guesses on how this prop shaft snapped?

[albinalaska]

any guesses how this prop might have snapped off? this isn’t my vessel, it’s a tour boat i drive in the summers - had another captain at the helm who reported no event other than clear loss of propulsion on that side.

not sure if we’ll ever truly know but curious if anyone has had a similar experience.

new prop, new shaft (1 3/8”), new bearings in the struts and engine was realigned at the beginning of the summer to within spec.

ive been told that the pressure needed to snap it was about 1300 foot pounds.

 

[rslifkin]

If I'm not mistaken, the top right part of that break isn't quite clean metal. So there may have been an un-seen crack there from a previous grounding, debris strike, etc. And things finally flexed enough under enough load for it to snap.

 

[SeaLion]

Agreed. It looks like crevice corrosion.

 

[psneeld]

I have heard of lots of snapped off props when some one goes from pretty high revs forward to reverse.

Of course any contact damage over time can cause cracks, I just haven't seen or read up on this.

 

[Diep]

Are you saying this shaft was new at the beginning of the summer?

It could have been fatigue propagating from some kind of flaw, either internal, or more likely in the keyway. Perhaps poorlay machined or damaged somehow during install. If this was a brand n shaft I would think the manufacturer would want to have a look at it.

 

[High Wire]

It looks like at least two cracks originating at the keyway. Hit something or corrosion? IDK.

 

[Frosty]

My instant thought when reading the title was "crevice corrosion." Though only four months would be pretty quick. Curious what type of stainless the shaft is made of (Aquamet, etc.)

 

[Shrew]

I recall many years ago an old mechanic warning me not to mark the shaft with a lead pencil. Did someone mark where the prop was supposed to end? That shearing looks very clean.

 

[Nick F]

any guesses how this prop might have snapped off? this isn’t my vessel, it’s a tour boat i drive in the summers - had another captain at the helm who reported no event other than clear loss of propulsion on that side.

not sure if we’ll ever truly know but curious if anyone has had a similar experience.

new prop, new shaft (1 3/8”), new bearings in the struts and engine was realigned at the beginning of the summer to within spec.

ive been told that the pressure needed to snap it was about 1300 foot pounds.

My interpretation is that progressive failure started at 12 o'clock, moving counterclockwise to about 4 o'clock at which point the remaining connected metal of the shaft failed suddenly. In the first area you can see successive bands - typical of a fatigue fracture advance. The bands are closure together at the start and then further spaced apart, indicating increasing rate of advance of the failure front.

The shape of the band's suggests to me a failure caused by torsion ((twisting), and not by bending.

What caused the failure to initiate? Perhaps a pre-existing flaw. There appears to be a cavity (shadow) visible in the second photo. Failure is also coincident with the forward end of the key.

I see no discoloration typical of crevice corrosion.

It will be interesting to know what alloy this is.

 

[Bacchus]

This looks somewhat similar to a shaft break I had inspected/analyzed by an employers metallurgy depth. A good metallurgist can inspect and determine the break origin. AFAIK the key / key way are frequent culprits. If shaft was new recently was the prop lapped to match the shaft Taper and the key / keyway fitted properly. I will dig for the related photos, analysis and articles I have found relating to "fitting" the prop / shaft.
[EDIT] Here are the related files

 

[O C Diver]

Without knowing engine HP, transmission ratio, shaft size, and prop diameter / pitch, it's hard to know.

One observation I would make, it seems like a lot of exposed shaft between where the propeller would be and the strut. Remember, the anode should be in front of the strut, or a collar anode.

Ted

 

[Comodave]

It looks like too much propeller overhang. It should be no more than 1 to 1.5 times the shaft diameter. So a 2” shaft should have between 2 and 3” sticking out behind the strut. Looks like more in the photos but really can’t tell for sure without measurements.

 

[albinalaska]

i really appreciate all this great insight. the shaft was installed in April and we are definitely going to reach out to the manufacturer. the overhang length definitely piqued our interest. the machine shop here in town said the same thing. again, appreciate the help! really helpful! thanks y’all.

 

[albinalaska]

This looks somewhat similar to a shaft break I had inspected/analyzed by an employers metallurgy depth. A good metallurgist can inspect and determine the break origin. AFAIK the key / key way are frequent culprits. If shaft was new recently was the prop lapped to match the shaft Taper and the key / keyway fitted properly. I will dig for the related photos, analysis and articles I have found relating to "fitting" the prop / shaft.
[EDIT] Here are the related files

thank you for taking the time to send this!

 

[Nick F]

This looks somewhat similar to a shaft break I had inspected/analyzed by an employers metallurgy depth. A good metallurgist can inspect and determine the break origin. AFAIK the key / key way are frequent culprits. If shaft was new recently was the prop lapped to match the shaft Taper and the key / keyway fitted properly. I will dig for the related photos, analysis and articles I have found relating to "fitting" the prop / shaft.
[EDIT] Here are the related files

Very interesting comparable! You posted the file of photos, but no report. Do you have the report/analysis?
Thanks

 

[Northern Spy]

I would ask the shaft manufacture if they would pay for a fractographic analysis. Acuren would be my go to for that. They do metallography in Richmond, BC. They have seen every which way shafts fail. I know as I've sent them enough to inspect and write reports for. It's amazing what they can detect with an SEM that you can't see with the eye or optical magnification. That said, it sure looks like it propagates from the keyway which are typical stress risers.

 

[KnotYet]

A big clue is that the shaft and prop were new. My guess would be that it wasn't machined
quite right; maybe the keyway was short or imprecise in some other way and tightening the
prop nut caused excessive strain at that spot.

 

[Bacchus]

Very interesting comparable! You posted the file of photos, but no report. Do you have the report/analysis?
Thanks

He simply noted the findings & conclusion as remarks noted on the photos. I have seen other notes that pointed to keys & keyway as a frequent problem area / defect initiation point. I decided that wire siezing was better than cotter pins for preventing any prop movement and did mine that way. The analysis listed the shaft as 304 SS which is not as good as 316 AFAIK. not sure what yours is?
Do you know if the prop was fitted / lapped to match the shaft prior to key & prop assy?

 

[TJM]

I am surprised that the shaft is 304 in salt water. Typically, 316 is used due to its crevice corrosion protection in salt water. Tensile strengths are close between the 2. What did the prop look like ? Any witness marks ?

 

[Bacchus]

I am surprised that the shaft is 304 in salt water. Typically, 316 is used due to its crevice corrosion protection in salt water. Tensile strengths are close between the 2. What did the prop look like ? Any witness marks ?

The shaft in my example was in fresh water and the prop was never recovered. Shaft broke underway when revenge up... our lake is 600 ft at deepest and several hundred ft when it broke off.

 

[jleonard]

Just ahead of the break I see what might be deformation of the keyway on the starboard side. A better picture from above focused on that area would help.

 

[albinalaska]

I am surprised that the shaft is 304 in salt water. Typically, 316 is used due to its crevice corrosion protection in salt water. Tensile strengths are close between the 2. What did the prop look like ? Any witness marks ?

the prop is currently sitting in the bay along the tour route between 30-90 feet of water but the owner is going to go dive to retrieve it when possible.

 

[albinalaska]

My interpretation is that progressive failure started at 12 o'clock, moving counterclockwise to about 4 o'clock at which point the remaining connected metal of the shaft failed suddenly. In the first area you can see successive bands - typical of a fatigue fracture advance. The bands are closure together at the start and then further spaced apart, indicating increasing rate of advance of the failure front.

The shape of the band's suggests to me a failure caused by torsion ((twisting), and not by bending.

What caused the failure to initiate? Perhaps a pre-existing flaw. There appears to be a cavity (shadow) visible in the second photo. Failure is also coincident with the forward end of the key.

I see no discoloration typical of crevice corrosion.

It will be interesting to know what alloy this is.

the shafts were AQ19 but i guess the machine shop is going to replace the shafts with AQ22.

 

[albinalaska]

My instant thought when reading the title was "crevice corrosion." Though only four months would be pretty quick. Curious what type of stainless the shaft is made of (Aquamet, etc.)

yeah AQ19 is what the shafts were but they are going to be replaced with AQ22.

 

[Bmarler]

I am surprised that the shaft is 304 in salt water. Typically, 316 is used due to its crevice corrosion protection in salt water. Tensile strengths are close between the 2. What did the prop look like ? Any witness marks ?

The aq19 material is essentially 304 but has some other proprietary ingredients. It’s commonly used on fishing boats and commercial boats in salt water. Aq22 is the 316 equivalent.

 

[Capt. Kenny]

At first glance, sheared keyway in concert with what looks to be lower than usual quality (or compromised) metal in the shaft.

 

[slowgoesit]

Albininalaska,
Was the other shaft by any chance replaced at the same time? and if so, did they by any chance pull that prop and look at the keyway area for any issues? If it were my boat, that is something I would definitely do.

 

[FlyWright]

Could you have been sold a used shaft at a new shaft price? Mmmmm...