There's a lot to figure in, with 415hp there's no reason to use those gears, prop slip on the active thunder is prob a bit higher than your boat, heavy boats that don't have much slip% need a smaller prop, light boat high slip will run a much higher pitch, a 292 formula with 6.2s runs a 28 bravo 1 stock and a 382 with 500s runs a 30 but with slip numbers in the high teens

4300 RPM with 1.36 gears only calculates to 69MPH with perfect solid medium. With a 7% prop slip it is 64MPH.
The RPM i waaay too low.
502 makes its best torque and HP around 4800-5200 RPM

5100RPM with 24 bravos turning 5100 is 85 perfect world.
72 is with a 15% prop slip.
The 15% is closer to realistic.
Plus the speeds have to be GPS not by the pitot tube style speedo unless the speedo is verified accurate with a GPS
My Formula running 1.5 drives with 27 Hydros turning them 5700 gave me 83GPS with a 14% slip.
So do the calculations on what you "should" turn and what your actual is. Figure the slip and that will tell you if you are getting the results you need/want

oops.....sorry....
here is my data calculator...
http://go-fast.com/Prop_Slip_Calculator.htm

My propslip is 10% for 74mph and 12% for 72mph. The speed and slip % has some variance based on fuel and passengers.

Whenever calculating slip for Bravo props, you need to deduct 1" of pitch from the stamped pitch on the prop. They are stamped heavier than they actually are.

My 1997 32 AT(8000# dry) and the 1994 35 Lightning(7800# dry) weigh virtually the same and have about the same amount of running surface. I'm pretty sure that his 1994 is also non stepped.

I'm betting the engines are not really making the amount power that they are supposed or were represented to be.

sounds like down on power to me...

I would try 21 props and see. The 23s may be loading the motors too much and keeping you out of the "powerband".

you should post this in the Fountain forum.....someone in there either has or has had your exact boat/eng combo.....