Prop Question

[NATO556]

Time to replace my 10 pitch on my 1984 Johnson 15 HP …….mainly used for moose hunting.
Should I go with a 8 or 9 pitch for better performance on my 19’ canoe ?

 

[Scott Danforth]

What RPM are you able to get the motor up to at WOT

 

[NATO556]

Low 6000 at the prop

 

[NATO556]

Time to replace my 10 pitch on my 1984 Johnson 15 HP …….mainly used for moose hunting.
Should I go with a 8 or 9 pitch for better performance on my 19’ canoe hauling a load .

 

[Scott Danforth]

well if you are hitting 6000 RPM with a 10p, you won't want to drop pitch and over-rev

 

[brodmann]

How are you measuring RPM's at the prop?

 

[NATO556]

It’s 6700 up on top
But I expect to lose some RPM by the time it gets to the shaft .

 

[jimmbo]

It’s 6700 up on top
But I expect to lose some RPM by the time it gets to the shaft .

I would love to hear the Rational regarding that one. I'm all Ears

 

[NATO556]

What RPM are you able to get the motor up to at WOT

By the 15 HP 1984 Johnson manual it’s a 7000 at the top !
At the shaft it drops down to like 6200 if my memory serves me correct with factory 10 pitch.

 

[Scott Danforth]

By the 15 HP 1984 Johnson manual it’s a 7000 at the top !
At the shaft it drops down to like 6200 if my memory serves me correct with factory 10 pitch.

what does the tachometer say you are actually getting.

Manual says RPM to be 5500-7000 max.

what the manual says doesnt mean squat-diddly. its what is the exact prop on your exact boat getting for a WOT RPM while loaded. you use that value.

so determine what that real RPM is with your combination of prop, boat, and load.....and let us know.

 

[brodmann]

He must be equipped with the limited slip differential driveshaft option!

 

[racerone]

The gear ratio is 2.42 : 1 -----So at 7000 RPM the prop spins at about 2900 RPM

 

[jimmbo]

He didn't say which shaft it was losing 800 rpm by

 

[racerone]

So many people today have very little knowledge of machinery.----Do they not teach this in grade 9 anymore ??

 

[jlh3rd]

no, they don't.....they're too busy telling young kids that they aren't what they were born as.........

 

[racerone]

Good one !!

 

[airshot]

The poster needs to understand that max rpm is the engine rpm. Adjust the prop pitch to get the max engine rpm....forget any other shaft speed !!

 

[jimmbo]

"It’s 6700 up on top But I expect to lose some RPM by the time it gets to the shaft ."
"By the 15 HP 1984 Johnson manual it’s a 7000 at the top !
At the shaft it drops down to like 6200 if my memory serves me correct with factory 10 pitch."

An 800 drop in Rpm? Maybe this particular engine has a Fluid Coupling between the Powerhead and the Driveshaft... Can't be the propshaft rpm as the Gear Ratio is 12:29

 

[ScottinAZ]

So many people today have very little knowledge of machinery.----Do they not teach this in grade 9 anymore ??

slightly off topic, but sadly, no. Shop class is mostly a thing of the past. they are now teaching "E-Sports" AKA video games, and "coding" so you can make said video games. The things that actually get us around and make the world work, AKA mechanical things are not taught anymore.... No love for the Trades, its College or nothing

 

[airshot]

slightly off topic, but sadly, no. Shop class is mostly a thing of the past. they are now teaching "E-Sports" AKA video games, and "coding" so you can make said video games. The things that actually get us around and make the world work, AKA mechanical things are not taught anymore.... No love for the Trades, its College or nothing

Actually, "nothing" is better than college is today!! Just look at the math abilities of todays college grads...they can't do the math to pay back their student loans !! Math a 9th grader should have no problem doing !!