1972 Johnson 25 HP prop

[cgwoods]

I have a '72 Johnson 25 hp (25el72) (9x9 prop) and have it on a 14' aluminum with a wood floor and center console steering. It's fairly light and gets out of the hole quickly but would like a better top speed. Last time I was out it was topping out at 21 mph with just me and my do in it. My previous boat was a 12' aluminum with no floor and 2 swivel seats and had a '76 Johnson 15 hp on it (9.5x10 prop) That boat would do 22 mph with just me in it. I understand this boat is heavier than the last but i would guess only 150-200 lbs heavier. I am just a little surprised that I don't have a more drastic improvement in top speed. Unfortunately I don't have a tach on it right now but it doesn't seem like it's over revving at wot.

Should I get a larger pitch or diameter or both? Also does anyone know of a composite pin drive prop? If anyone has any suggestions or has experience with this motor I would greatly appreciate it.

 

[gm280]

Re: 1972 Johnson 25 HP prop

I would get a tach and know the RPM's before laying out any money on any prop. Also, what are you using to verify your speed...?

 

[cgwoods]

Re: 1972 Johnson 25 HP prop

Im using a GPS to verify speed

 

[jestor68]

Re: 1972 Johnson 25 HP prop

Assuming a 2.15 gear ratio; a 9" pitch should produce about 5802 rpm using 8% slip at 21 mph.

With a WOT rpm range is 4500-5500, you can afford to step up to an 11" pitch and still be around 5400rpm, and pick up several mph.

That assumes that the motor is running properly.

 

[Howard Sterndrive]

Re: 1972 Johnson 25 HP prop

check compression, check spark with a spark tester, install new plugs, check coils and points, and do the link n synch.

 

[steelespike]

Re: 1972 Johnson 25 HP prop

The 71 Brochure indicates a 1.75 ratio the 9" prop at 21 mph using 10% slip gets about 4800 rpm.
Came with a 10" prop in 71 so the 9" appears to be a concession to the weight.
Speeds on a light reasonably fast boat is rated at 32 mph Yours is not a light reasonably fast boat but
we may be able to improve your speed.
You should get a tach for the health and best performance of your motor.An induction tach like a Tiny tach will fit almost any motor
land or sea.You need to do a tune up with a compression check.
Be sure the throttle opens all the way.The anti vent plate (just above the prop) should be about even with the bottom of the transom.
If your prop appears to be original simply replace it with a 9" pitch Solas Amita prop if available may improve things.

 

[cgwoods]

Re: 1972 Johnson 25 HP prop

Ok I got out to the lake again today and I was between 5500-5700 rpm at 20.5 mph.

 

[steelespike]

Re: 1972 Johnson 25 HP prop

Calculator says your slip is 23%,very high.Is the prop original,badly worn?Speed at 5500 would be around 24. with 10% slip.
I think your tach may be picking up some stray pulses.You need to hook it up carefully according to the directions. Probably on the bottom plug wire and get it out from under the hood as quickly as possible.