Out drive Propeller Situation-gctid821377

[RCF50]

I have a 1987 2855 Contessa powered by a 5.7 liter with vortec heads. The out drive is a OMC geared for a 5.0 liter. I was in need of a drive a few years ago so I purchased this drive because it was advertised a V8 drive. Long story short the drive runs the boat with no problem. I installed my aluminum prop with is a 15 x 15. The boat runs great with this drive it gets up on plain and cruises at anywhere from 18 to 22 mph at half throttle. Here my question the other day I had the boat out the weather got bad and I was rushing back to my marina. I decided to run at full throttle which is a rare but when I got it opened up I noticed that the rpms were just at about 4000. With the old drive WOT rpm was 4800. My question is do I leave it alone. Or do I go to a smaller prop. I saw a 14.75 x 15. Considering it. Before you ask the boat running perfect. Engine is strong and the bottom is clean. I know the gearing in the 5.0 drive is slightly different then the 5.7. Any suggestions? Oh yeah at WOT it did 30 mph speed over ground on the GPS.


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[Pcpete]

Sounds like you have "taller" gears in the drive. There should be a tag or serial number based way to find out your ratio. Failing that, you could get a laser tachometer. These use a reflective strips that are more like dots so you could stick one on the harmonic balancer and another on the prop. Diameter doesn't matter, one rev is one rev. Then is some simple division to get to your gear ratio. From there a competent prop shop can get you to a size that will get your full performance back.