the same load as a stainless steel prop of the same pitch.... Your 5 litre will have had a different drive ratio, so you are not comparing apples with apples....
How does an aluminium prop spin easier? Again, if the props are the same style, pitch and diameter, they will spin the same..
Mik, you have next to no knowledge in this area (and that's obvious by the posts) and are sending threads in the wrong direction. With a 19" prop it only makes 4100.. Putting a 21" on will REDUCE that... The only way to get up in the 'over-rev' range would be to put a 15" on...
OP... Just catching up on the thread. How well does the boat accelerate? If it's slow out of the hole and has difficulty maintaining plane, then the prop is too big. Se if you can borrow a prop of a smaller pitch and see how that goes. All this assumes the engine is in good working order and proper tune. Have you at least checked the compressions?
Chris......