Cavitation and planing?

Discussion in 'Bubba's Outboards' started by sandiego, Jun 25, 2008.

  1. sandiego

    sandiego New Member

    Messages:
    2
    State:
    california
    i have been rebuilding a 17' rib-rigid inflatable boat.it is rated at 90 hp and 12 person compability-i put a 85 hp suzuki with 20" transom. the boat is a deep v and in the last 18" the bottom has a small triangle to plane on. the problems: should the cavitation plate be underwater? it planes now. Does the motor need to be deeper? the bottome of the boat is the same height as the cavitation plate.
    at full blast when i go across a boat wake the motor will cavitate and high rev. i put a dolphin fin on it and didn't seem to work to any better. I have a 15 pitch 3 blade propeller and have a light boat with little load shoud i use a different propeller?
    thanks for any input
    gene b.
     
  2. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    A doel fin is like an airplane wing. It creates lift using water instead of air.
    I've seen hydro foils on the tillers of sailboats for the simple reason of getting the stern up out of the water to create less drag.
    This is how the doel fin works to get a boat up on plane quicker. Its lifting the stern of the boat.
    In the same sense it will help prevent prop blow out too especially when turning.
    The cavitation plate should be about an inch above the bottom of the V. Thats a starting point.
    Too low and it causes tremendous drag.
     

  3. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    How many RPM's are you turning at WOT.
     
  4. justwannano

    justwannano Active Member

    Messages:
    1,003
    State:
    SE Iowa
    "Too low and it causes tremendous drag"
    So is this true with a jon boat also?
    I've always thought I ought to have more speed at full throttle.
    I also don't get very far on a gallon of gas.
    have a good one
    just
     
  5. sandiego

    sandiego New Member

    Messages:
    2
    State:
    california
    i am running about 4600 with a 15 pitch prop no load, the boat motor is rated between 4600-5200 rpm.
    thanks,
    gene
     
  6. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill


    If you are running a 15 pitch prop and it is a 90 h/p and ain't turning but 4600 i would do a spark test and a compression test because that motor should be burning the paint off that prop.






    And Bob that hold s true about the drag also on a jon boat or any other boat.
     
  7. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    Yep, ever seen someone put a 25" shaft motor on a 20" transom?
    People do it all the time:eek:oooh:
    They are dragging alot of motor under the boat.
    Cavitation plate should be at or a little above the bottom of the boat ideally.
    Any lower is like dragging an anchor. The shaft itself is plowing water.

    You'll notice two things that are industry standard on the stern of the boat.
    The heights of the transoms 20" or 25" and the angle of the transom.
    Not all stick with the angle to a T but most manufacturers are right there in the ball park if not on the money.
    The motor brackets are built with an industry standard angle built into them to compensate for the angle on the stern of the boat. Makes the motor hang straight in the no trim condition.
     
  8. justwannano

    justwannano Active Member

    Messages:
    1,003
    State:
    SE Iowa
    Thanks Mark

    Gotta re do my transom.:roll_eyes:
    At least I have experience lol.

    thanks again
    just
     
  9. justwannano

    justwannano Active Member

    Messages:
    1,003
    State:
    SE Iowa
    I've a 15" transom and a 18" distance between clamp and cavitation plate.
    Just so folks don't think I'm a complete idiot thats the same distance on the motor that was on the boat when I bought it. I still have that old motor.
    I've a new winter project. I believe I'll save the new/old bolt on transom.

    have a good one
    just