Prop on Johnson 70hp help

[DAN M VAGOS]

This is like my fourth post on my new boat. The answers I have been getting from all of you have been greatly appreciated and it helped out a lot. My question now is the boat is it 1999 Key Largo 160cc and the motor is a 1999 Johnson 70 horse. The tachometer is a US Marine which I know is not factory the boat but in trying the boat yesterday it seemed to tac out at around 6300 RPM. The two props the guy gave me with the boat are both the same and are three blade 13 3/4 x 15. Does this sound right to you or is this not the size I need. Thanks

 

[eavega]

Re: Prop on Johnson 70hp help

Your RPMs are too high at 6300. I believe the Johnson 70 HP should be 5000-6000 RPM at WOT. You should probably step up to a 17" pitch. That should lower your RPMs into the right range.

Rgds

Eric

 

[jestor68]

Re: Prop on Johnson 70hp help

Using the following weights; dry wt of 860, 500 lbs people/gear, and 12 gals of gas; the prop selector says you should be able to turn a 19 pitch at about 5500 rpm @ 41 mph.

A 17 pitch would provide better acceleration and over all performance with a heavier load, but would likely run closer to 6000 rpm with a slower top speed.

So it depends on what load you would normally be carrying.

 

[DAN M VAGOS]

Re: Prop on Johnson 70hp help

Ok so a new 160cc is 1100 and my boat I would think is a little more. Plus I would think I'm going to have another 600+- of gear and people.

 

[DAN M VAGOS]

Re: Prop on Johnson 70hp help

Any thoughts

 

[dazk14]

Re: Prop on Johnson 70hp help

Any thoughts

The 17" is the simple answer and pull the RPM's into the correct range and give you load carrying capacity as stated above.

Are there any characteristics that you would like to improve? Top speed, lower planing speed, strong acceleration for watersports, etc?

FYI- The tach sounds about right for a 15" pitch prop on that rig.

What is your budget? Is a $300 stainless too high or you can run an Aluminum for ~$110ish.

17" pitch is a common size and you may be able to find a nice used stainless from ebay, and even an 18" should work well - maybe better depending on your needs.

It often takes trial and error to get the right prop, but this should easily be a "one and done" prop selection, given your starting point.

Edit: You can get into a great stainless prop for $273 delivered.

 

[DAN M VAGOS]

Re: Prop on Johnson 70hp help

I just had my prop guy make it a 15 pitch and he cut it down to a 13 1/4 . He told me that's what it should have been.

 

[eavega]

Re: Prop on Johnson 70hp help

So, what, he trimmed down the diameter by a half inch? You started with a 15" Pitch prop, and you were under-propped by at least an inch of pitch. Diameter is not going to have the same effect that pitch is going to have to bring your RPMs into the acceptable range for that motor. I would suspect that trimming down the diameter of the prop would, if anything, INCREASE your RPMs as now there is less blade to push against the water = less resistance to rotation = spin faster. I may be wrong there, but that is what I would figure. You can pick up a brand new 17" Pitch three blade Solas right here on iBoats for less than $130 shipped, and if you wait for a sale you can pick it up for about $100.

 

[Rkrough]

Re: Prop on Johnson 70hp help

I have a Johnson 60hp pushing a 2800# semi-displacement hull and 13.25 x 15 is what most use for my boat. I can't imagine you needing that small of a pitch on a boat that weighs nearly 1/2 as much and is a planning hull.

I have my boat under propped (14 x 11- 4 blade) because of it's weight, shape and I like to go slow. I installed a new ignition module from CDI Electronics with a 5800 rpm rev limiter to avoid over-revving the motor.

 

[marcusknight]

Re: Prop on Johnson 70hp help

I have a 1979 Johnson 70 hp on a 16 foot starcraft. The prop I have used is a 14 diameter x 17 pitch. Although I am thinking of changing it out. There was another prop that came with the boat that was 13.75 x 15. But I haven't used it because it's too chipped. However, I might get another one similar to that so I have two different types and we will see how the performance shakes out.

 

[bill king]

I just recently ordered an impeller for my Johnson 65hp motor, but it came with the wrong key.The bottom of the drive shaft is flat, requiring a pin key or square key.Can the impeller be modified and can a square key be self made to fit the impeller which has a slit for a woodruff key. Any advise would be highly appreciated.
Bill

 

[jbcurt00]

Bill
Please take note of the helpful tip at the top of each forum page. Then start a topic about your Johnson, its impeller and key.

After 90days w no new posts, treat inactive topics like library books: for research only and not to be written in

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