RPM too high!

reka12

Petty Officer 2nd Class
Joined
Feb 25, 2005
Messages
185
Hi all: We finished our conversion and took it out for it maiden voyage. It is a 14' old run of the mill fiberglass fishing boat that has been converted to a small flats boat. It ran beautifully in the water! (Pics will be forthcoming in the boat restoration section.) Beautiful running little boat! The tach is showing 7k rpm (tach was set at 4Pole during install)at WOT and I know this is too high.<br /><br />A little history on the motor: It is an '03 25EL Merc. It was used previously on a pontoon. The original aluminum prop has 13P stamped on it (assuming 13 pitch) and the radius from center of hub to tip of blade is 5" (10" diameter). I know I cannot go any larger on the diameter (the blades are close to the motor now). Any suggestion on what I try next?<br /><br />I also noted that the anticav plate is two inches below the hull. We will move it up an inch; however, from reading previous posts, we may gain even more rpm. Looking forward to the answers from you wizards!
 

north40

Petty Officer 1st Class
Joined
Apr 13, 2004
Messages
331
Re: RPM too high!

First make sure that hub is not slipping. Put a mark with a sharpie on shaft and outside of hub. If marks do not line after a run, the hub is spun and needs to be repaired. If this is ok, try higher pitch prop. Not sure what WOT is for your engine but every increase in pitch decreases RPM approx 200.
 

north40

Petty Officer 1st Class
Joined
Apr 13, 2004
Messages
331
Re: RPM too high!

Your max RPM should be marked on the serial plate on engine.
 

reka12

Petty Officer 2nd Class
Joined
Feb 25, 2005
Messages
185
Re: RPM too high!

6000. You might be onto something. When removing the prop, I noticed some rubber "strings" from the hub area. Upon closer inspection, it does appear that it may be slipping. I will check it out tomorrow. Thanks, Mike
 
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