Outdrive Gear Ratio Question

[NorCal Boater]

I guess I really have more than one question....

My boat is a 1985 Cobalt CM23. It is 23' and the listed dry weight is 3900 lbs. With 50 gallons of fuel, a small cooler and 2-4 people on board the weight in the water is probably closer to 4900 lbs. The boat came originally with a 260 Mercruiser/Alpha I Gen I package but it is a Jackshaft model. In other words, the engine is mounted in the center of the boat and has a driveshaft connecting the engine to the outdrive. It's kind of cool and the boat has always planed very easily.

This is a boat that I purchased with a partner in October of 1989. I exited the partnership in 1998 and bought the boat back 4 years ago in pretty rough shape. I'm just finishing a restoration/refurb. With the original engine package top speed was about 45 MPH trimmed with 2 aboard. After I exited the partnership, my old partner had an engine built for the boat. The current setup is a GM small block 383 Stroker of undetermined horsepower but estimated to be somewhere in the neighborhood of 325-375. Currently, the motor turns about 4400 rpms at WOT with a top speed of just over 40 MPH. And I mean like 41. The prop is a new Solas SST 14 3/4 x 17. I tried a 15P and RPMs went up to 4700 but the speed went down to 39.

This brings me to my question(s). I believe this engine should easily be turning 5000-5200 RPMs at WOT as that is what is recommended for the Merc 6.2, but it appears re-propping will not make the difference. I need to replace the SE106 outdrive and I am wondering if a different drive ratio would be helpful? It is currently a 1.50. Will a Gen II (1.47) drive bolt to the Gen I gimbal housing. If so, would I see any difference? There is also a 1.62 available or would that be going the wrong direction?

I don't run my boats hard and I am not looking for 60 MPH. In fact, I very rarely run my boats WOT but I am concerned about the motor not reaching maximum RPMs. I'm curious to see what the tech guys have to say.
Thanks, Shawn

Here are a couple of pics

of the boat....

 

[NorCal Boater]

Wow.... not even a, drop dead. No problem, I figured it out.

 

[Chris-380]

Pretty complex situation, way beyond my capabilities. Care to share with the ones that are curious.

By the way, your boat is sweet, looks great

 

[dlvwill]

300 HP is about the limit for an Alpha; increasing the ratio going from 1:50 to 1:62 will only compound the problem with torque multiplication. You would want to go the other way with the ratio if possible to help the drive live, but you are living on the edge regardless, so drive it easy. You don't say whether the the engine has the later roller camshaft or flat tappet, but 4400 probably isn't out of line. The mid-engine arrangement makes this a cruiser, not a speedster. You have more weight amidship, so a lot of wetted surface to overcome. Same basic issue as with a direct drive ski boat which never really get past mid 40's mph.