Help! May drop prop pitch.

cat toon

cat toon

Seaman
Joined
Sep 20, 2010
Messages
61
I an running a 70 horse Johnson on the Cat Toon,I took off a 19 pitch prop and put on a 15 pitch prop,I did not run the motor on the toon with the 19 pitch prop because I was thinking that pitch may be to much for the toon,well with the run with the 15 pitch I got 23 miles per hour,showing on my GPS, with 5000 RPM's showing on the Tach,this is wide open trottle,should I drop to 13 pitch to get the extra RPM? Is the 5000 RPM to much strain on the motor? Your knowladge will be appriciated.
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,557
Re: Help! May drop prop pitch.

I wouldn't be the least bit concerned about your rpms. Sound great to me regardless what the recommended WOT is and it probably is 4500 to 5500. Even if it's 4000 to 5000 as some OMC's are, you are still good to go.

Mark
 
cat toon

cat toon

Seaman
Joined
Sep 20, 2010
Messages
61
Re: Help! May drop prop pitch.

Texasmark said:
I wouldn't be the least bit concerned about your rpms. Sound great to me regardless what the recommended WOT is and it probably is 4500 to 5500. Even if it's 4000 to 5000 as some OMC's are, you are still good to go.

Mark
Click to expand...

Mark, I am told that my motor should run at 5000 to 6000 RPM with 5500 being the best,So I should be ok?
 
jestor68

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Help! May drop prop pitch.

cat toon said:
Mark, I am told that my motor should run at 5000 to 6000 RPM with 5500 being the best,So I should be ok?
Click to expand...

That's because the motor's rated HP is at 5500 rpm. I think you know what you need to do.
 
S

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help! May drop prop pitch.

Is this motor a 2 stroke? If so it is rated 5000-6000 rpm according to my info.What year?
Marine motors are straining like they are going uphill all the time I think its better to let them rev easily within their range.
At 5000 there is little room if you start adding weight. A couple of hundred pounds and your below the rating.
Using a ratio of 2.23 your slip is a little high for a pontoon at 27%
It is possible that a 13" could add up to 400 rpm might lower slip a little.
A 4 blade might improve slip a little more and could be more responsive.
Change in speed isn't likely to be much but speed with a load may hold up better.
 
cat toon

cat toon

Seaman
Joined
Sep 20, 2010
Messages
61
Re: Help! May drop prop pitch.

steelespike said:
Is this motor a 2 stroke? If so it is rated 5000-6000 rpm according to my info.What year?
Marine motors are straining like they are going uphill all the time I think its better to let them rev easily within their range.
At 5000 there is little room if you start adding weight. A couple of hundred pounds and your below the rating.
Using a ratio of 2.23 your slip is a little high for a pontoon at 27%
It is possible that a 13" could add up to 400 rpm might lower slip a little.
A 4 blade might improve slip a little more and could be more responsive.

Change in speed isn't likely to be much but speed with a load may hold up better.
Click to expand...


I took off a 50 horse Merc and put on a 1987 70 horse Johnson, the info I gave you came on the second run of the motor on the water, as i stated before, I am told the motor is rated at 5000 to 6000 RPM with anything over 6000 in the red line,I only got 5000 RPM out of the motor at wide open open trottle,My worry is am I putting to much strain on that motor,when it will not get up to the 5500 RPM crusing RPM's. I am happy with the speed of the toon now but worry about the motor,If I drop to the 13 pitch will I hit that 5500,with some left over?
 
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