[jrd22]

Kind of a long story so I'll try to be brief. I delivered my brother's 34' Northsea pilothouse from the San Juan Islands to Lake Washington (Seattle) last month. Motored all the way (no wind), spent the night at Pt. Townsend, picked up my brother at Shilshole Marina, through the locks and five bridge openings and then the prop fell off about two miles from the slip (just off Billy Gates place for those that know the lake), had to be towed the last mile after trying to sail (at least we didn't have any prop drag).
After being hauled out it was obvious the key sheared off (half was still in the keyway) and due to the design of the older Autoprop it spun itself off the shaft when we put the transmission in reverse while we were trying to figure out why we had no propulsion. We didn't hit anything that we know of, never heard or felt even a slight bump and we were at cruise speed in flat calm water at the time.
I'm guessing either something was hit at some point and the key just finally gave way, but the real mystery is that there were cracks in the fiberglass on either side of the shaft log that weren't there a couple of years ago at haulout. The shaft is true and the cutlass bearing and shaft tube are in good shape so what would have caused these cracks?

Thanks for any thoughts or WAG's

 

[Faster]

Bit of a head scratcher! btw is there a strut also?

The remaining key piece looks kind of odd too..

The cracks look to be caused by some sort of lateral loading that wasn't normal .. esp if the shaft is still true. Motor mount issues? but then I'd expect you noticed abnormal vibration...

Whatcha gonna do??

 

[RichH]

Cant tell by the photo angles if the shaft is straight or tapered for its connection to the prop.

In either case, the ultimate cause is probably 'insufficient press fit' of the prop onto the shafting. What 'should' be resisting the prop from turning on the shaft is the friction generated by that press fit: prop hub bore slightly smaller than the OD of that prop shaft mating area of the shaft. Keys should never be used to stop rotation of rotating parts ... theyre there only as a 'back-up' in case the prop no longer has a good 'PRESS fit' onto the shaft .... as would happen by wide temperature changes in the metal. I like to have a minimum 0.002" smaller prop bore diameter per INCH of shaft diameter ... and then use an arbor press to force the prop onto a straight shaft. Tapered bores are always preferred over straight bores.

Also, could be photo angles again ... but if the prop was a normal right hand prop, those right hand threads on the stub end of that shaft will simply allow the stop nut to spin loose and shear off the cotter pin when doing so .... if the prop loosens on the shaft and now with the nut off there's nothing holding the loose prop on, the prop moves aft and then off. Right handed props should always have left handed threads on that stub nut - IMO.


With tapered shafts, the mating surface should be 'honed' so that most all the internal surfaces of the prop bore and the mating surface of the tapered shaft are in CONTACT (for greater friction).... and then the prop hub is fully torqued to forcibly 'drive' the prop onto that taper (again, a press fit).

 

[knuterikt]

Assuming the propeller was installed correct.
The order of events is something like this
1) Split pin securing the nut fell off
2) Nut securing the propeller worked loose/slack (and fell off?)
3) Propeller have slowly worked loose on the taper
4) The key broke (when the gear was put in reverse or forward.
5) If the nut was not already lost - he spinning prop helped it to unscrew the last bit.
6) propeller fell off

I had a propeller shaft drive flange split in a similar way once.
The installers did a bad mod on the flange.
I don't know for how long the system had been working before it failed.
But the nut got loose, the flange worked loose and the flange broke at the key way.

 

[boatpoker]

It may be just odd lighting in the photo but that key looks more like brass than bronze.

The fracture running forward from the log would give me serious concern. I'd be grinding out that crack and taking a look on the inside to see how deep it is.

 

[sony2000]

The flattening of the two threads just prior to the slot, could indicate the key was lying there when the prop was put on to the taper. The prop was never fully seated on the taper. Subsequent left and right rotation of the prop made a greater and greater looseness and this slapping of the key over time, set up the metal fatigue.

The lock nut kept trying to undo and cut into the cotter pin.

The slapping of the key, would be the same as hitting the log with a hammer, each time, and the result was cracking of the fiberglass.

Then in one fell swoop, everything let go.

 

[boatpoker]

Is that a crack in the shaft ?

 

[desert rat]

The Sherlock Holmes award is to be shared equally by sony2000 and boatpoker.

 

[christian.hess]

The flattening of the two threads just prior to the slot, could indicate the key was lying there when the prop was put on to the taper. The prop was never fully seated on the taper. Subsequent left and right rotation of the prop made a greater and greater looseness and this slapping of the key over time, set up the metal fatigue.

The lock nut kept trying to undo and cut into the cotter pin.

The slapping of the key, would be the same as hitting the log with a hammer, each time, and the result was cracking of the fiberglass.

Then in one fell swoop, everything let go.

I concur...

if I may add, people often underestimate the damage even a slightly bent shaft or unbalanced prop can cause...the stress cracks arent even excessive for what happened

and can be FIXED.

 

[jrd22]

Thanks for all the ideas, I'll try to answer the questions in order.
Fast- there is a strut with a cutlass bearing that was in good shape. There was no vibration, very smooth and the shaft was measured for run out and it is almost perfectly straight.
Rich- tapered shaft. I'm sure the prop was torqued on properly a couple of years ago, my brother's business is pumps and he's anal about the fit of impellers to shafts and has all the proper tools.
Boat- I agree the key looks more like brass, but I'm sure it was the one supplied with the Autoprop. The cracks are getting ground out as we speak and I'll know more after talking to my brother.
Sony- I think that initially the key was positioned properly, not sure what caused the threads to be flattened. Would the bronze key be hard enough to flatten the stainless, I thought that was the reason to use a sacrificial bronze key to save the stainless shaft? There is no cotter pin used with the Autoprop, just the nut and key. The shaft is straight and the cutlass bearing and shaft log are in good shape so I don't know how the shaft could have caused the cracking in the fiberglass. If the shaft was slapping the log from distortion seems like there would at least be wear spots on the shaft and interior of the log, but there aren't.
Boat- I thought the same thing and my brother looked at it closely and it was a bit of grass.

One thing I failed to mention was that when the prop let go the engine rpms went up briefly and then there was a "clunk", kind of the opposite of what you'd expect if it hit something. The design of the old Autoprop doesn't allow the use of a cotter pin (bad design if you ask me). Which allowed the prop to spin off when we put it in reverse after the key gave way. I still have no clue what would have caused the cracking, especially since it was not there a couple of years ago at haulout and the boat has hardly been used since. I'll try to get some pictures or info. when they grind out the cracks.

By the way, BoatUS is picking up the tab for the whole shebang; towing, haulout, repairs and a new MaxProp.

 

[RichH]

The design of the old Autoprop doesn't allow the use of a cotter pin (bad design if you ask me). Which allowed the prop to spin off when we put it in reverse after the key gave way.

I think we're close to a 'bingo' here. If you are choosing a MaxProp, they have solved this cotter pin problem by the use of a special castellated nut which uses 'pins' that are inserted into the sides of the castellation and the pins are positioned parallel to the shafting .... very ingenious, no need for cotter pins, 'works well'. With all the feathering props ('cept Auto-Prop) when those blades 'reverse' they impact on their end-stops as set by the internal gearing. Over time this 'hammering' when changing from fwd to reverse can put a hell of an impact strain on a cotter pin.

Good luck. ;-)

 

[Stumble]

What I see is a lot of old damage that is coming home to roost. The key, or what's left of it, looks like it has been cracked long enough for the bright metal to tarnish. This indicates that is has been split for a while. Likely in the same incident that caused the crack in the fin. Do you know when the last time the prop was pulled and inspected was?

 

[jrd22]

Rich- I think you're right, the repeated hammering over time probably accounts for the key shearing. Glad to hear that MaxProp has come up with a better solution.
Greg- the prop was pulled and bearings replaced about three years ago (last haulout).

 

[sony2000]

The fiberglass cracks and possible log crack, can be unrelated to the prop. During the last haul out the strap from the Travel-lift slipped off the hull and onto the log, cracking the fiberglass. Having just happened, it would have been too early to see the damages that now have been highlighted by the development of the osmosis, seen in the two pictures, where the cracks exist.

 

[boatpoker]

it would have been too early to see the damages highlighted osmosis.

What "highlighted osmosis" ?

 

[Fstbttms]

It may be just odd lighting in the photo but that key looks more like brass than bronze.

The key is likely dezincified. This results in brittleness and could be why it failed.

 

[captflood]

GREETINGS EARTHLINGS ; check that the Engine has not moved or now missalined dew to one of the engine mounts may have collapsed or gone hard. Also check for forward and backward movement on the mounts or engine bed. While your at it check the engine bed mount to the boat bolts or screws, and when you get boared check the coupleing on the shaft to the gearbox and bell houseing to the engine. AS ALWAYS GO SAFE

 

[ind46igo]

One very plausible explanation for this event has to do with the installation of the prop. When installing any prop, the possibility exists wherein the key can be dragged forward enough by the prop (when sliding it onto the shaft) such that the forward end of the key starts to raise up due to it riding up on the fillet at the forward end of the shaft keyway. The end result is that the prop never lands on the taper but instead lands on the key. When the nut is torqued up, it feels like all is OK but all you are doing is making a nice dent in the top of the key. Such an installation will operate satisfactorily for some period of time but eventually the prop will loosen its bite on the key. Once that happens, the nut is now loose and the prop will work radially, repeatedly beating up the key. Eventually, the key will shear, and the prop is then free to spin relative to the shaft and nut. The nut will then get unthreaded and the prop is lost.
The higher torque on a prop with an engine with a reduction gear, i.e. most all diesels, would make this a more likely scenario. A cotter pin would have prevented the loss of the prop in such a scenario but would not have prevented the failure of the key.
The key is not really the torque carrying element in the propeller shaft interface, it is the tapered fit between the prop and shaft. Anyone who has ever removed a properly installed prop knows that a significant amount of effort is required to remove a prop from its shaft once the nut(s) have been removed.

 

[dabnis]

Maybe some blue Loc Tite on the shaft threads at the next prop install? Not quite sure how the other "Locking" washer or whatever it is works?

Paul T

 

[jrd22]

The cracks were ground out and it was wet inside so either some kind of damage caused the cracks and allowed water to enter, or water intrusion around the shaft log caused expansion and resulted in the cracks. Guess we'll never know for sure. At any rate it's all back together and should be splashed fairly soon. Thanks for all the feedback, this one really had me scratching my head. Here are a couple of pics of the finished job and new prop.