I just bought a 2000 24' Stepbottom Superboat with a 225 mercury promax just rebuilt new pistons and cranks and new lower unit.

I took it out for the first time today and something is wrong because with myself and 4 others our max speed was 43(GPS).
The boat had 30gallons of gas in it and I would think we would reach about 60mph. The racing unit was replaced by a non-racing lower unit(talked to John coen and he said this shouldnt change the speed). The rebuilt motor has about 2 hours currently on it.

Anyone have any idea what's wrong?

You have to let us know what rpm at wot (wide open throttle) and what prop are you spinning before you could get any help.

about 6500rpm at wot and spinning a Michigan wheel Apollo xhs A series

did you change the engine height? the change of lower unit will have an affect on speed if it has a different gear ratio. what pitch is the prop?

Break in mode?

i thnik a good prop might help not sure about it getting you 17 mph what kind of speedo you running gps?

Definatly a Prop/Gear Ratio problem, You have plenty of rpm, Now you need to convert in to forward motion.

Thats what I am figuring....if that engine is spinning up to 6500 rpm that means it is generally healthy...the hub might be bad in that prop or you dont have enough prop. Do a search on outboard 24x7's and see what guys are using for props. That should give you a baseline. Scream and fly might have a tad bit more info on the outboard performance boats.

prop pitch is 14x22

Those Boats Dont Like The Extra Weight ,Meaning 5 People And 250 Lbs Of Fuel Try Running It By Yourself And See What The Speed Is ,,And A trophy plus Prop Works Best On That Boat