Cavitation Problem

Discussion in 'Bubba's Outboards' started by ooonnnery, Mar 16, 2006.

  1. ooonnnery

    ooonnnery New Member

    Messages:
    35
    State:
    Bartlesville, Oklahoma
    I have a 25 horse Johnson on my fishing boat, I have acouple of props, the one that is currently on the motor is a 6x6, the other is a 9x10. Last time out on the water I noticed after getting up on plane, it start caviating till I cut the trottle back. It would do this each and every time I went about 100 yards or so , Do I have the wrong prop on ?
     
  2. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    You didn't say what kind or size boat the motor is on, but ?6X6? That's tiny. The 9X10 sounds more appropriate, unless you're pushing a barge. At least that's my opinion. How long has the 6" been on there? It could be the problem. How many rpm's are your turning. At any rate, I would check the condition of the prop for excessive wear, also for damage to the motor's cavitation plate, boat transom, etc for anything out of normal. It sounds like you might be experiencing 'blow-out' with too small a prop, but it's hard to say without knowing a little more. I'd sure like to know what you find out, though.
     

  3. ooonnnery

    ooonnnery New Member

    Messages:
    35
    State:
    Bartlesville, Oklahoma
    My boat is a 14 foot V-botom, as for the RPM, no Idea. The boat has had modification to it, installed carpeted wood floors, and a deck up front ( 3/4 marine plywood. As for which prop I had on there, sheeesh, I dont remeber. To me, the 6x6 prop looks large ( prop blades ) than the 9x10 blades. The 9x10 is a steel prop, and the 6x6 is aluim. both props look good, no dings or bend blades.
     
  4. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Bob, I'm confused a little. Those numbers are supposed to represent prop diameter and pitch. The 9X10 should be significantly larger than the 6X6. In fact, I've never heard of a 6X6 for that motor. Let me check some listings for the motor and get back to you. That number doesn't sound right. Could it be '9X9'?
     
  5. Matt Smith

    Matt Smith New Member

    Messages:
    119
    State:
    Tennessee
    Turn the prop over and read it. Does it now say 9x9?

    What year is your motor? If you don't know, then post up the model number. Some 25's came with 9x10 as standard and some came with 9.25x 11. Newer ones come with an even higher pitch.

    Post up the year of your motor or the model number, and someone will be better able to advise you.
     
  6. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Is the motor speeding up and the boat slowing down?

    You may have a prop slipping on you. If its not slipping then try lowering the motor a little and get the prop deeper in the water.

    I am with blackwater on this the numbers don't compute in this old head.
     
  7. blackwaterkatz

    blackwaterkatz Active Member

    Messages:
    3,659
    State:
    Andrews, SC
    Bob, I did a quick check on cabela's listings. The 9X9 is the smallest Michigan Wheel listed, then 9X10, then 9-1/4X11, like Matt said. Try the 9X10, it's kind of an all-purpose prop. The 9X9 would be for a heavier load, and the last for a light load. What I don't understand is that the 9X9 appears larger to you than the 9X10. The diameter should be the same on both, and it's almost impossible for most of us to see the difference in 1" of pitch with the naked eye. Does your motor use a shear pin, or is it a splined shaft? The clutch could be slipping, like Bubbakat said. I've had that happen before. You go a little ways, then it starts slipping, pretty soon you can only idle to get back to the landing (hopefully).
     
  8. ooonnnery

    ooonnnery New Member

    Messages:
    35
    State:
    Bartlesville, Oklahoma
    Okay, heres the info you requested: 1976 Johnson 25 horse, it has the long transome ( long shaft ). the prop uses a shear pin. Thanks for everyone responding to this.
     
  9. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    From what you describe, those are symptoms of a slipping prop hub.

    Sometimes you can tell by looking at the inside area of the hub and see little grains of rubber from the insert against the prop housing.. It's hard to tell by trying to hold the flywheel and turn the prop, but if you can't get on plane without it cavitating, your prop would be slipping bad and easier to tell.

    If your other prop doesn't cavitate and the 9x9 does, then it's a slipping hub.

    I've also had broken shear pins that would hold enough to move the boat, but under more pressure, would slip at the break, much like a slipping hub.
     
  10. ooonnnery

    ooonnnery New Member

    Messages:
    35
    State:
    Bartlesville, Oklahoma
    Just alittle update to this problem.
    I had a boat mechanice come bye the house to work on another moter ( 18 horse Johnson motor ), and while he was working on this motor I descibed the symptoms of the 25 Horse johnson. I told him recently I had the lower unit off, and installed a new water pump. He asked, did the caviation problem start after doing this, I told him yes, he then said, it was likely that the shifting rods may not be alined correctly, he said just being off alittle, it would or could jump out of gear when giving it full throttle for a few seconds. He is coming back over tomorrow, and he will look to see if this might be the problem. I did check what some of you meantioned, about slippage of the prop ( shear pin ), and didnt find any evidence of this being the problem. I will post what we find out, and I thank you all once again for your responses.
    Bob