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Replacing props underwater. Lesson learned?

Discussion in 'Props, Shafts & Seals' started by Pascal, Jun 2, 2018.

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  1. Pascal

    Pascal Senior Member

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    I increase power in 100 rpm increments over 1000, pausing for 4 to 5 seconds and there is really no vibration. Ive always done that on every boat i run, There is no need to stress anything

    Boat has 2300 rpm 1650hp C32s, pretty sure the ZFs are 2:1, with 3.5" AQ22. The C30s 84s use 3" shafts..
  2. Capt Ralph

    Capt Ralph Senior Member

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    Pascal
    We know you are a very conservative operator.
    Yep, no need to stress anything.
    I'm convinced an event happened long ago and you ended up on the bottom of the scrodem pole, Last guy with throttles in hand.
    But when all else fails, it's a boat, Crap (no relation) happens.
  3. PacBlue

    PacBlue Senior Member

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    That remains to be seen. Riding the keyway is still a possibility. Putting the key upside down with the wrong radius in the bottom of the keyway is another possibility. Removal of the broken shaft in the propeller will help complete the story.
  4. PacBlue

    PacBlue Senior Member

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    If the shaft is a 3.5" diameter AQ22, the installed Safety Factor is 3.51. Or another way to look at it, for a recommended Safety Factor of 5.0 (minimum for highly loaded recreational diesel installations), the Shaft Diameter for AQ 22 would be 3.932", round up to 4.0". The AQ 22 Torsional Strength for shafts from 3" to 12" Diameter is only 36,600 psi.

    If the shaft is a 3.5" diameter AQ22 High Strength, the installed Safety Factor is 6.72 or for a recommended Safety Factor of 5.0, the Shaft Diameter for AQ22 HS would be 3.168", round up to 3.250". The AQ22 High Strength Torsional Strength for shafts from 2-1/4" to 8" diameter is a more robust 70,000 psi.

    If you do not have AQ22 High Strength Shaft Material, you have a design/engineering problem as well!
  5. Pascal

    Pascal Senior Member

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    It is AQ 22 HS...
  6. PacBlue

    PacBlue Senior Member

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    Good news from a design / engineering perspective. It is a conservative approach and well within industry norms. As a side note, the corresponding recommended spacing between bearings / supports would be 119" for 3.5 diameter AQ 22 HS Shaft.

    A standard SAE Propeller Hub Length would be 3 x the Shaft Diameter, in this case 3 x 3.5 = 10.5" Long. A look at Propeller Diameter would yield a max of 15 x shaft diameter, 15 x 3.5 = 52.5". A more conservative approach would only allow for 13 x shaft diameter, 13 x 3.5 = 45.5".
  7. Pascal

    Pascal Senior Member

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    Prop is 40" diameter. Hub is 10.5". The whole shaft is about 11' long so we re well within support requirements

    The prop shop doing the shafts told me AQ22 only comes in HS for 3.5" shafts
  8. wdrzal

    wdrzal Senior Member

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    From OP's first post...........

    "Needless to say the other shaft is going to get inspected and the prop resinstalled when we haul out in Miami. "

    Have not only the other shaft mangafluxed, have both pieces of the broken one done as well. It will "enlighten" all of us, (pun intended) . Its cheap, simple, quick and very telling when looking for stress cracks
  9. PacBlue

    PacBlue Senior Member

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    If you don't mind, do you have the following:

    Full Load Displacement?
    Full Load Speed?
    Pitch?
    Brand / Style of Propeller?
  10. Pascal

    Pascal Senior Member

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    Veem 40x49 5 blades no cup

    32kts at 2300 rpm

    Full load disp 137 000 lbs
  11. PacBlue

    PacBlue Senior Member

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    That is a nice turn of speed for that size of boat and power.

    Based on the information, the Cavitation Potential rates in the Medium range. Somewhat normal for a production boat.

    I personally would have gone with a 2.25 or 2.5 / 1 gear ratio to reduce the tip speed a bit and still keep a 3.5" AQ 22 HS Shaft, but that is "Monday Morning Quarterbacking" for you. At least you know it is not a design problem, would check the Port Shaft thoroughly as well.
  12. Pascal

    Pascal Senior Member

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    Yeah we re going to pull the port shaft and get it magnafluxed.

    The boat performance is pretty good... in calm waters she ll do 20kts on 70gph and 50% load. We rarely cruise over 22/23kts so altogether there is little stress on the drivetrain Now i ve only been running her for a couple of years so who knows how she was run before that.... as I mentioned on survey sea trial the captain (not full time but "management company") scared everyone with a heavy hand. With a softer touch she ll get on plane quickly with very little bow rise. No trim tabs btw

    The price we pay for the lightness and economy is in head seas... she s no hatteras :)