Correct prop for a 74 Johnson 50

dmb

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Hello all. I am going to get a new prop for my 74 Johnson 50 twin. It is on the back of a 74 Starcraft 18 ft Super Sport ( I know it should be running a larger motor). It is currently running a 15 pitch aluminum but I don't think it is reaching top RPM and gets me going at about 24 mph, also when I have another couple in the boat, it won't get up on plane.
Would anyone recoment a 13 pitch and should I go to a 4 blade or stick with a 3?

Thanks
 

Texasmark

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Re: Correct prop for a 74 Johnson 50

I was going to say 15P till you said 18' boat even though it is a Starcraft and assumed to be alum (light weight). 15 is what the guys used to use in that day on 15' tri hulls with a 50 horse looper and a couple of guys and gear. On that boat I'd say at least a 13 for a SWAG. An 11 probably will be overkill. You will have too many rpms and speed would be adversely affected.

Mark
 

steelespike

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Re: Correct prop for a 74 Johnson 50

Guessing your slip at 14% at 24 mph your turning about 4700 rpm.
Your rpm range is 5000-6000 according to period literature.
Would be best to prop close to the 6000 figure.
You should really have a tach.A induction tach is reasonable and easy to install
can be used on almost any motor.
A tach will get us your accurate rpm now and after the new prop.
Without accurate rpm and gps speed you can't tell if your setup is working.
I think if my figures are accurate a 12" prop could give you about 5300 rpm.
 

dmb

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Re: Correct prop for a 74 Johnson 50

Thanks much: Not a big hurry right now, still have 2 1/2 feet of ice on the river and from the looks of things, it won't be melting soon! I think I am going to shoot for the 12 pitch and put a tach on her also.

It appears a 12 pitch is hard to find, I think I will end up with the 13 pitch and hope for the best!
 

steelespike

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Re: Correct prop for a 74 Johnson 50

I guess if the 13 is too much you could have a shop remove little pitch.
 

dmb

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Re: Correct prop for a 74 Johnson 50

I suspect that the 13 will do, the hull is alumium. I had a 74 supersport 16 ft years ago and it had a S/S 15 pitch on it. I wish I never sold that boat.
 

jestor68

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Re: Correct prop for a 74 Johnson 50

My vote is for the 14 X 11 Solas Rubex.
 

steelespike

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Re: Correct prop for a 74 Johnson 50

My vote is for the 14 X 11 Solas Rubex.
The 11" will give excellent hole shot and stay on plane easier at reduced throttle.
If its a little light you can still use it by watching the rpm.It will likely be slower than the 13
but better with a load.
 

dmb

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Re: Correct prop for a 74 Johnson 50

I see that Michigan Wheel has a 11 3/8 x 12 that is rated for this engine. Not really knowing anything about these, I would be concerned that the prop would be too small diameter wise? Any help with the diameter is appricated.
 

Texasmark

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Re: Correct prop for a 74 Johnson 50

At the top of the page on this site is a picture of 2 propellers. Click on that and answer the information. They will give you a rather large list of 11 and 13P props in the 13 1/4 to 14 dia range. You don't want to cut diameter AND pitch so stay away from the prop you referenced.

I am unaware of Jestor's background in the area, but from my personal experience, coming down from a 15 to an 11 is a big step; too big for me unless you really have a tub to push around.......however, your comment about not getting on plane with an additional couple on board speaks of drastic measures, possibly what caused him to prefer the 14 x 11P.

HTH,
Mark
 

dmb

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Re: Correct prop for a 74 Johnson 50

Thank you very much Mark. I will stay away from that prop. I suspect I will end up going with a 13 pitch, I figure it wil be a good comprimise between the 15 I currently have and the 11 which may be a little too light, and having another couple on board is a regular occurance. Would you suggest a 4 blade or stick with a 3 blade?

Dan
 

Texasmark

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Re: Correct prop for a 74 Johnson 50

I got to thinking about these numbers we are kicking around so I went to my BAM prop slip calculator and ran some numbers.

I took WOT which is where you rate a prop, of 5500 rpm. I think JR ran mostly 4500-5500 WOT rpms back then.

I looked up the gearbox for that engine turning the 13+ dia prop and it is 2.67:1 Data from iboats forum in 2005.

I plugged in with 11P and the max estimated possible speed was 21 and with an assumed realistic 20% prop slip, the actual speed was 17.

On the 13P, max was 25 and with the same slip was 20.

Planing speed is going to be right there 13, 15 mph or thereabouts. You are going to be running half in and half out of the water and the engine will be lugging all the time

When you add this extra couple you are just asking too much from that rig my man. I'd get the 13, or get some more ponies, or have your friends get their own boat.

My 2c,
Mark
 

jestor68

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Jun 12, 2012
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2,308
Re: Correct prop for a 74 Johnson 50

Guessing your slip at 14% at 24 mph your turning about 4700 rpm.
Your rpm range is 5000-6000 according to period literature.
Would be best to prop close to the 6000 figure.
You should really have a tach.A induction tach is reasonable and easy to install
can be used on almost any motor.
A tach will get us your accurate rpm now and after the new prop.
Without accurate rpm and gps speed you can't tell if your setup is working.
I think if my figures are accurate a 12" prop could give you about 5300 rpm.

And the 11 pitch should get him to about 5500 rpm; which is where his motor's HP is rated at.

50 HP at 5500 rpm; rpm range 5000-6000 rpm.

That boat/motor has a dry weight of about 1300 lbs. The calculated operating weight is about 2068 lbs, with your 4 adults and gas.

Combine that with the fact that a riveted Star Craft is not the easiest hull to push, and the fact that his motor is a 1974, resulted in my recommendation for the 14 X 11 pitch prop.
 
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