need prop size help

mudlakeladyjane

Recruit
Joined
Aug 4, 2019
Messages
1
We have a 1999 Mercury 40H 4 stroke 747 cc. Had 1 carb replaced & 2 carb kits recently replaced. The tech said it didn't run at top rpm. So he ordered us a 12.5 X 8 prop. At idol vibration is bad. Kick it in a bit & it smooths out. Original prop was 12 X 10.5 & it labors a touch. We have it on a 20 ft pontoon. What would be the right prop?
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,782
A new prop shouldn't vibrate. Going from a 10 to an 8 is a 300-400 rpm at top end increase in rpms which supports what the mechanic said. If the new prop has a separate hub, maybe it didn't get installed properly. Have that checked. I just installed one on my boat and found that even though I thought I had the insert all the way in, I didn't and the prop was loose on the shaft after my first run up with it...prop is supposed to be tight on the shaft, no slop. Easy fix, the insert had seated itself so I just tightened the prop nut to the required torque and problem solved.
 
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