| On a recent trip down to the South Coast from Lowestoft, we stopped overnight at Burnham-on-Crouch. On the following day, whilst travelling down the Crouch on our next leg to Ramsgate, we were just speeding up to get on the plane when we heard a "thump" and the port engine suddenly speeded up. We dropped the speed back immediately to investigate. My first thought was that I'd either lost a propeller, or that one was slipping. Realising that we certainly weren't going to be able to get back on the plane, we crept back to Burnham Yacht Harbour to establish what the problem was.
I lifted the drive to the beach position to allow me to see it, and it was clear that I'd lost the rear propeller, nose cone, and part of the inner driveshaft.
One thing that struck me (as a metallurgist with hundreds of failure investigations under my belt during the early part of my career), was how clean the break looked - there was no obvious initial defect and the fracture surface looked very dull and smooth - normally indicative of ductile failure.
I spoke to engineers at the Burnham marina, and also to the chap who normally services my engines. Both advised, with the usual free advice disclaimers, that as I wasn't leaking oil and as long as I took it easy, I was unlikely to do any damage limping the boat back home.
After discussion with our friends who were accompanying us in their own boat, we decided that this was the best course of action, particularly since we could do it in stages and still enjoy our holiday. Obviously our trip to the Isle of Wight via Ramsgate was off the table, so we changed our intinerary to include a few more days in Burnham and then visit Woolverstone and Woodbridge on our way back to Lowestoft. The boat managed this fine, easily achieving 8 knots through the water and about 10 with a favourable tide.
We arrived back at our marina in St Olaves about 10 days later and once the boat was lifted, it was easier to assess the damage.
I had to wait several days for our insurers' surveyor to examine the shaft before we could instigate repairs. I agreed to meet with him to assess it together.
Examination of the shaft in-situ was quite difficult due to the presence of calciferous deposits that had built up in the 10 days exposure to seawater. It's likely that this was caused by cathodic over-protection of the exposed duplex stainless steel shaft, as there was quite a lot of anode loss as well.
At the time, I was fairly convinced that darker region at the bottom of this photo looked like a pre-existing fatigue crack, though it doesn't look as convincing in this image as it did to the unaided eye.
I acknowledged this to the surveyor, who appeared appreciative of my openness, though as a chartered engineer I don't really have any option but to call it as I see it. However, after seeing the photo much larger on my computer, I did email him later to say it wasn't nearly as clear that this was the cause as I had thought during our visit.
Obviously my insurers wrote to me quite quickly after this to say they were denying my claim as there was evidence of fatique and my policy does not cover "wear and tear". An additional bit of evidence supporting this viewpoint was actually the lack of damage on the front propeller.
Anyway, I pressed on with the repairs, asking Peter at Northgate Marine to try to preserve as much of the fracture surface as they could. This was always going to be difficult as, in order to remove the shaft, they had to drill and tap it so that they had something to pull on.
Once the shaft was removed, I took it to ASAMS so that the end could be cut off and ultrasonically cleaned. This is when things became very interesting:
The distinctive feature of this fracture face is that it has the characteristics of what we call a "cup and cone" fracture. It also has lines radiating out from the centre, suggesting that the initial part of the failure is in the centre, not at the outer edges, which is where any pre-existing defects such as fatigue cracks would be expected.
The raised shear lips (11 o'clock clockwise through to 10 o'clock) are associated with ductile tensile overload. However there was still an area between 10-11 o'clock where the failure looked to be normal (ie. 90 deg) to the surface. Here's a close-up (rotated to the top):
Again, there are no signs of "beach-marks" or anything that would indicate fatigue or a pre-existing defect.
My conclusion is that there was no evidence of wear-and-tear, so I sent the specimen and driveshaft to the insurer's surveyor to look at it again. He sent the components to another laboratory for their examination (my association with ASAMS obviously necessitates a completely independent 3rd party) and I'm pleased to report that my insurers have contacted me to say that they will accept my claim.
It's still quite a mystery as to how this occurred, mainly because of the lack of damage to the forward propeller. I did recover quite a bit of thick fishing line (or similar), and my initial thoughts were that the props had pulled up something that had got between them. I still think this is the most likely explanation, though the clean break with minimal other damage suggests that whatever got caught distributed the load very evenly, causing a uniform tensile load to the shaft. You probably couldn't recreate this condition if you tried, and I consider it a freak accident.
18th September 2023 | 
Kevin Millican
