Want to make the fishing boat go...
 

Have 23' Bayliner Trophy hardtop (you can stop laughing now...) that I replaced the V6 with a 270hp magnum v8 and rebuilt the alpha outdrive with new gears at the 1.35 ratio. Experimented with 3 blade and 4 blade stainless props. Currently running a 3 blade 19 pitch Mirage and get about 46 mph at 4900 rpm. The old drive train did about 34. Is there any cheap magic I can do to do this rig to get above 50 mph. Specifically the drive lower unit and prop.
Is there a bolt on lower unit that would be more efficient?

I put a NOS unit on a 270 Magnum using the smallest jets. We upped the prop pitch 2"s. Gained 7 m.p.h. on the gas. Made a mean roar!

The right 4 blade stainless propeller may get you closer since your slip % is almost 30!
We do have these propellers for testing.

Brett

Until I saw it was a sterndrive, I was gonna say that a worked over 2.5 would do the trick as far as "making it go" haha

https://fbcdn-sphotos-a.akamaihd.net...23065669_o.jpg

A Diamond Marine 2.5 SS on a john boat?

Thats cool!

I've got a SS 2.5 as well on a little padded V. Fun river runner.

Brett

The four blade I have looses about 200 rpm and a does about 44-45. However it comes out of the hole better and cruises faster for the same rpm. How is it working this well with 30% slip or am I missing a bunch still?

Are you sure about the gear ratio? Sounds almost like a 1.47/1.5 or 1.65 set.

Your speed/rpm combos fit with a 1.5ish gear set. IF it's actually a 1.65 set that would give you about 13% slip which is more in line. IF you have a 1.35 set then as BBlades noted the slip is through the roof.

Will verify with my drive guy what gears he put in it. The ratio was changed from whatever the 4.3litre engine had with it. I did some checking into the the prop position with the drive level with the boat. The cavitation plate is over an inch below the center of the keel. Is there a bolt on lower unit that will raise the prop up a couple of inches like the ones on my other boat's Bravo XR drives? Does SEI make one? If I need to ask this question in the drive forum, I will re post there. The last part of this is the exhaust or lack of free flowing exhaust, any changes to it short of going out the transom for any gain?

Verified the drive ratio - it is 1.5 according to the drive technician. I will post my drive and exhaust questions in the other forums. I am assuming I can't do anything else with the prop for more top end.

Your slip % is still a little high.
Gear case mods can sloe you down or speed you up depending on the issues and the current performance. Adding a nosecone would most often slow you down with the top speeds you are currently running unless the the case is causing the prop to ventilate now.

Labbing the prop can give speed results but needs to be done with care with your current slip %.

Call us anytime and we can discuss options further.

Brett