Cavitation Ventilation Problem 80hp outboard

[imported_Cpt_Quint]

I have restored my 18ft aluminum Bluefin and have a cavitation/ventilation problem.
The boat had this problem before the transom rebuild but is worse now. I have a 20in shaft on my Mercury 80hp 4cyl but the transom is now 20 ? inches.
The motor runs at 5900 to 6000 RPM WOT that?s just right according to the book I have. The prop is a new aluminum three blade.
The prop cavitates when?The RPMs are at approx 3500 and I hit the gas kind of hard and any time I make a significant turn.
I have tried all tilt positions.
The cavitation plate is maybe ? in above the hull line. I?ll bet if it was 2 inches lower this prob would go away, but that?s not an option.
The hull is not perfectly flat and I?m sure delivers a lot of air near the prop but I cant fix that either.
Would a four blade prop help? What about one size bigger ? the hole shot is not a concern. How about a hydrofoil?
What do you guys think? Help??.

 

[WillyBWright]

Re: Cavitation Ventilation Problem 80hp outboard

A cupped stainless will help no matter how many blades. I can't tell for sure, but the aluminum looks uncupped. Cupping that would help as well. Prop shops do that.

 

[steelespike]

Re: Cavitation Ventilation Problem 80hp outboard

Not an expert on these Mercs but the rpm range in my book is 4,500 to 5,500.
I see that the 85 hp is in the 6,000 range.
You could try a prop with some cup in it that would bring the rpm down a little and may help the venting.
The propulse prop is a 4 blade adjustable prop that I have heard does well.
4 blades is supposed to help low end performance and decrease venting.
Generally top end will be somewhat less though I have seen some that increased top speed signifigantly.

 

[JB]

Re: Cavitation Ventilation Problem 80hp outboard

I think this would get more attention in the Prop Forum.

 

[Esox]

Re: Cavitation Ventilation Problem 80hp outboard

What is that mounted on the right side of the transom that is close to the center. Transducer? If thats what it is moving it may help.​

 

[WillyBWright]

Re: Cavitation Ventilation Problem 80hp outboard

Good call Esox! That would be disturbing the water flow to the prop. Anything like that should be at least a foot either side of centerline.

 

[imported_Cpt_Quint]

Re: Cavitation Ventilation Problem 80hp outboard

There are two items showing in the picture. One is a cord attached to the drain plug and the other is a temp sensor that has been removed. The transducer is on the other side 14 inches from center.
I will be checking with a local prop shop about cupping this prop or going up one pitch on a cuped prop.