85 HP Johnson pushing 24 Foot Pontoon

[jiju1943]

I am totally overwhelmed with all the high tech information about props, it is way over my head. I just need to know which prop, of the two I have, will work best for mileage and one that will have enough power to push the toon back on the trailer. We don't plan to do a lot of riding around because we fish a lot and don't care a lot about cruising around, so speed is not a concern, mileage is. The prop on the 85 hp Johnson now is an aluminum 13 3/4 X 15 3 blade. I have another 85hp Johnson on an ole bass boat and the prop on it is a 4 blade composition, there are several numbers on the 4 blade 4X323X483 also 4X127X19, there is another number but I can't read it. Do these numbers mean anything to anyone?

By the way, the first time out the motor was running really bad and we burned 5 gallons of gas in 4 or 5 miles, I am still trying to get the engine straightened out but that is another story. I really appreciate and help anyone can give me.

 

[steelespike]

Based on your requirements you can consider the 15" prop a starting point.
The 19 is of no use here.
Regardless of your needs the prop needs to allow the motor to operate within its wot rated rpm range.
Once set you can operate at what ever throttle setting you prefer.
Once you have the motor running right you need to do a lightly loaded wot test run ,get us the rpm and gps speed.
Try to set trim and weight for the best speed.I believe the rpm range is 4500 - 5500 with the hp developed at 5000.
Your motor will use about 10 or 11 gallons per hour at wot.An hour at 1/2 throttle could be 5 gallons.
Obviously Messed up carbs could cause trouble.You could be down one cylinder and not realize it.
A leaky fuel pump diaphram could waste fuel.

 

[jiju1943]

Thank you steelespike, I appreciate that information, I need to get a GPS and hook my tach up.

 

[jiju1943]

That motor is now a parts motor, I have another 85 hp Johnson on the toon now. Haven't been to the river with it yet but I can tell you it sure runs much better than the other one. After I took the engine off and put this one on, I found a few problems. The old one, I was having so much trouble with, had two coil wires crossed, a hole in the fuel pump diaphragm and the prop shaft was bent as well as the prop. I am amazed it ran on two cylinders, no wonder it used so much gas.

As I said in my first post above the motor on the toon now has a four blade prop and has the numbers 4X323X483 also 4X127X19. This motor pushed my old bass boat pretty good except it would cavitate when I nailed it. I don't care about speed or I wouldn't have bought a toon. LOL We just want to fish, not burn a ton of fuel, like it did on the first time out and have enough umph to push the toon back on the trailer. Any recommendations on the prop size?

steelespike, I still don't have a GPS and I have been too busy trying to get a motor to work on the toon, hopefully now I can do that.

 

[Sea Rider]

wot rpm is more important than gps readings, gps reading is only a speed reference as how fast you're going at a given rpm. Go for a wot spin very lightly loaded on calm flat waters with engine trimmed at 90? and weight evenly distributed on deck. Check achieved wot rpm and compare them against factory stated safe wot range. Will know if actual prop is doing its homework or not and go from there maximizing your prop for best overall water performance at any throttle range with efficient fuel burn.

Happy Boating

 

[steelespike]

You need the gps reading to help evaluate the tachs accuracy. The gps gives us a 3rd number we know is accurate. gear ratio,prop pitch,gps speed.
As you say rpm is critical but only as good as its accuracy.You can check it against another tach and/or the gps speed.

 

[steelespike]

Your 19" is to much prop A 13 or 15" would be a good starting point.If the motor is really healthy a 15 should work a 13 if it is a little tired.
As Sea Rider suggests its important that your motor is able to operate within its wot rpm range.
Once setup you can operate at what ever speeds you prefer.

 

[jiju1943]

Wow, thanks guys, I have a tach, now to figure out how to hook it up. I will let you know what happens. Thanks again.

Jim