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Prop Cavitation/Ventilation or Transmission Slipping

Discussion in 'Hatteras Yacht' started by mattm, Apr 18, 2015.

  1. mattm

    mattm New Member

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    How can I tell if I'm suffering from prop cavitation/ventilation or a slipping transmission? I have a 1988 Californian with Caterpillar 3208's. I'm unsure of the prop size and pitch. At about 1900 rpm's the starboard engine revs to about 2200 rpm's then returns to 1900. I've not pushed it further but my mechanic has recently run it WOT (about 2400 rpm) without experiencing the issue.
  2. PacBlue

    PacBlue Senior Member

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    Cavitation and/or ventilation will usually show physical signs on the propeller itself. This usually requires a haul-out or a diver and clear visibility, maybe a GoPro camera for a first try. A haul-out would also give you the dimensional information stamped on the propeller. I do not think the speeds are very high for your boat at this rpm, so I am suspecting something mechanical without a picture of the propeller itself.
  3. NYCAP123

    NYCAP123 Senior Member

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    What physical signs would show on the prop?
  4. SFS

    SFS Senior Member

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    Pitting damage, usually (but not always) on the back of the blade in a radial pattern. The negative pressure on that side of the blade causes any gases present to come out of solution, and those bubbles have impact effects on the blade material.
  5. PacBlue

    PacBlue Senior Member

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    Yes you would see physical erosion / pitting on your propeller. Also look at the propeller hub where the blades originate from. You may even see some discoloration of the blade material. You have mechanical throttle linkages for this era of boat, unless someone has upgraded to electronic throttles, which would still have some mechanical attachment point to a potentiometer to run a mechanical 3208 throttle position. If it is only one engine experiencing this, compare the two set-ups, port and stbd.

    Also, check your gear oils, port and stbd, see if they look same / smell the same.
  6. Bamboo

    Bamboo Senior Member

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    The above posts are describing a condition commonly referred to as "prop burn". If there is a lack of smooth surface in front of the prop/shaft- ie hull protrusions- then prop burn can be found sometimes. Viking built some hulls with larger engines in the early 2000's which needed a slight hull alteration to accommodate those engines; prop burn was common on those hulls while being non existent on the same hull without those alterations.
    However that is but one reason for prop burn- and unless most of the same hull/engine combination have prop burn I'd cast doubt on identifying this as the issue here.
    Transmission slipping would result in slower prop rotation- not prop burn or cavitation.
    Last edited: Apr 21, 2015
  7. PacBlue

    PacBlue Senior Member

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    The propeller burn is a reference to the physical state on the prop. It can be a on the back side of the blade as a result of true propeller cavitation, or it can be on the prop hub or front side of the blade as a result of an air bubble from the disturbed inflow exploding / vaporizing on the surface. I have seen and fixed this many times.

    The OP's application is not a high hp / high speed vessel, so I would guess, due to lack of data, that he is referencing a mechanical problem. Pictures of the propeller would go a long way towards eliminating any issues concerning his propellers.
  8. Bamboo

    Bamboo Senior Member

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    I agree that it is likely a mechanical issue somehow given the information we have.
  9. Capt Fred

    Capt Fred Senior Member

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    From my experience, when I have cavitated, there is a lot of noise and vibration along with an increase in engine rpm. When the tranny slips I do not hear or feel much in the way of noise or vibration. When the tranny slip all I detect is the engine increasing in RPMs. I second PacBlue in checking the oil for level, color and smell. The oil should be clean and no smell. Make sure you follow the procedure for your tranny to check the oil, many require the engine to be running in neutral. Also make sure that the tranny gear selector is hard against the stop when in forward and reverse.