Why is it that two boat just alike,same drives, gear, props, One with 500hp and the other with 800hp turn the same rpm. Run different top speeds.

They can't. Unless a major hull difference. In theory a given rpm with exact ratios and props is identical speed. Speed increase from hp comes from the ability to spin a larger prop.

i may get in trouble for telling you this, but actually it's magic that causes the speed difference. i recommend trailering your boat to Louisiana and having a vodoo princes polish your prop for more top and AND a better hole shot... i usually give her three live chickens and an eagle claw for doing this to a pair of my props, but for the love of god do not give her any purdue chickens or she will put a hex on your junk, ... good luck, rm

If the 500hp boat is over-trimming to get max. RPM and causing excessive slip % and the 800hp boat keeps drive tucked in- since the additional power lets it pull the max. RPM without over-trimming; the 800hp could easily result in more speed.

Yep, 5000 rpm with 1.5 Drives and xx Prop means the same speed (If the motor can turn the prop to 5K) Now say they are 19" props, 250 hp will get you the same speed as 1250 hp. HP only shines when you can turn a more aggressive Prop or a Lower Gear ratio to that RPM. Otherwise Everything is Linear....

someones gears are diff and I bet its the 800hp boat

Not possible if both boats are being driven correctly unless there is something substantially wrong with the hull on the 800hp boat.

My $$ is on the 800hp having different ratio outdrive gears as well.