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Pontoon/ Tritoon update: what is cavitation caused by? Mercury service bulletin and fix

larrywittlarrywitt Posts: 2,841 Moderator
The pictures will show a new part that fill the gap between the prop and housing. The motor is a 2018  mercury 150HP  it is supposed to solve the problem. Motor is brand new;  why did he have to buy the part? He did buy 2, one is in and one as a spare.
It is a snug fit. He has not run the boat since it was installed. 


  • tilemantileman Posts: 1,141 Officer
    WHAT!!! Gotta see this!   This is Mercs answer to his problem?  
    Here's ta swimn' with bowllegged women!
  • kmagnusskmagnuss Posts: 3,237 Captain
    I'll be surprised if that fixes it.
    Tarpon... everything else is just bait.
    Captain Keith - Crooked Rod Charters
  • livebaitlivebait Posts: 793 Officer
    $12k plus on a motor and gotta buy an o-ring? WOW!
  • K-DawgK-Dawg Posts: 235 Deckhand
    I think that part should have been on the engine from the factory.  It's meant to fill the gap between the 150's 4.88" casing and props for a 4.75" casing. It keeps the exhaust gases inside the prop rather than escaping around it.

    And it's called ventilation, not cavitation.
  • larrywittlarrywitt Posts: 2,841 Moderator
    K-Dawg said:
    I think that part should have been on the engine from the factory.  It's meant to fill the gap between the 150's 4.88" casing and props for a 4.75" casing. It keeps the exhaust gases inside the prop rather than escaping around it.

    And it's called ventilation, not cavitation.
    In any case it did make a  and a.lot of bubbles behind the boat and a top speed of  16 MPH  wide open, last time out.
     The water was well ventilated.
    I am very interested to see how the fix works. I hope it is a perfect fix for his sake
  • larrywittlarrywitt Posts: 2,841 Moderator
    oes Your Prop Have Secret Cavitation? - 01/23/2019

    Have you ever run your finger along the face, or edge, of a prop and felt a pitted area? If you have, then you know what cavitation can do to a prop. Given enough use, that pitting will only get deeper and you will end up having to replace the prop. The causes of cavitation have always been somewhat of a mystery to us, so we asked some prop experts to give us the technical explanation for this phenomenon.

    Does Your Prop Have Secret Cavitation Cavitation 1
    With a prop turning 4000 rpm, how can a boater know if he or she has power-robbing and prop-damaging cavitation?

    Cavitation very commonly occurs on damaged, or imperfect, propellers and can occur if an attempt is made to transmit too much power through the prop. At high rotating speeds or under heavy load (with high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapor pressure of the water, resulting in the formation of a pocket of vapor, which can no longer effectively transfer force to the water.

    To our thinking, cavitation could be defined as the phenomenon of the formation, rapid collapse, and subsequent implosion of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. As these bubbles collapse, energy is released in the form of a shock wave that can, and often does, damage the surface of the material in question. (Stretching the analogy to a screw, we might say the water thread 'strips'.)

    The true cause of cavitation can arise from several factors, but we see them occur most often as a result of leading-edge imperfections, such as nicks, dings, scratches, bends, etc. That said, they can also be caused by improper geometry or upstream imperfections related to the hull, such as hull design/configuration, hull imperfection, incorrectly installed accessories/appendages, etc. Since there are so many possible causes, diagnosing the problem should really be done on a per-case basis.

    Does Your Prop Have Secret Cavitation Cavitation 2
    This drawing shows where cavitation can occur. 1.) Generated cavitations 2.) Cavitations coved backside of the propeller 3.) Cavitations vanish away from propeller 4.) Turn 5.) Ship's driving direction 6.) Water stream based on a propeller.

    Cavitation wastes energy, makes the propeller "noisy" as the vapor bubbles collapse, and most seriously, erodes the prop’s surface due to localized shock waves against the blade surface.

    A similar, but quite separate issue, is ventilation, which takes place when air is introduced to water around, or to the water flow fed to, a propeller. This can be caused by a number of different sources, such as porting, over-and-through hub propellers, tunnel hulls, extreme motor elevation, excessively high trim angles, stepped hulls, or even hull appendages (transducers, pick-ups, and the like). As the propeller meets the aerated water, the water's flow attachment to the propeller's respective working surfaces is detached. As the water detaches, the propeller's loading is reduced, and the torque applied to the propeller through the shaft can more easily spin the propeller to a much higher rpm even at the same given power input. This not only causes the motor to rev easily, but it reduces the thrust provided by the propeller, slowing the boat's speed and/or acceleration, as well as reducing the propeller's control over the boat (blow out on turning, on holeshot, etc.).

  • tilemantileman Posts: 1,141 Officer
    I still think that motor is up to high, someone needs to go to the rear of the boat while this problem is happening. That's the only way to make me believe that's not the problem. Sorry I'm a thick headed Italian. I think that's where the starting point. Now I have been known to be wrong so I hope this isn't the 2nd time 😰
    Here's ta swimn' with bowllegged women!
  • Circle-HookCircle-Hook Posts: 503 Officer

    I have purchased 2 of the same motor, a 2017 and a 2018. Both came with the ring installed in the housing when I removed the shipping box. It probably did come with one.  These rings fit sloppy- however at 175 hours the on 2017 the ring is still there.

    It is my understanding it is there to help with an exhaust over prop issue. I agree with motor too high comment.

    Obviously I think these are outstanding motors or I would not have bought a second one. 

    I paid $9999 for 2017 in the box with full warranty plus 2 year extended,  $10,500 for the 2018.

  • tilemantileman Posts: 1,141 Officer
    Wow beat the hec out of me, I was wrong....... remember that happened to 
    me in 2001!  🤗 
    Here's ta swimn' with bowllegged women!
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