rudder failure

Clean up the ends with some pickling solution ( brush it on and hose it off don't use a wire brush) and post some higher res photos of all the shaft ends and we can tell you more.

Looking at the PDF there appears to be an old fracture ( dark stain ) . One of the Jpegs shows what appears to be a classic torsional shear with stepping failure which often originates at a weld or keyway.

The FOS of 2 . Is that 2X the class requirement or 2X the strength for yield on a combined torsional & bending calculation? If the latter there's insufficient fatigue allowance. Usually we send the material away for testing there's a lot of substitution goes on with SS grades. It's also possible that touching the mud simply finished off the compromised shafts. They may also have been compromised by numerous groundings and this was the last straw.

 

I have a pretty healthy collection of power transmission shafts that failed simply due to torsional overload...and the fractures look almost exactly like those in the OP pics. However..I assumed (a lot of that in this thread) that it would be highly unlikely that a simple ultimate strength torsional stress failure could be produced in a rudder. Not unless it was one horribly bad design with a very large unbalanced blade in combination with a very robust and powerfull steering actuator.

But I've not seen everything yet...and I've seen a lot.

 

It is entirely possible that these are four examples of failed rudder shafts that did not happen at the same time, but were what happened to the replacements. Though it appears the OP statement (however unclear) was meant to say they occurred at the same time. I have done my share of failure analysis when I worked for our local, very large, aircraft manufacturer.

I would not characterize the lines as "classic beach lines" because they are too irregular, you can get that kind of mark from the natural grain of the metal in a simple overload. However it does appear to have failed after a few large cycles.

But I want to suggest something to the OP, this is presuming they did not fail from striking something, and the appropriate size shaft was originally designed. That is that the rudders are experiencing flutter. Their shape, shaft location along the cord, or their location on the hull sets up a violent harmonic that overloads the shaft very quickly. Flutter on aircraft is not easy to predict, and is always dangerous.

You may have indeed sized the shaft properly, but something about the hull, prop wash, or rudder shape or geometry causes a harmonic at certain speeds, and they fail in a matter of seconds. You may have to change the size, or location of the rudder to prevent future failures if flutter is indeed the culprit. Use a much more stout linkage to get as much spring out of the the systems, redesign the rudder to balance the mass around the axis, or to change the location of the shaft relative to the centroid of the rudder area. Go to a lower aspect ratio rudder might help too, makes the surface more stiff, less flex.

Good luck.

 

I'm thinking if they were successive replacements for a one rudder boat, they wouldn't all be covered in fresh grease that seems to be in the same, freshly removed condition. I'm thinking the shaft on the left has had the yoke removed. I'm thinking the yokes on the left side of the shafts are for the rod that connects them all together so they act in unison and the extra yoke on the right side of the shaft with the red yoke is the one that connects to the steering actuator. I'm thinking some grease has gotten on the broken ends that might be misleading (dark stain). I'm thinking french44 is sleeping because it's 3;39 am in France right now.

 

more details

Hi all,

Firstly thanks for your lights. The 4 rudders were on the same ship, it's a pusher boat. Lenght about 30 m speed around 6/8 kts. draft 1 m . From the crew no noise ?? The rudder was new and the shaft of 80 mm of diameter is in C45 (re 375 mpa Rr 660 mpa) . the calculation give a Von mises load of 160 Mpa so nearly 2 times under Re. the failure appear just below the hull and the rudder aera is 1.2 m² (each) . we have 2 bearing and the lenght below the hull of a rudder is 1 m (short) After checking this morning we have bending on the two central rudder from aft to fore so may be in backward operation ?? and the ship left the shipyard since 2 days so 10/20 hours of navigation before the failure . I think that the ship hit someting and some replies are in this way.

French 44 in DRC on congo river

 

Being on the Congo River helps explain why the crew is mute. I guess on a boat that big with a bunch of engine and other noise going on, like a lot of disorganized yelling and hollering, you might not hear anything happen, and with a big enough mass in motion you might not feel anything either. Hydraulic steering wouldn't transmit anything to the steering wheel. I imagine the Congo River is pretty full of silt and opaque and full of sunken logs, ships, trash, etc.

Maybe you need some sort of guards back there to prevent that from happening again. Something similar to what semis have



to keep cars from going underneath the trailer. (but more streamlined for going through the water)

 

With only 10 to 20 hours on those shafts, I think we have enough info to conclude the operator smacked into something. He needs to call his insurance agent and get it repaired.

The only exception is if the boat was of poor design.

french44: Some pictures of the boat in question would be helpful to all before final opinions can be made.

 

Reading more into the "pusher boat" this is your typical river barge pusher/tug boat. The quadruple rudder you describe, if similar to this design, sounds as if they just smacked into something and it tore off the rudders.

http://www.marinelog.com/DOCS/NEWSMMIII/MMIIINov24.html

Knowledge of the river channels and a sonar shallow alarm may have prevented the incident. Unless there is more evidence, I suspect whoever piloted the vessel was in error. Being a new boat it may have lacked a sonar so it would have been risky to operate.