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  1. #1

    Default Prop Cavitating (catch air?)

    Have a new 11 ft Saturn SD330 inflatable, and used, ~2000 Nissan/Tohatsu 8hp 2-stroke, that we took on the water for the first time. Have an issue I'm hoping someone can help us with.

    Bear with my terminology as I have almost zero experience with outboards.


    For some reason the prop seems to cavitate (catch air?!) when we try to get up on plane, like a clutch slipping on a car. If I give it full throttle the prop will cavitate, rev very high, than catch water for a moment, drop RPM's, then air again.


    I've tried all 6-7 engine "trim" angle positions with similar results. Fwiw, the "top" part of the prop's outer turning radius is about 3" below the transom so I don't believe there's any issue with the Nissan's shaft length. The Saturn rear transom is about 14-15" deep. I believe the Nissan is a short shaft motor.

    If we take the time to adjust the engine trim angle just so AND my wife positions herself perfectly fore-aft we can prevent the prop from cavitating and can apply full throttle; however, the bow is high and it won't quite get on a plane. Plus, the boat feels like it's dragging an anchor. Clearly something is wrong.



    It cavitates riding solo in all engine trim positions. I only weigh approx. 140 lbs. My wife only weighs 110 lbs.


    Just to be clear, the issue is not so much not getting on a plane, but the fact the prop appears to cavitate (and over rev) under med/high throttle conditions.


    Do I have a bad prop? Current prop doesn't appear bent. There's some light nicks and wear on the leading edge, but that's about it.


    Thanks in advance for any help!

  2. #2
    Join Date
    Aug 2017
    Location
    South Africa
    Posts
    101

    Default Re: Prop Cavitating (catch air?)

    Inflatables does this, please post a pic of your setup, then I can steer you in the right direction ....

  3. #3
    Join Date
    Mar 2010
    Location
    Buffalo, NY, USA
    Posts
    2,073

    Default Re: Prop Cavitating (catch air?)

    Do you have trim tabs?
    The tilt should be adjusted so that the motor is vertical at speed.
    If you are still ventilating, then you will need the prop lower in the water.
    Paul
    Buffalo NY USA

  4. #4

    Default Re: Prop Cavitating (catch air?)

    Thanks for the replies!


    After a bit of troubleshooting we've discovered increasing air pressure in the floor/keel area reduces the cavitation/ventilation quite a bit. We also noticed the floor underneath the rear seat area bulges upward quite a bit when applying a lot of throttle. If I push down in this area with my feet, the rear of the boat rises up and it gets up on plane easily. According to my 12vdc air pump I have the correct air pressure, but you never know. Ordered a K-Pump air pressure gauge to verify the pressure. If it's low, I'm hoping the higher pressure solves the problem. If not, not sure what options we have left other than to constantly stand on the floor where it bulges---not convenient to say the least.

  5. #5
    Join Date
    Aug 2017
    Location
    South Africa
    Posts
    101

    Default Re: Prop Cavitating (catch air?)

    I thought as much, that is why I wanted to see a pic to see what we are working with .... I would suggest going to the hardware store and cutting yourself a marine ply floorboard that fits snug on the floor, it will solve this issue, your ideal pressure for the floor is 14,5 psi and the main pontoons at 4,35 psi.

  6. #6

    Default Re: Prop Cavitating (catch air?)

    Problem solved. Discovered the 12vdc Boatstogo air pump wasn't putting out the correct air pressure. Although it's supposed to be capable of producing up to 12 psi (80 kpa) via an adjustable knob, it was only putting out a fraction of that even when adjusted for max output. Received the K-Pump air pressure gauge and pumped up the boat using a manual K-Pump to the pressures recommended by Saturn. Also installed a hydrofoil. Boat now gets up on a plane with no problem with zero cavitation/ventilation issues.

  7. #7
    Join Date
    Aug 2017
    Location
    South Africa
    Posts
    101

    Default Re: Prop Cavitating (catch air?)

    Great! Glad you got it sorted.

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