Zodiac acti-v cavitation

[BCRyan]

Hi, hoping to find a solution for my cavitation issue. I’m new to the SIB inflatables, always had Jon boats previously. So, I have a zodiac club fastroller acti-v that was given to me and a 9.9hp 2stroke I had already, so when I get the boat just on plane it starts with the cavitation. Rpm revs and prop seems to be pushing air and then doesn’t wanna go anywhere . Now I am using a 9.9hp long shaft that I have extended the transom to where the ventilation plate sits about 3 inches below transom base. I know the suggestion hight is 1-2in bellow transom but wouldn’t being even lower prevent the cavitation issue? Should I raise it more or lower it? I’ve tried all the different trim positions and same if I’m solo in the boat or with a passenger.. would a hydrofoil help solve the problem?

Not sure which direction to go here and help and insight would be much appreciated.
Thansk

[Fenlander]

Hi and welcome to the forum. Air floor setups can sometimes be quite tricky to get right to avoid the problem you're having.

I've owned two 360 Fastrollers and two 340 Fastrollers and run them with both 10hp and 15hp 2-stroke Mercury motors. My motors were short shaft and set with anti-ventilation plates about 1" below the transom bottom.

We ran loads between two adults and two teens plus dog and kit... to one adult and one light teen. 90% of the time these boats were free from prop grip problems but certain wave conditions or perhaps crew in the wrong place could spark off a brief slip.

The basics are really important with these boats that you have a good pump to get full up to pressure on the air floor in particular and that your pressure gauge is accurate enough to know that pressure is correct. Also important to recheck pressure once the boat has been on the water for a while as it's likely the cooling effect will have dropped the pressure a bit. Also you can't afford to have even the smallest leak say from the valve cap on these air floors as they will soon drop below critical pressure while still feeling quite hard to the hand.

The only time I suffered constantly from prop slip was when I bought a brand new spare prop which was a Solas brand not an original Mercury. The blade design was a bit different and it caused frequent and annoying slip so I got it refunded then bought a new OE Mercury prop and that was OK.

So I would check your prop type/make and blade condition... plus of course making sure the rubber prop hub isn't slipping.

Regarding your use of a longshaft I guess in theory it ought to grip more deeper in but I would be tempted to bring it up to a standard setting if your transom mod allows... or consider swapping for a shortshaft.

[homer]

Fenlender - is there a mount you can attach to the Zodiac transom and then attach the Mariner so you can adjust the motor up and down.

[Fenlander]

If you Google outboard or transom jack plate there are both fully adjustable and cheaper multi position devices available.

But they are a bit over the top for a SIB and 15hp. Usually the tried and tested method with various thickness packers until you find the best height works fine.