Prop Size? 30HP Johnson on 20' Pontoon

[shawnswan]

I lost my prop this weekend, luckly I was just pulling into the marina. I need a recommendation on what size of prop to get. I've got an '85 30HP Johnson on a 20' Pontoon boat.

Thanks.

 

[steelespike]

Re: Prop Size? 30HP Johnson on 20' Pontoon

It is difficult to know what prop would be best without some sort of base line performance.My guess is about a 7" pitch.It may be a little light but its better to be light cause you can still use it and just throttle back so you don't over rev.A four blade pontoon specific prop will perform better than most 3 blade props.

 

[shawnswan]

Re: Prop Size? 30HP Johnson on 20' Pontoon

The guy at the local marina recommended a 11" pitch 10" dia. 3 blade. Does this sound right?

 

[steelespike]

Re: Prop Size? 30HP Johnson on 20' Pontoon

I'm not an expert but::
Doesn't sound right to me based on what I've compared it to.
The present day Evinrude 30 on a 900lb 16ft pontoon runs a 9" pitch
at 5,900 rpm and 17 mph. Same motor on a standard 600lb 16ft Starcraft
outboard runs a 12" pitch at 5,800 rpm 27 mph.
Your 20ft pontoon has to be 1200 or 1400 lbs dry.These motors are similar
though gear ratios may be slightly different Your motors hp is rated at 5,500, like the Evinrude, your rpm needs to be between 5,500 and 6,000.
There is no way a 11" prop will do that.If he will let you try props to get the right one you could start at a 9" and go from there to get the right rpm.
If you run with the family most you need to shoot for the higher rpm so
the motor will stay within its rpm band when loaded.
Again my thoughts on a 7" were you could run it even if a little light where a 9" if to much you shouldn't run it at all because it will make the motor lug.
Could cause premature engine failure.A 4 blade pontoon prop will deliver the best performance.