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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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    Over the years, i ve heard a few stories of shafts snaping after the prop was installed underwater but always thought it was pretty rare and heard many experienced folks claiming it was safe... well, i am now in the "don't do it" group.

    Cruising at 22kts and 1800 rpm last week in 20'+ on the exuma bank off Hawksbill, i fell a small vibration coming out of nowhere. Within 30 seconds as i was slowing down, i fell a bang and lost drive on starboard.

    It was too windy and rough to do anything so i marked the spot and returned a week later on the way back home. Spent a couple of days searching before finally finding the wheel in 24' of water. First day it was still too rough to see from above th surface so i had my girlfriend pull me behind the dinghy.... hours of playing human lure. Second day was calmer so we started doing passes with the boat watching over the side and spott s it on th second pass, about 40' from the orignal trwck.

    Raising it without diving gear wasnt easy as it was as deep as we could get... first we used one of the aft deck winches to bring it close to the surface then lifted on deck it with the davit It has two dings, i guess from hitting the rudder (found scratches on teh rudder close to the hull) although i dont quite explain the chunk missing off. Blade, especially away from the tip.

    So.... no wheel install underwater for me or if we really have have to, then i would do a short haul asap to reinstall. Needless to say the other shaft is going to get inspected and the prop resinstalled when we haul out in miami.

    Pics and videos:

    https://instagram.com/p/BjiA8x6DHfX/
  2. Seasmaster

    Seasmaster Senior Member

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    I'm not a material science engineer, but I find the stress marks on the shaft fascinating, as well as the discoloration on the outer edge of the shaft. To my untrained eye, it looks like the metal fatigue was on-going for sometime.
  3. Capt J

    Capt J Senior Member

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    That has nothing to do with replacing the prop underwater. That has to do with either vibration issues for a long time or weak metallurgy. It broke clean far in front of the keyway on a solid piece of shaft. If it had to do with the prop being loose, it would've snapped at the key way.
  4. Capt Ralph

    Capt Ralph Senior Member

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    I'm with Skippy J on this, That was not a propeller installation issue.
    In the pictures the key way is perfect, no distortion of shaft material here.

    Also, that is an old crack.
    That shaft hit something a while ago, maybe years.
    Last edited: Jun 2, 2018
  5. Capt Ralph

    Capt Ralph Senior Member

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    I assume you have 2 piece shafts? Spare on board? Maybe a dive team already scheduled to replace the shaft?
    Great work finding the wheel.
    Last edited: Jun 2, 2018
  6. Pascal

    Pascal Senior Member

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    Good point on the crack being an inch or so in front of the key... there is a rust line along the edge of the crack

    No they re one piece shafts... not that long about 10/11' unlike the 20' sticks on my hatt.... Anchored off cat cay, crossing tomorrow and hauling out monday or tuesday. We re due for bottom paint anyway...
  7. PacBlue

    PacBlue Senior Member

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    That is a classic propeller fit / key way induced shaft break. The crack most likely propagated from the keyway, the break was clean and happened very fast once it began to fail.
  8. Capt Ralph

    Capt Ralph Senior Member

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    I really disagree.
    In the picture of the prop with the shaft end still attached, The key way was barely cut into the shaft at the break. There is no deformation or pattern around the key way.
    You can see the C patterns across the shaft from the splinter.
  9. PacBlue

    PacBlue Senior Member

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    Run it by Frank and Jimmies or some other prop shop, the fact that it is right at the end of the key way cut says it all. I have seen this exact pattern on many occasions, always at the most forward end of the keyway cut-out.
    The stress riser most liked occurred at one of the corners of the keyway, could have been from a past grounding or the poor fit of replacing a prop,
    A material defect in the exact same spot as the keyway? Just randomly? I don’t think so.
  10. Capt Ralph

    Capt Ralph Senior Member

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    Are we looking at the same picture?

    Attached Files:

  11. d_meister

    d_meister Senior Member

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    My guess is the prop didn't get all the way onto the taper by binding on the key. The half moons radiating from the edge opposite the splinter broken off suggest impact vibration causing the damage. Very likely, the telling evidence will be on the shaft metal inside the prop. If the shaft stub just drops out when the nuts are removed, that would tell the whole story.
  12. Capt Ralph

    Capt Ralph Senior Member

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    Did anyone notice the ding / repairs on the blades?
  13. Capt J

    Capt J Senior Member

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    It's a Lazzara. Lazzara has always built their boats without enough blade tip to hull clearance as well as putting the underwater exhaust too close to the propellers. Getting on plane with lazzara's, if you don't advance the throttles ever so gingerly induces caviation that feels like you're driving over railroad ties at 50 mph throughout the entire boat. Has nothing to do with the prop or how the prop was installed, it's forward of the keyway. If you look at the severed piece that sticks out beyond the clean break in the shaft, the s/s shaft material is very porous. This is a cavitation issue over years and years, many cycles of whipping the shaft, and most likely a prop that was off/damaged prior to this one, and subsequent stress cycles. I think the prop had a log or other similar shaped floating debris get between prop and strut as you can see one blade has a lot of damge very close to the hub...... I know of several Lazzara's that have flung the prop and shaft off in the same area. A keyway break usually results in the shaft split in half across lenghways. Lazzara's are also famous for having breaking and having to change motor mounts way too early.
  14. Pascal

    Pascal Senior Member

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    IMG_9070.JPG Here is a close up of the break which is about 1/2" forward of the end of the key way. The key can be seen about an inch inside

    I ve never had an issue with cavitation when accelerating as I always try to be gentle. But I suspect the previous owners captain was as on sea trial he jammed the power so hard even the surveyor told him to chill :). Boat was shaking and vibrating like a car on rail road ties. Good analogy
  15. Capt Ralph

    Capt Ralph Senior Member

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    Just like a fell tree. A lil abuse on the left who knows when (last week, 3 years ago), them TIMBER.
    A diver nor key way cut caused that.
  16. SeaEric

    SeaEric YF Historian

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    The fact that they were able to locate and retrieve the lost wheel - That's the headline!
  17. Capt Ralph

    Capt Ralph Senior Member

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    Yep, You notice 40' off line and when it landed, it's spinning/flying when it found the bottom near covered a blade? A week later.
    Another few days and it may have been lost.

    Another thread on YF a few months ago, the owner could not find his old wheel.

    Hours after one of my customers lost a wheel at the Riviera Beach bridge (yes, many years ago it was a draw bridge), I found his 26" wheel barely. A blade tip was just showing.
    It screwed itself down nicely.
  18. PacBlue

    PacBlue Senior Member

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    The vast majority of these breaks are due to a previous grounding incident, soft or hard.
    Cavitation is not the cause nor is whip, as the shaft was sufficiently supported by the cutlass bearing. 10% prop clearance is done all the time, not a factor on the shaft breakage.

    Shaft diameter size when compared to horsepower and reduction gear, safety factor, and material selection combined with a physical impact all have a role to play in this failure. If we knew the hp, rpm, shaft diameter, shaft material, could easily calculate the safety factor which has to be above 5.0.

    Other factors - was the prop barrel a full length SAE size in relationship to the shaft diameter and propeller diameter.

    The clear break exactly atthe exact location forward of the cut key - not a random event.
  19. Capt J

    Capt J Senior Member

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    Have you ever been on a Lazzara when they're trying to get on plane and the amount of vibration they have? I did a seatrial on a 68' where BOTH surveyors came running out of the engine room and screamed ALL STOP as the motors were vibrating so bad they thought they were going to tear the stringers right out of the boat. They didn't realize it was a common Lazzara deal and if you try to come on plane other than really slowly it whips the shafts something crazy, and has torn several engines right off of the mounts on several of them..........Lazzara came up with an add modification on the hull to try to shift the exhaust gasses elsewhere which helps some but is not an end all to end alls. The engines vibrate so much they literally look like you're running a prop with 3 majorly bent blades at cruise speed. It is something fierce. Every Lazzara regardless of size that I've been has exhibited this, including Pascals (which I've never been on).
  20. Capt Ralph

    Capt Ralph Senior Member

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    I'm not sure you have studied the pictures either.
    BUT, The main O P concern, Did a diver install the propeller wrong causing this failure.
    I say NO and NOT the cause for this failure.