Johnson 60 hp rpm vs speed

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Corky123

Cadet
Joined
Jun 11, 2011
Messages
7
Hi all,

1988 johnson 60 hp with trim all or part way up from stopped or slow wot 5500 rpm 30 mph (sounds like motor is over revving during acceleration) if I trim down a little rpms drop to 4800 (sounds like motor has changed gears) and speed is still 30 mph on gps. Would this be cavatation. 13.25" prop 17 pitch.

Thanks,
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Johnson 60 hp rpm vs speed

If you read the stiuckies at the top of the page and get us as much info as possible we can tell better whats going on.Cavitation Is a different animal.You don't feel cavitation.It occurs in the low peressure areas of a prop and etches the surface.
Ventilation is when the prop gets airbound and loses grip.Years ago it was incorrectly refered to as cavitation even by the manufacturers.You may be trimmig up too soon or too far.Your prop may be in poor shape or lacking cup.
Is the antivent plate just above the prop about even with the bottom?
 
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Sixmark

Master Chief Petty Officer
Joined
Jul 11, 2010
Messages
890
Re: Johnson 60 hp rpm vs speed

So do you have this motor attached to a boat or are you just driving it around by itself, knowing what size and style boat it is on can make a lot of difference.
 
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boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: Johnson 60 hp rpm vs speed

Sounds like you're blowing a hole in the water by having it trimed up to high causing ventilation. Depending on the boat that mtr should run 5000 to 6000 rpm with the correct prop and trimmed correctly.
 
C

Corky123

Cadet
Joined
Jun 11, 2011
Messages
7
Re: Johnson 60 hp rpm vs speed

Thanks for the info, I will try a new prop any suggestions on pitch? I was thinking a 21 would help wot rpm.
-16.5 ft Sylvan classic fisherman
-Prop is new and no etching on the prop
-Antivent plate is even with bottom of boat
 
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hidef

Lieutenant
Joined
Aug 22, 2009
Messages
1,465
Re: Johnson 60 hp rpm vs speed

Actually you should go to a smaller pitched prop. If you are turning 4800 with a 17" prop a 21" will knock your RPMs down to 4200 and you really be lugging the engine. You are looking for 5500 RPM or so properly trimmed out no ventilating or cavitation. Two inches of pitch will change the engine RPM by 300 RPMs.
 
S

Sixmark

Master Chief Petty Officer
Joined
Jul 11, 2010
Messages
890
Re: Johnson 60 hp rpm vs speed

17 or 19 is the range you want to be with that motor and boat combo, if you are expecting to hit 50mph, then don't.

Trim your motor once you are on plane and adjust from there.
 
C

Corky123

Cadet
Joined
Jun 11, 2011
Messages
7
Re: Johnson 60 hp rpm vs speed

Okay new prop 13.5" 15 pitch arrived and will post once I try it out. Local OMC tech suggested the 15 pitch.
 
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