Prop Selection - 24 ft pontoon 85hp Johnson

[clemsonteg]

I have a 1988 24 ft pontoon with a 1978 85hp Johnson. I took it out this weekend with my recently installed tiny tach but I think partly I did my max rpm wrong. You can read my current thread in the Outboard forum:

http://forums.iboats.com/showthread.php?t=498166

The short of it is I could only get the boat up to 3500 rpms. The issue is the boat was not "lightly loaded" as I found from the guy at the local marine shop said it should be, which is basically just me and the gas in the boat. Instead I had myself and 3 other adults plus the cooler and 24 gallons of fuel on the boat. Being an '88 it also has the smaller toons on it.

The current prop is a slightly beat up (edges are a little rough) 13 3/4 x 15. The guy at the marine shop suggested a 13 or maybe even an 11 pitch due to the heavy boat, unfortunately I don't know the weight. With the current prop I could get the aforementioned rpm and around 16 mph with the load described above

Basically I wanted to see if that sounds around in the ballpark. I hope to get time tomorrow or wednesday to make another run with just me in the boat since I replaced the plugs and de-carbed the motor.

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

Yes a lightly loaded test will help figure a prop better.Be sure any weight is carefully balanced.
Toons don't allow much trim usually just try to get it with the least ventilation.
Check that the throttle is opening all the way.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

Well for a short time today it hit 4600 rpm and 21 mph until whatever is causing the low rpm came back. That gives me 36% slip with my 2:1 gearing. I'm thinking a 13 pitch will get me closer to 5000 and I shouldn't lose any speed

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

My info indicates 2.29 gear ratio.That would account for some of that high slip number.
Of course pontoons do have high slip numbers. I think your tach may be off 21 mph is about right and I don't think you could make 21 at 4600 rpm.thats about 900 rpm below the max rating.
As far as losing speed you may be losing a cylinder.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

For the gear ratio i put it in gear and with the plugs out and the prop marked I was able to make exactly 2 rotations to get the prop back in the same spot. As for the tach I'm using a tiny tach so I'm not sure how accurate they are

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

Maybe your motor is a 79 or has a 79 lower as according to my info they went to 2.00 in 79.
As far as the tach goes if you can do slip numbers you should be able to figure your speed
(with a planing hull) using your 2.00 and your rpm.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

The serial tag shows it as a 78 but I'm not sure what has it as 2.00 I read conflicting stories in the outboard forum some people said the manual had it at 2.00 and websites at 2.29. I took it out again today and got a better run at it. 4600 rpm 20 mph via gps and tiny tach. I'd like to go to a 13 pitch from my 15 that is beat up on the edges. This test today was also done with 3 adults and 10 gallons of gas.

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

I have no issue with your 2.00 ratio and going to an 11" pitch seems reasonable.Assuming the tach is accurate.Usually tinytach errors are very obvious.11" should put you near 5,500.
Your 20mph speed with your load at the time seems reasonable.A 4 blade prop will resist venting and have slightly better slip numbers Also usually respond better at the dock.
Shoot for a larger diameter if given a choice.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

I have no issue with your 2.00 ratio and going to an 11" pitch seems reasonable.Assuming the tach is accurate.Usually tinytach errors are very obvious.11" should put you near 5,500.
Your 20mph speed with your load at the time seems reasonable.A 4 blade prop will resist venting and have slightly better slip numbers Also usually respond better at the dock.
Shoot for a larger diameter if given a choice.

So if I go with a 4 blade I should still do the 11 pitch?

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

In a planing hull they usually suggest dropping an inch of pitch when going to a 4 blade.
I don't know with a pontoon.If you don't drop the inch it could lower the rpm by about 100.
I would be inclined to stick with the 11"
Keep in mind any prop change is an educated guess and actual results will vary
but I think the boat will feel a lot peppier though in a pontoon any increase in speed will likely be small.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

I'm having trouble finding a prop. I'm looking for around a 14" diameter and 11 pitched 4 blade for the above mentioned motor but so far it's no dice. Any suggestions on where to look?

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

Bump

So far the only thing I have really found is a 13 1/4 x 13 4 blade. It's a solas amita

 

[steelespike]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

There are 2 choices get the 4 bld. 13 1/4 The diameter is probably reduced due to the 4 blades.
Test it and have it repitched based on the results.
Or go with the 3 bld. 11" pitch.

 

[clemsonteg]

Re: Prop Selection - 24 ft pontoon 85hp Johnson

Boat did 4600 rpm and 21 mph today with the 4 blade 13 pitch prop. I guess my next step is to first clean the carbs because until the end of the day it would only do 3500 (I think something was stuck in the carb and then it broke loose) then it kicked up to 4600 and threw us all back in the seats as the speed jumped from 15 to 21 mph. The maybe have this prop re-pitched to an 11?