21ft pontoon prop question

Joetomass

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Joined
Jul 16, 2012
Messages
26
I have a21 ft pontoon with an Johnson 90hp want to try and get little more top end speed current speed is 24mph at 5200rpm with a 14x11 prop. With 2 people and very light gear. I had a 13.75x13 prop and was getting 5000 rpm and 24 mph. GPS. Any ideas on this. had motor gone over and they said everything is fine, also installed a water glide this summer but didn't give me the increase speed we hoped for.
 

Rancherlee

Chief Petty Officer
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Jun 6, 2006
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621
Re: 21ft pontoon prop question

I have a21 ft pontoon with an Johnson 90hp want to try and get little more top end speed current speed is 24mph at 5200rpm with a 14x11 prop. With 2 people and very light gear. I had a 13.75x13 prop and was getting 5000 rpm and 24 mph. GPS. Any ideas on this. had motor gone over and they said everything is fine, also installed a water glide this summer but didn't give me the increase speed we hoped for.

what year Johnson? 5000rpm sounds right for an old Cross flow 90hp but is way off for a 90hp 4 stroke. 24mph would be dang good for a 21' with a 90 without the waterglide but with a PROPERLY setup water glide should see close to 30mph I would think. Got Under skinning? that makes a pretty good difference with a 3rd toon or waterglide by keeping the crossmembers from catching the spray. I've seen one pontoon with a waterglide and the owner said it took 3 or 4 trys to get it working well and that it was pretty finicky about how he loaded the pontoon with passangers.
 

steelespike

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Apr 26, 2002
Messages
19,069
Re: 21ft pontoon prop question

Your just about dead on for your setup. 23-24 mph.11"-13" props.It appears that the 11" has a very good slip at about 11%.
Perhaps you were running down wind or current.Still a very good number.
You could try raising or lowering the motor.but don't expect much.
The 11" slip makes me wonder if a 10" or 12" might do better. If they did I wouldn't expect more than 1 mph or so.
So are you still at 24 with the water glide?What year is the motor?
 

Joetomass

Cadet
Joined
Jul 16, 2012
Messages
26
Re: 21ft pontoon prop question

It's a 1997 90 johnson 2stroke
It is skinned and I am getting the same rpm and mph with or without the glide. I'm more around the 23mph but if I'm luck I can hit 24 mph. Was thinking same thing as more glide around but when I spoke to them the said prob can't go up or down due to I have smaller U shaped toons.dont know about moving motor up I think it might cavitate. I would have thought the same 30 or so with the glide. What else can I try
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: 21ft pontoon prop question

I don't see you getting much more than a couple of mph.If you have no venting(cavitation) now, raising the motor may be a possibility.
If the motor turns out to be excessively low could add some meaningful speed.
A 4 blade prop or a specialty pontoon dedicated prop. A 10" pitch should put you at about 5500 rpm.
Might continue the good slip numbers of the 11.You might try careful weight placement or elimination.
Motor trim is usually strictly for the least ventilation but might get a whisker more speed at just the right point.
 
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