'72 merc 1150 RPM "gain"

monk-monk

Chief Petty Officer
Joined
Aug 27, 2006
Messages
642
when i was on the lake the other day i had a hard time holding a steady throttle. i did have a "full boat" people, gas, gear, etc in this 16.6' Glastron Bowrider. i did not lose RPM's or speed but it would increase the RPM's and speed and i would have to keep backing off the throttle to hit a happy medium and then have to get back in the gas to get back up to a decent speed. it didn't happen suddenly it just starts increasing, but it made more work than pleasure trying to maintain a cruising speed! your thoughts?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: '72 merc 1150 RPM "gain"

10 to 1 on... Your prop's too big... Get the next size small (in pitch) prop.... That will solve that problem... If you have a tacho in the boat that engine should be making 5000-5500rpm at full open throttle... If it's not getting to 5000 (and I'll bet it's not) you can destroy pistons, real quick...

Chris...........
 

monk-monk

Chief Petty Officer
Joined
Aug 27, 2006
Messages
642
Re: '72 merc 1150 RPM "gain"

thanks Chris, i have the merc manual on order...plus, i've read clams_canino's timing method, and im going to perfect that FIRST!! this is the original prop on the engine, but you think it is too big for the boat its on...max hp for this boat is 100hp (not original motor)...smaller prop, steadier throttle??
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: '72 merc 1150 RPM "gain"

...smaller prop, steadier throttle??

Exactly.... You're putting less load on the engine, so it'll be easier to hold a speed... It's like the difference in your car of going up and down hills in 3rd as opposed to 5th...
 

monk-monk

Chief Petty Officer
Joined
Aug 27, 2006
Messages
642
Re: '72 merc 1150 RPM "gain"

yeah, im starting to learn about this now. my prop has the numbers 17P A4 stamped in it, i can only assume this means a 17 pitch...now when just me and my brother were in the boat i recall having it a W.O.T and it was only at 4000 RPM's...how much pitch should i remove...from other articles i've read it sounds like i need to back off at least two sizes (calculating this at about 300-400RPM's per size...i might even need to go as low as a 14P and how does diameter affect this formula...your thoughts?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: '72 merc 1150 RPM "gain"

If you're only getting 4000 with a 17" prop, then look for a 13" prop.. Diameter is sort of fixed and doesn't really come into the equation much for outboards.... Diameter is more of a concern for inboards.... (not sterndrive inboards)

Chris.........
 
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