Interesting to hear that Norwegian Jewell is reported to have lost her propeller at sea.
This link notes a “failure”, but the second says. “a propeller fell off the ship”.
“As our cabin was near the front, we heard nothing. Friends closer to the back, were woken up by a loud bang in the middle of the night. The propeller, hurled up by the rotation of the engine shaft hit, the ship’s hull. Fortunately, it did not slice through the metal and make a big jagged hole before sinking to the bottom. In the morning, we noticed just one water trail instead of the usual two. Our ship was a lame duck limping across the calm Pacific. This did not add to feelings of confidence.”
Well who knows what really happened? In the case of this ship, it has a azipods rather than conventional shaft and screw arrangement. If I recall, these pods had a design issue with the thrust bearing when they first came out. That was some time ago.
The same way anything else falls off. More likely and significantly more common is a CPP blade coming off. ‘You’re about to lose a prop’ is a difficult discussion to have with most captains. Vibration can tell you a lot but unless you’re diving down there it’s a hard point to get across.
Surprisingly, no, at least the one time I had that very nearly happen. Not only did we not return to port we rode out a 70 knot 35 ft sea storm way out on the Russian border post discussion.
That being said, we didn’t lose that blade. You could wiggle it back and forth 5 inches or so like a bad tooth once she was up on drydock and it completely destroyed the prop hub but it stayed attached. Barely.
The thread title is a general question, so the answer below is a general answer.
One common cause of lost props are annular microfractures in the tail shaft nearest the propellor, caused by the vessel going through repeated “soft groundings”. This can mean the vessel sitting on the sea bottom for a few hours between tides at a dock. It can also mean repeated passages through a shallow channel and running over a sandbar, where the prop is not stopped but hindered by the sand.
Doing this once or twice will not lead to prop loss. The damage slowly accumulates over years on vessels whose routes include these conditions.
The damage is undetectable by normal means, even if the tail shaft is pulled and given a normal inspection. You really have to look for it.
When the prop does fall off it can happen in deep water and “for no reason” (often while the vessel if backing and filling), leaving the investigators scratching their head.
As I said, I am not proposing this as the cause in this particular case. I am just answering the general question.
It was a long but interesting read JD. Know that area all too well, especially with N and NE winds. Kicks up in a heartbeat. And not 4-6’ in October. Add a bit to that. Missing initialed chart? That’s a goodie. Once you swing out a bit past Jupiter Light hold on. Those were almost impossible parameters for Reinhaur to meet. Whoever wrote that contract wasn’t too bright. Thx for sharing.
Virtually all mechanical failures will be in some part due to fatigue. The polished areas are where the crack propagated slowly. The dull gray area is where the metal finally fractured.
Loosing a prop is fairly common on small boats with keyed shafts. The key way acts as a stress riser.
Yes. But they are microfractures, so you have to be far more diligent when you carry out the test, and do it repeatedly. A matter of assuming the fractures are there, and then performing the test repeatedly, magnifying glass in hand, until you are absolutely sure they aren’t there.
For this cause of prop loss, the trade route of the vessel tips you off ahead of time. Do you navigate shallow channels with sand bottoms? Do you occasionally touch bottom at the dock, between tides? When you haul her out, does the bottom paint look like it’s been sandblasted a bit? Those would lead you to increase scrutiny of the tailshafts. There can be zero vibration before the loss.
Yes, most of the time for a shaft subject to a bending moment.
It is important to remember when approaching a design problem the science of fatigue is relatively young. It was only at the turn of the last century that fatigue was discovered. And, it was the failures of the liberty ships where they’d break in two that really launched the research that has lead to our modern understanding fatigue. I think that investigation is also one of the first to look at stress risers too.
The crack propogated slowly across the section as indicated by the “growth rings” that show a progressive failure. Finally, the load on the shaft was more than the material strength and the final stage was the sudden fracture of the remaining intact portion of the shaft that shows up as a rough surface.
There are probably a few ways. If the coupling to the transmission is bad, hard reverse can suck the prop and shaft aft and maybe right out of the boat.
There is always the danger of stray current corroding the prop and/or shaft to the point it fails too.