I have a marine crate engine 435 hp 502, i would like to know what its the maximum rpm i should be running this engine before the hp starts to go down?
thanks in advance

Max horsepower will be between 4600-5000rpm.
The engine can safely handle about 5600rpm though....

So anything above 5000 rpms would be of no speed gain? im really confused when it comes to this.

Well probably not??? You should try some different props with your engine max rpm in the 4600-5000 rpm range.
I tried a number of props on my boat last Summer.
My boat went faster with a 26p spinning slower than a 24p faster. I am assuming that the engine went past it's peak power with the 24p.

I will try different props in the next few weeks, hope i gain some mph. Thanks for the info Pat.

i found that the best rpm spped came at propd for 5150 only spent about 4k on props to get to that #

Do You have a 24' Pantera?
Talk to Jo at Pantera.
He'll likely tell you to start with a 25p Mirage plus or a 26p Bravo one.
You should run about 73mph in fresh water and about 75mph in salt.
Good Luck!