I had a new engine rigged in the boat. The lower large crank pully was an approx 8" diameter pully. The engine dynoed at 765hp. The 8" pulley wouldn't clear the engine area in the boat so instead of trimming the area out to make the large pully fit they put a 6" diameter pully on the motor.

Did this bring horse power and the performance of the engine down? All of the calculations that the everyone tells me is the boat should be turning a higher rpm and should result in a higher top speed.

Any help...

The smaller pulley will spin all the accessories faster. It might take 5-10 hp to spin them at the higher rate.

Is this a motor with a blower? If you are putting a smaller bottom pulley then you are going to make Less boost, less HP & need smaller prop

Smitty is correct.

The smaller crank pulley is going to spin the accesorys 25% slower,if its normally aspirated the only problems it might cause would be low alternator output at idle or low water pump output but it should actually free up some horsepower. If its blown it will be a different story,my procharger has approx a 8" crank pulley,if i were to put a 6" pulley on it would drop peak boost from 10 psi to 7.5 or from 8 psi to 6 psi which would take a serious amount of power away,Smitty

Yes it is a blown motor... 496mag HO ported polished all tricked out with the best of everything. The motor was built by Larry Peto and has a 3300sc whipple on it. It just seems to me it isn't putting out 765hp. My old motor was a stock 496 with a 2300ax whipple, 620hp and seemed to be just as fast.

Are you saying you decreased the size of the blower drive belt pulley?
If so, YES!!!!!!!!!!!!!!!!!!! That will decrease boost and hp. You can comopensate by using a smaller top pulley if available.
Gary