New prop still ventilating

[Barroga]

I have a 20’ 1997 Tahoe Alpine with a 2002 Mercury 50ELPTO 20” shaft. I bought a 3 blade10.8 11P propeller. The motor is as low as it is going to go on the bottom screw hole. WOT the cavitation plate is about 1” below the surface and a couple inches below the motor pod. I hit around 15-17 miles and then it ventilates. I have tried raising it to the last bolt hole and still cavitates. The Mercury prop finder tool suggest a 10.8 11P four blade prop. Before I go spending more money. Is this accurate to stop it from ventilating. Or is the three blade prop fine and I need to get the propelller below the pontoons. I can do a 5 inch extension from Bay Manufacturing.

 

[jbcurt00]

cavitation or does the prop ventilate?

suspect its ventilation

 

[Capt dave 258]

Is it possible you have a spun hub on your prop and it's slipping under load?

 

[Barroga]

It’s a brand new prop. I look at new pontoon boats with 200hp and their props are below the bottoms of the toons. I bought a 4 blade 11p prop. If this doesn’t work, I might fabricating a homemade manual jack plate to lower and raise my motor before spending money on a 5” extension from Bay manufacturing.

 

[jimmbo]

How are you trimming the Engine?

 

[Silvertip]

I take it this is a pontoon and if so, you really should have a "Big Foot" motor which has better gearing and can swing a much meatier prop. Your current gear case has 1.83:1 ratio whereas the Big Foot would have 2.33:1. 14x9 props were the general prop for those engines in pontoon service.

 

[roscoe]

The motor is as low as it is going to go on the bottom screw hole.

s. I have tried raising it to the last bolt hole and still cavitates.

This makes no sense.
Why would you raise it?

You need to lower the motor.
What was your original prop?
Why did you replace it?

 

[Barroga]

This makes no sense.
Why would you raise it?

You to lower the motor.
What was your original prop?
Why did you replace it?

It has three holes. Tried all three. I must have misspoke. Motor is as low as it will go. Resting on the engine pod transom. No original prop. Mercury prop finder indicated I should use this prop.

 

[Barroga]

I take it this is a pontoon and if so, you really should have a "Big Foot" motor which has better gearing and can swing a much meatier prop. Your current gear case has 1.83:1 ratio whereas the Big Foot would have 2.33:1. 14x9 props were the general prop for those engines in pontoon service.

This is what I got so trying to make it work.

 

[Barroga]

How are you trimming the Engine?

Seems happiest when near level.

 

[racerone]

Need to lower the motor / prop more.-----Add one of those whale tail gadgets.-----Will keep air away from prop.

 

[oldboat1]

Does the hull design create turbulence around the motor (possibly a bracket or brace)?

The pontoon may need weight distributed toward the stern.