Rudder Shaft Failure on 15 M sailing Catamaran

Solid Aluminium Rudder Shaft Failure

We are searching for answers here
I made these 17years ago
70 mm OD Alloy Shafting not just round bar from Repudiable Australian Extrusions supplier at the time not cheap rubbish.

Marine Grade Hard anodized after machining the stock and welding a C/L

Web for the HD foam Rudder Blade with epoxy skins.

To me it looks like the billet was stopped at this point during the extrusion

stage and it is a cold joint however small it was in temp it has taken this

long to show up

The vessel is a 13000 kg 15m sailing cat Lead was around 15% Chord at top 750 mm depth 650 mm not that big of a rudder as there are two to do the job
Any thoughts welcome as we are building two new ones prior to Xmas.

Cheers Specmar Aus

 

What is being asked?

specmar.aus,
I read the post a few times and have to confess it was so cryptic I have no idea what you're attempting to learn or ask?

Usually pictures help but these didn't do any good for me. I'm sorry but I can't make much of anything from the post's pics?

Are you showing a failure in an aluminum rudder shaft? IF so, where are the two sides of that failure? Sorry to be dense... but the rounded over section is not clear to me? what is it? why is it here, did the aluminum shaft (alloy not given) round off in the failed state?

What does the several images of the two shafts show? I see two cylindrical pieces of material but have no way to infer is this is the failure state or the good rudder with the failed one?

I think your post is a bit obtuse, sorry to be slow on the uptake, what are you trying to find out from the Forum?

I'm used to having a pic, giving its topic, perhaps some added arrows or text and then maybe a bit of narrative about the image to help see its relationship to the post?

color me confused.

cheers,
Kevin Morin
Kenai, AK USA

 

Hi Kevin
I guess some arrows and text on the Pic would make it easier to see, later today or tonight I will address this.
The Rudder shaft let go just inside the blade and the stump is the bullet shaped object in the latter photos the colour is dark grey as we hard anodized the shafts prior to attaching the foam core shells of the rudder blades themselves. I could really delete the first 3 photos they don't mean much. Thanks for your reply Cheers Specmar Aus

 

Also please post a photo of the stern in the water, and tell us or show us how the boat is stored most of the time. What sort of electrical grounding plate does the vessel have, if any? I would want to know that the cathodic protection had been maintained without a lapse all those years. Is it an aluminum hull?

 

In addition to Kevin's comments

Can you explain what you mean by this:

Define alloy shafting..what is it when compared to round bar? As you're implying a difference if so, what is it you image the difference to be and actually what IS the difference. Most shafting material/grade is no different from solid bar in its composition and metallurgy.

 

an idea to consider: stress corrosion. I am not that familiar with marine alloys, but I know on aircraft alloys, some combinations of heat treat, stress load and nearby chemicals (like deicing fluids used on aircraft or runways), intergrainular corrosion occurs along the microscopic grain boundaries. These can not been seen with the eye when it is occurring, until the corrosion plane makes it through the part and it fails, almost falls apart under a light load.

High strength alloys can often have this annoying tendency to get a very thin corrosion plane working its way through the part at the grain boundaries (you can only see the action with an electron microscope), the chemicals start the process on the surface, and tiny tendrils work their way down deep into the part over a long time. Sometimes the internal stress gets so large that the part can just "pop" apart without any load.

the irregular jagged edges of the failure plane on those pictures suggest to me that is exactly what has occurred with your rudder shafts (failure plane is along grain boundaries, not in a shear plane). Taking a number of years to show up is also typical of this kind of problem.

There are several ways to deal with it: use a different alloy, change the chemical environment around the part (regularly soak the part in LPS or Corrosion-X or similar product). after anodizing, paint the part with hard coatings to prevent the corrosion from starting on the surface, etc. It is a difficult and complex problem deal with, go read up on stress corrosion in alloy parts.

good luck.

 

Gentlemen
Thank you for your keen interest here I managed to track down the production manager where the Owner purchased the Rudder shafts in 1996.
He explained that the Material is 6082 T6 and is today still supplied to InCat Crowther for the rudder shafts of some of their vessels. Whether or not it is classed as shafting or round bar is as was pointed out possibly immaterial, however they were Machined / Ground round and true when we took delivery.

My first thoughts are looking like the likely culprit.

“To me it looks like the billet was stopped at this point during the extrusion

stage and it is a cold joint however small it was in temp it has taken this

long to show up”

Well almost right, when they push the aluminium billet through the die to form the extrusion they add new billets as required and this causes piping see extract below
For the technical explanation.
In other words the hot center of the extrusion runs faster than the cooler exterior at the die face and lags behind the core when the new billet is pressed into action the combination of different temperatures and the lagging outer surface form this cold join.
The solution is to end check both ends with acid where the flaw can be seen see Fig-3 (b)

Reference:
PRODUCT DEFECTS IN ALUMINUM EXTRUSION
AND THEIR IMPACT ON OPERATIONAL COST
A.F.M. Arif1, A. K. Sheikh1, S.Z. Qamar1, M.K. Raza1, K.M. Al-Fuhaid2



Page 4

Pipe Formation, also called extrusion defect, fishtailing, or back-end defect, is the most
common flaw encountered during aluminum extrusion, especially in the 6xxx series
alloys: an annular separation in the cross section (due to division into an inner core and an
external zone) in the rear third of the extrusion. Billet-container friction results in the
billet surface layers remaining stationary at the container wall while the billet core is
sheared past. The region of dead metal that exists in the center directs the flow forwards
giving rise to a cone shaped defect [Sheppard, 1999] as shown in Fig-3 (b) in the report Page 15

 

It will also depend on the size of the original billet and the type of extrusion ie pushing the die or the molten billet, usually one way is better than the other. The ammount extruded in one 'hit' will make a difference, the larger the KG throughput the more reliable generally. As it happens 6082 is pretty hard to clean from the die so generally the extruder would want to run the largest ammount in one straight go as possible for good consistency. I'm actually surprised that the material where the 'join' was, should be considered sufficiently homogenous to that either side. One might have thought it could be 'marked' as it runs through (maybe a dye) cut out (later when cool) and it get remelted, or at least separated from the better part of the run.

I realise it actually moves at a very, very quick rate out of the die but not impossible to monitor. The very first few metres and the last would be suspect for quality and I would expect them to be cut off and recycled at least. Different extruders will have slightly varying ways of achieving the result, maybe contact the extruder directly?. Most are pretty helpful, in my experience.

 

Hi SukiSolo

thank you for your reply

During the conversation I had with the production manager he explained that a good deal of waste material is produced pushing these and heavy sections approximately 10.4 KG Lm
They usually recycle 30% or more mainly from the last push of the billet through the die.
This is caused by the higher temps in the core area hence the domed shape of the inclusion we have.
Given they Machined / Ground round and true prior to delivery it would have been very difficult to detect the flaw.
We are never too old to learn and I will now insist on Certified Extrusion that has been End Check Tested mim.
On an upside I am pleased it lasted 17 Years as it was goes to show even with this flaw the shaft sizing at the time was adequate for the use.