Advice on prop for pontoon

[speedmerchant]

I have a new to me 1989 20' Lowe pontoon (Fish'n Fun model) with a same year Johnson 40hp . I have had a few issues. The current prop is a 13 x 11, which was ventilating. The previous owner had a hydrofoil on the leg. I have no idea why on a pontoon boat. Anyway I removed that and it is much better but still suffers from some ventilation and over reving. If the tachometer is reading right, with the engine trimmed all the way down (cause if I trim it up it is more suceptible to ventilate) it is running at 6200 rpm. I know the range for this engine is 4500-5500. That being said this prop seems all wrong for this boat. My thoughts are to go with a 12 1/4 x 15 or a 12 1/2 x 13, based on my findings. I was intially leaning towards the 15" pitch due to getting the rpms down (and that 6200rpm is trimmed all the way down don't forget) but I read a post from silvertip saying about another pontoon that 15 pitch would be way too much pitch for a pontoon. Any thoughts are appreciated!

James

 

[steelespike]

I think your gear ratio is 2.42 I think your 11" is closer to what you need than a 15.
What is the best gps speed at the 6200?
With your prop on the edge of ventilation your slip is probably high and the tach may be off
as well. Can you lower the motor one hole?

 

[speedmerchant]

I am not sure if I can lower the motor anymore or not. Will check tonight. I'll run some speed tests as well. I did get a prop to try from a dealer friend of mine that just happened to have some old stock of a new 12.5 x 13 prop that has cupping. The current prop isn't cupped. Will try it all and get back to you, Thanks for the help so far.

 

[speedmerchant]

So got some testing in today and here is the results on this breezy day here with a GPS. (should be noted that this pontoon boat has a hard top over 1/2 the deck that catches the wind pretty good). I did check and the motor is lowered to it lowest hole. With the 13 x 11 prop 11 mph into wind and 15mph with the wind at 6200rpm. Basically anything over 5000 rpm and you can feel the prop slip and if i accelerate too hard it ventilates at nearly any speed. Put on the Johnson/Evinrude Hydrus prop 12.5 x 13. 16mph into wind and 18 with the wind and no ventilation at any speed at 5350rpm. Both prop tests were with 2 people on board full fuel and some gear. I left this prop on for about an hour cruise and it ran consistent 17mph. Seems like all is well. Any thoughts? think there is anything left in this boat? The previous owner said he had it to 20-21 on GPS with a prop he dinged and lost and didnt know what specs it was. Not sure I believe that. This boat also has the "U" shaped pontoons which I understand aren't the most effiecient.

 

[steelespike]

Calculator indicates 33% slip with the 13. Can't argue with 18 mph but I wonder if there still may be a better
setup.The Hydrus seems a good match
Pontoon slip seems to usually be about 20%.
Maybe check to see if the motor appears to be too deep at speed,maybe move the weight around.
If the boat is squatting just moving weight forward may help.
Maybe play with the trim a little. On our 18 footer trim just helped to reduce venting.

 

[speedmerchant]

Thanks, I'll experiment with the weight. I don't think the motor is running too deep as wide open I can see the cavitation plate just barely under the water but maybe that doesn't indicate that?. I tried the trim but anything other than full down you can start to hear the exhaust and she'll rev up and loose a hair of speed. I wonder if that silly hydrofoil might help to put back on if it is running too deep?

 

[steelespike]

Some folks hate the foils but I don't think they will arrest you if you try it.
They are supposed to raise the stern and reduce bow lift.
Be sure you have tried all you can, note the best speed and rpm.
It's possible a little more weight toward the stern will allow you to trim out a notch.
Have your crew start amidships move a little at a time toward the bow then toward the stern
You may only need to gain 1/4 to 1/2 an inch. you may need only 25 lbs at the stern or 100 lbs
5' from the stern.
Give the foil a try.I'm pretty sure it will cost some speed and in the process use more gas,
longer to get from here to there.