Pontoon Boat Cavitation I/O

[morgan.rinehart]

I am having an issue with Cavitation on 1996 Harris Pontoon with Mercruiser 3.0 I/O and Alpha One. This boat is new to me this season but have always been around Runabouts with the same motor so am familiar with the 3.0. The first run after purchase this year I experienced cavitation when passing through wakes or anything above 3000 RPM. I experienced cavitation all the time unless it was fully trimmed down.

Reviewing the setup I decided to remove the Stabilizer installed by the previous owner as on a non-planning hull it seemed incorrect to have a stabilizer. This resolved the cavitation issues with passing through Wakes and was able to maintain forward motion all the way to WOT 4400 RPM which was about 22MPH GPS with two people on board and I could trip up without losing grip.

Loaded the boat with about 6 persons on the next trip out and cavitation had returned with the added load however not all the time but certainly when passing through a wake it would trigger a blowout. Through a tube on and cavitation started at 2500RPM and could never get fully going.
My first thought was a Spun Hub so I marked the hub and jumped up and down on the prop and sure enough it moved but the whole shaft moved. So thought my day got a lot worse and it had to be engine coupler so I marked the pulley on the engine and sure enough all my weight was turning the whole engine so the coupler looks okay.

Running out of ideas I keep thinking that the boat could not have come from the factory this way and had to work correctly at some point. There is no moving the Outdrive the height is fixed and would say the cavitation plate is about 6" above the bottom of the pontoons.

Could it be the prop selection? Currently it has an OEM prop 48-78114A40 13P which looks to be 16" with 13 pitch and it's a three blade. Mercury shows it as one of 3 available for Non-Plane / Heavy boats and the A40 I think indicates added Cup.

Does anyone have any ideas? I am starting to pull my hair out or what is left of it. I thought about getting a 4 blade but don't like throwing money at something that is not fully understood only to have the same results.

 

[MaPaHa]

When it was loaded it sounds like the weight got shifted to the front since it ran fine unloaded. I think the RIGHT 4 blade prop would make an improvement. I have a 4 blade on a fish and ski boat with a 175 hp and also on a 22 foot tri-toon with a 150 hp. They seem to get a good bite but I don't think you can pick just any 4 blade and stick it on there. I've found that most boat shops that sell props don't know as much about application as you would expect. I use Power Tech props and they have always been willing and helpful to select the right prop for your application. They won't sell directly to you so you have to find a dealer but that's not hard. My last one I bought from them for my mid 90's Evinrude 150 on a 22 foot tri-toon and it's their stainless "big ear" Off Shore series. I think they are made to hold their grip in rough water but it's what they recommended for my boat and it works great. I told them what the boat is and the performance I was looking for and they gave me the specs on it and I ordered it through one of their dealers. If you get a dealer that wants to change the spec, I wouldn't do it. Just go with what they say. Ask about their return policy within 30 days if the rpm isn't exactly right the first time. Good folks down there.

 

[BigDfromTN]

I have a 90hp fourstroke OB. Last season experienced much of the things you relate. Prop was original and had some dings in leading edge. Tried a couple 3 blade props and then went to the 4 blade with good results.

Find a local prop shop that will sell you a prop and let you return it if the results do not make you happy. That is what I did and it was worth the little extra I had to pay for each of the props. Actually just bought the 3 blade and then traded for another 3 blade, then paid the difference in the 3 and 4 blade prop. No regrets here!

 

[morgan.rinehart]

Thanks for the advice. Going to give them a call and start down the path to getting a 4 Blade. Looks like that will solve my issue.