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just got a new upper unit for my 86 searay 268 weekender with a 260hp mercruiser 350 engine. Short story on the boat. I bought the boat from a private seller. the boat would hit 5000 rpm at wot with a 14.25X17 ss 3 blade prop.The boat would hit a top speed of 9 mph after trying many props found out the boat had the wrong ratio drive 1.98. So i ordered the correct 1.47 ratio drive. Installed the drive and broke it in and changed gear oil. Now with the same prop i'm only able to hit 3500 rpm at wot but now the boat moves at 19 mph the boat still doesn't plane yet but it's at the brink of planing if i can get some more rpm's out of the drive it will plane.I know it's not an engine issue cause i was able to hit 5000 rpm prior to the drive replacement.this engine was just rebuilt and only has 20 hours on it. I'm at a lose here.
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Are you sure about the speeds you listed? Go to http://www.go-fast.com/Prop_Slip_Calculator.htm and enter your information. This will tell you where you should be with your prop and drive.
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