Prop issues with pontoon boat

[Bnewton1951]

This is a great site! This is also my first time posting a question. I'm sure someone here will have the answer I'm looking for.

1) Performance issue you are trying to correct.
Need to increase WOT RPMS
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Michigan Wheel 4blade aluminum originally a 15x16, repitched to 15x14 and cupped
3) Current prop diameter and pitch (required).
15x14 with added cup
4) Wide open throttle RPM and speed with an average load (very helpful)
4600 at 27mph
5) Engine/drive make, model, year, and HP
1988 Evinrude 150 XP
6) Boat make model, year, length and weight
1986 StarCraft Pontoon, 24' (SD 240) 3200lband
Test run with one person, 20 gal fuel, calm water
RPM by in dash tach-new
MPH by GPS

The boat was ventilating consistently at about 15 mph prior to repitching and cupping. No ventilation now, but RPM'S too low. I don't think I can repitch this prop any lower. Suggestions to prevent ventilation and get RPM'S into the 4500-5500 range? Thanks for the help.

 

[steelespike]

I'm thinking that when you added cup to the prop it tests like a 15" so you only went down an inch in pitch.
If you want to work with that prop have them remove some cup.
Your within range at 4600,granted at the bottom end I have seen some amazing results removing some pitch.
How is the performance,keeping in mind your all up weight must be close to 4000 lbs.
Using 15" pitch the calculator says about 23% slip at 27 mph.Fairly typical for a pontoon.

 

[Bnewton1951]

Thanks for the quick response. The two goals for depitching and cupping the prop were to get RPM's into range and stop the ventilation problem. Ventilation cured. If I have some cupping removed do I risk ventilating again? The motor is set in the lowest position. Some posts I've seen recommend raising the motor to decrease the drag from the lower unit and prevent ventilation. What do you think? Any suggestions for a different prop if this one can't be made to work? Thanks

 

[steelespike]

Before going any farther you may want ti verify your tach is accurate.It wouldn't be unusual for a 26 year old tach to be off a bit.
Anytime you tweak a prop there is the chance of unwanted results whether buying new or a modification.
While your original 16" is within typical pontoon application a brief search produced a slightly more common 13" application.
That boat is sure no fly weight.
Pontoon prop sizes and speeds are all over the place I do wonder your rpm and speed when the original prop had a good grip.
​ If the 16 was indeed a little too big it might introduce some ventilation problems and perhaps higher slip numbers.
The motor develops its hp at 5000 so it seems you really need to be there or more when your done.
I think a 13" pontoon dedicated prop may be the way to go.
Keep in mind nothing is carved in stone in prop selection.

 

[Bnewton1951]

Been out of place for a bit. Update on prop-Prop shop was able to go one more inch down on pitch. Should now be about the 13 that you mentioned. WOT now 4700 at 24 mph by gps. Tach is new, but could still be inaccurate, I agree that 5000 rpm should be the minimum target. Next change will be to raise the motor to see if reduced drag will improve rpm's without ventilating. BTW-no ventilation now. Thanks for the comments Everything helps. It's getting better step by step.

 

[Bnewton1951]

Update. Raised motor one hole and it began slipping too much. Got 5200 rpm but only 20 mph. Put it back down and will use as is. I'm open to any other suggestions.

 

[steelespike]

Repitching that far seems like a real stretch, literally.
I would definitely feel that would be questionable compered to a natural prop.
I think you would have been better off with slightly less cup.
I've seen some amazing results on here with reduction in cup.