Hi all,
I have a 1993 Formula 336 SR-1. I recently repowered it with 632 ci 5600 RPM 725 HP/760 lbs of torque motors. I have XR uppers and -2 sport master lowers on it. A friend of mine gave me a set of Bravo 34 pitch props worked to about 32P. The boat ran at about 4500 RPM and 74 mph. Obviously too much prop. Or so I thought. So I got a new set of Hydromotive IV-X 28 P. The RPM went up to only 4900-5000 RPM and the boat slowed to 68 MPH.
My question is with the -2 lowers what type of prop should I be running? I am also running Marine Max cavitation plates on it.
1:1.5 gear ratio.
I think I should get close to the RPM I got on the dyno but obviously pretty short of those numbers.
Thoughts?
Thanks in advance.

To start you want to know Where the prop shaft is in relation to the bottom of the hull.

you had 21% slip with hydros

7% with bravos. Something is very odd about the drastic difference in slip numbers.

Also engines are very different in the boat than on the dyno. The dyno is a good baseline but you need to make sure you verify tune in the boat.

this may sound dumb but if peruse the forum its happened. You are trimming the drives up correct?

Yes I am trimming the drives correctly. Those RPM and speed numbers are with the drives trimmed to max RPM. Without any out trim the RPM is much above 4000.

My 311SR1 was very similar to your 336. The hull centerline dimension was essentially the same, but the 336 had a "moulded on" swim platform vs the "add on" platform of the 311.

My boat had 410hp 502 mags, 1.36:1 B1s, and I ran 23p solid hub Mirages. I got 4900 rpm and 71mph gps with light fuel load on a 75 degree day. Tabs full up. Drives about 6 on the indicators.

Something is WRONG with your numbers.
Grab a GPS unit for checking your speed.
Make sure your engines are running properly (one bad plug wire is 40rpm). Make sure your throttles are opening all the way up. Make sure your timing advance gives you 34-36 degrees above 3,000 rpm. Make sure your flame arrestors are flowing enough to not be starving the motors for airflow. Make sure your motors are getting enough air from "outside" by temporarily rolling up a couple beach towels and closing the hatch with one on each side (chocking the hatch up with a couple inch air gap from fully closed. Make sure your carbs are not starving for fuel - the factory fuel lines/pickups/anti siphon valves aren't rated to flow the 140 gallons/hr that two 750hp motors are asking for. Make sure the carb jetting is correct as installed in the boat - jetting on the dyno is not optimized for your installation. Also, if your motors made 750hp with velocity stack on the carb and 2" dyno headers, your stock Mercruiser center-rise exhaust manifolds are a different story.
Finally, your Bravo drives will grenade in short order behind those motors without significant upgrades.

Good luck.

I’ve had the boat for 17 years. Prior to these motors it had 592/500’s. With standard bravo 1’s and 25p mirage plus 3 blades it turned 5100 rpm and about 74mph.
plug wires are all tight. Carb is a Holley 1050 Dom on Snyper tall polished Intakes with a 1.5 inch spacer. I replaced all the fuel lines from the tank to the motors with 1/2 inch lines and big fuel pumps. I forget the rate right now but they were the ones my builder ( a boat racing engine builder) told me to get. Timing is 34* There are 3 inch flame arrestors on them. All the speeds are from a gps. I will double check it with another unit. I can’t push the throttles up any more but I know that doesn’t mean much. I will check to see if the carbs are fully opening and I’ll try putting more are to them. Thanks for your suggestions. They are much appreciated. Those numbers don’t sound right to me either. That’s why I ask questions.

What does it run with the original props? Did you just changeover the lowers? Sounds like a gearing problem?