Correct prop size for best performance on a 1971 50 hp johnson hyro electric drive?

[ChaseBoats22]

I have a 1971 50hp johnson hydro electric drive motor and wondering what size prop gives it the best performance safely withought putting to much strain on the motor.
-I have a 13 3/4 x 15 on it right now. I bought it to replace the prop that was old and chipped pretty good 13x6 is what i believe it was and tossed it in the trash. The prop on it now I think the dia. is too big and not letting the motor turn at its max rpm? (is noticably slower)
- Any input or experiences? THANKS

 

[eavega]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

The correct prop is going to depend on your setup; weight of the boat, height of the motor, trim position (assuming you don't have a power tilt, but I could be wrong) are key. start with 1. verifying that your boat is not severely underpowered with the 50 (what is the max rating of your hull? ) or waterlogged. Then verify the position of the motor (height, and trim angle) to make sure they are optimum.

for the record, I had a 1972 50 HP on a 1976 YarCraft that was rated for 70 HP Max, and had to step down to a 13" pitch prop to get motor spinning at the recommended WOT RPMs.

Good luck.

Rgds

 

[steelespike]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

I believe the Hydraulic shift appeared on 73 and 74 motors. No such thing as a 6" pitch for a 50, your 15" pitch seems right for the typical boat the 50 would be applied to.Please read the stickys at the top of the prop forum and get us as much info as possible.
Typical speeds would be about 30-35 at about 5500 rpm.
A healthy older motor would produce speed close to that. You could have issues with the boat,motor or both.
A gps is good for speed.Lacking rpm and speed a detailed description of the performance will help.

 

[ChaseBoats22]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

My motor is a 71' model and it is a hydro electric drive.
- I ment to type 13x....16...not a 13x6 sorry

 

[ChaseBoats22]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

The motor is on a pre 70's model 16' quachita flatbottom.
- Im not sure what the boat weighs but its build like a tank, not real real heavy but probably weighs more than your average 16' flat bottom.
-I do not have a tach set up nor have i ever gps the speed i just know that when I took the 13x16 off and put on the 13 3/4x15 the top end speed decreased considerably.

 

[steelespike]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

Are you saying the motor slowed and thus the boat also?
Or does the motor seem about the same but the boat has slowed.
Generally a prop with less pitch might have a larger diameter.
Usually diameter doesn't affect rpm.
Normally a drop in pitch would increase rpm 150 to 200 rpm per inch of pitch.
If the motor was near the top of its rpm range with the 16 then the motor might over rev with the 15 and lose some speed.
From your description it does appear the 15 some how is too much.Diameter might knock 50 rpm off the the change but you should still have 100 to 150 rpm increase.
Perhaps in the process of changing the prop you changed the remote cable adjustment so its not getting full throttle.
Perhaps the old prop was more primitive or damaged enough that it let the motor rev easier.
New the motor came with a 13 3/4 X 17 prop though the final selection would depend on the boat.
Prop up to 23" were available in 71.
The present day prop chart of available props the 15 is near the bottom with only 4 props smaller,
there are 8 props bigger. All this indicates the 15 should work with your boat assuming it hasn't
picked up a load of water in the foam under the floor over the years.

 

[ChaseBoats22]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

I believe the rpm is the same. The speed is what has changed and I'm sure I haven't messed with the throttle at all. Someone gave me an 13x18 and it the prop would slip/spin out when I made any type of semi sharp turn and was also slower. I replaced the floor several months ago and there is no foam in the floor. I have had to drill holes to mount new a new steering bracket a while back on the side of the boat and still did not see any foam shavings.
I'm almost positive the problem is in prop selection and I know a 16 pitch is hard to come by these days. Just before I purchase another I wanted to see what the next beat prop to a 16 pitch would be.
Thanks

 

[ChaseBoats22]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

Correction... the prop that slips/spins out when i make sharp turns is a (13x19)...not a 13x18.

Its possible i made another mistake and the the fist prop i had on the boat was a (13x17) and..13x16??

I reckon i will see if I can find a used 13x17 at a local marine shop used and try it out.

 

[steelespike]

Re: Correct prop size for best performance on a 1971 50 hp johnson hyro electric driv

Ok I just want to be sure I make myself clear. We can't do this right without the rpm and gps speed.
An induction tach. is reasonable easy to install and can be used on almost any motor.
We need the rpm and speed to tell if your setup works.You need to know the rpm of the motor to keep it healthy
Too low causes lugging which can lead to premature repair problems too high can lead to destruction.
The 15 should have had more rpm than the 16. Your" flat bottom"should produce good speed and a 17 might work
but will produce less rpm than the 15.I wouldn't do any more prop changes until you have a tach and the gps speed.
To get us started the gps speed and a detailed description of the performance will work.
When you did the floor were the framers underneath in good shape?
Does the boat respond to up trim by the bow coming up or
does the boat lay down like its glued to the water?