What prop size for 20ft pontoon with 70hp Johnson?

[88fish"n"fun-toon]

Hello everyone, this is my first post and of course its a prop sizing question. I have been trolling this forum as a guest to try and find an answer and have come close, but need some reassurance.
First off my motor is a 85' 70hp johnson and my boat is an 88' lowe pontoon. I really want to change the prop since the ends of two of the blades are chipped off. My problem is I cant find the pitch number on the prop anywhere. I think its in too bad of shape and has worn off. My second problem is I dont have a tach, so I cant say what rpm im running at.
However through my searching on here, I have come across a few older post with setups similar to mine. Most seemed to be going with a 3 blade 13.75 x 13.

Does this seem about right? Thanks for any input and this forum is great. I have picked up a lot of information just searching as a guest. Glad to be a member now.

Tj

 

[steelespike]

Re: What prop size for 20ft pontoon with 70hp Johnson?

Welcome, Have you looked under the prop nut for numbers.Also a prop shop may be able to repair the prop and tell you the size.
A 13 does seem reasonable Don't worry about the diameter though on a pontoon if there is a larger diameter available it may help.
Also consider a 4 blade as they should reduce slip numbers.
It is important for the health of your motor to be able to rev freely near the max rated rpm at wot. unconfirmed 6,000.Over reving can cause premature massive failure.Lugging at too low an rpm can lead to premature repair problems and poor performance.Without a tach and gps or speedometer you can't tell if your setup is working.There are induction tachs that are reasonable and easy to install.you may be able to borrow a shop tach if you have a regular marina you patronize.

 

[88fish"n"fun-toon]

Re: What prop size for 20ft pontoon with 70hp Johnson?

Thanks for the reply. I was able to take it out this weekend and I used a gps to tell the speed of the boat. With 3 people on it, I averaged about 15-17mph and with 5 people about 12-15 mph. Im guessing the prop that is on it now is pretty close to right.

I was thinking about a four blade prop, but dont really know much about them. I know they only come in a 13 1/4" x13 which was a smaller diameter than what I was originally looking at. Is there any benefits of the four blade prop?

Of the three blade props i was looking at either the Solas Amita 13 3/4" x13 or the Turning Point Hustler 14" x13. What would be the better choice?

 

[steelespike]

Re: What prop size for 20ft pontoon with 70hp Johnson?

A 4 blade will be much less likely to ventilate(cavitate)Also should have lower slip numbers.
Boats like pontoons tend to have high slip numbers.I think you may be able to read about slip on the prop store page.All props have a certain amount of slip.I think your speed is fair but near the bottom for a 70 on a 20ft toon.I've seen tests with a new 50 on a 23 making 19 mph and on a 20 making 21.Of course weight balance is important and motor trim though on a toon that is pretty limited.Again without rpm and prop size it is pretty meaningless.The slow speed could be an indication you are over propped but without that info we don't even know where to start for sure.
And with no tach there is no way to know if the new prop is right.
Also would be good to check that the throttle opens all the way.
As far as the Hustler I don't think its a good bet for a pontoon.Solas amita 3 or 4 blade is ok.
Don't worry about diameter on the 4 blade except if there is a choice go for the bigger.With the 3 blade definately try to find the largest diameter.
Without the prop size and rpm its a real crap shoot.

 

[88fish"n"fun-toon]

Re: What prop size for 20ft pontoon with 70hp Johnson?

I will try and get the prop size this week when im off and can go back up to the lake. I will also see if my marina has a tach I could borrow. Should I just ask if they have an induction tach to borow? The the throttle plates are opened all the way. The balance of the boat isnt what I would like. All the weight is in the back. It use to be a fishing pontoon, but I took the two fiahing seats and livewell off the front. Sometimes I have to get someone to shift to the front to plane the boat out. Im liking the sounds of that four blade prop though.

 

[steelespike]

Re: What prop size for 20ft pontoon with 70hp Johnson?

Check out the TinyTach.Just be sure you have all the info about the tach before ordering.
There are are other induction tachs as well.
You could also use an optical tach. Maybe your marina will have a tach you could buy or borrow.

 

[88fish"n"fun-toon]

Re: What prop size for 20ft pontoon with 70hp Johnson?

Well I didnt get to go up to my boat today because of my work schedule getting changed. Looks like it will be saturday before i can get us some prop numbers and a tach reading.

I do have a couple of questions on just some general prop selections. I really like the things you said about the 4 blade props, but the smallest pitch for my motor seems to be a 13, so hopefully that wont be overpropping the boat. ( I know we wont know anything until further testing on this)

My other question is about the hydrus "pontoon prop". I see its designed for pontoons, but i cant seem to find any reviews or feedback on it. Its fairly expensive and that kind of makes me nervous that there isnt really any consumer info on it other than whats advertised. Ever had any experience with these props or heard anything in general about them??

Thanks again for your time and advice!!!

 

[88fish"n"fun-toon]

Re: What prop size for 20ft pontoon with 70hp Johnson?

I took my prop off today, but there are no identifying marks on it. The only numbers on it are the ones under the prop nut and they read 40409w and 01238f. Looks like it might have said michigan on it at one time, but its too worn to tell.

Since I dont know what im starting with, would you agree that a 13 pitch prop would be a good starting point?

 

[mikeygeo]

Hello .. I just bought a 70 horsepower Johnson motor for my 22 foot pontoon boat I took it out yesterday and could only get up to of 4300 RPM with a max speed of 17 miles per hour my prop size is 13 3/4 by 17 I no I need to go smaller I just don't no how small any help

 

[steelespike]

This is a 5 year old thread.
I do welcome you but you need to start your own thread. Should be a tab at the top of the prop forum.
A 13" prop is a reasonable starting point. but I do suspect your tach may be reading lower than actual.

 

[Rebelpeb]

Is the tach you were talking about a electric or water, we have a 24 foot fiberglass Pontoon Skipper Craft with a 70 hp Johnson the prop it has is a 3 blade #5179, what prop and tach do you recommend?

 

[GA_Boater]

Is the tach you were talking about a electric or water, we have a 24 foot fiberglass Pontoon Skipper Craft with a 70 hp Johnson the prop it has is a 3 blade #5179, what prop and tach do you recommend?

As steelespike said, this is still a 5 year old thread. Please start a new thread with your questions. Go to the top of the page and click on "Prop Questions and Topics" above the blue Shop Now banner, then click on "+ New Topic" to start a new thread. Thanks.