Arcona 460 sinks

[geem]

I doubt if my rudder would float, so surprised that the Arcona rudder might be bouyant. It would explain the excessive amount of movement that was ripping the hull apart in the area - but no water could be seen entering so the break must be some distance inside the hull. Looking at how deep the rudder is, could it have struck a submerged object in the past to start the break? No doubt the video will be studied in some depth as it is now the only record of what has happened to the rudder stock.

The rudder on my current boat certainly wouldn't float but some spade rudders don't have a stock that runs the full length of the blade. The idea being if you hit something, the bottom part snaps off. There may not actually be much weight in the rudder but reasonable buoyancy. Just a thought

 

[dunedin]

Rudder buoyancy? Aluminium shaft and rudder cored with foam?
I had to remove a rudder off my Catamaran in 2005 after being hit by a local boat. We did the job in the water. It was really hard to do because of the buoyancy of the rudder

I was wondering that - easy to suggest sitting ashore with coffee in hand, but could the rudder shaft have been hammered down till the rudder floated free, then attempt to plug the hole. Generally there is a tube of some sort above a JEFA bottom bearing that is above normal sea level. Presumably the loose rudder broke this off. If it had been possible to push the rudder down and out the boat Mia’s have stayed afloat. Then would come issue of getting to harbour with no rudder.

 

[dunedin]

The rudder on my current boat certainly wouldn't float but some spade rudders don't have a stock that runs the full length of the blade. The idea being if you hit something, the bottom part snaps off. There may not actually be much weight in the rudder but reasonable buoyancy. Just a thought

The full engineering diagram of the Arcona 460 rudder stick is shown in the link in post #20.
The stock covers the majority of the blade - but suspect like other Jefa rudders (and other Scandinavian boats) the bottom few centimetres of the rudder tip are indeed designed to be sacrificial if have a grounding on a rock or similar.
Jefa rudders in general will have been well tested with rock groundings, being so common on Scandinavian and Baltic built boats- as well as hundreds if not thousands having done trans Ocean voyages.

 

[geem]

The full engineering diagram of the Arcona 460 rudder stick is shown in the link in post #20.
The stock covers the majority of the blade - but suspect like other Jefa rudders (and other Scandinavian boats) the bottom few centimetres of the rudder tip are indeed designed to be sacrificial if have a grounding on a rock or similar.
Jefa rudders in general will have been well tested with rock groundings, being so common on Scandinavian and Baltic built boats- as well as hundreds if not thousands having done trans Ocean voyages.

I had a look at the jeffa site. I don't know which cruising rudder the Arcona would have but choosing one for a 42 to 50ft boat, the rudder weighs 22kg and has a volume of 77 litres. The aluminium shaft can't extend close to the bottom of the rudder due to their shape unless they machine a lot of the stock away. Either way, it's possible that the rudder is bouyant.

 

[rotrax]

The autopilot arm can clearly bee seen above the break.

I am sure the early info on the Hanse failure suspected poor fitting/alignment of the autopilot arm.

 

[matt1]

Yes, the megawatt sinking was attributable to a retrospective autopilot install from what I recall of the MAIB report

I dropped my Jefa rudder a few weeks ago and with all the top assembly off it was very difficult to lower it. It took a lump hammer and timber and came down mm by mm. It was very snug in the bearing .

 

[C08]

I sold the lightning burnt out wreck of Stingo to an american couple. When they hauled her out, they insisted that the lifting straps be placed about a foot astern of the lifting marks that were clearly marked. Idiots! The straps ended up under the saildrives. That "wisdom" ended up ripping the saildrives off the engine block and breaking the saildrive seal, leaving a gaping hole in each hull of several square feet... Glug, glug, glug, said the catamaran formally known as Stingo.

It took several scuba divers to place the lifting straps in the correct position and then another four hours to slowly lift (and drain) her.

Both engines and saildrives and all the electrical looms had to be replaced.

I used to follow your exploits in the Pacific with great interest and wondered what happened to "Stingo". Good to see you survived a lightening burnout which must have been a sobering experience. Good luck with your new boat search.

 

[Grace/Arcona-460]

There is a newer version of this boat, the Arcona 465, which is made entirely out of carbon fiber. I will definitely look into getting a carbon fiber rudder as a replacement.

I looked into the design of the rudder for the "all-carbon" Arcona 465, which is a more recent version of the Arcona 460 which IdaLina was, and Grace is. Interestingly, this new all-carbon boat has a nearly identical rudder, and a rudderstock of aluminum! They are both Jefa rudders. They have the same exact dimensions, with the same diameters and taper schedule, with the difference that the 465 rudderstock is 77mm longer (seems to be at the top, near the top bearing). The 465 also has a different, taller bottom bearing (but the same top bearing). Most importantly, they both have the rudderstock of the SAME material, Alu6082. The design of the 460 rudder is from 2005, while the 465 rudder is from 2016. The 465 is said to have a bit taller freeboards, so I assume that's the reason for the 77mm longer rudderstock.

This all tells me a few things:

1) Although the designs were made 11 years apart, for all practical purposes they are the same. So, if there were some "bad calculations" done for the 460 rudder design, it didn't dissuade them to still use the same design again 11 years later... although they had every chance to change things for the new design (as they did with the bottom bearing).

2) I'm likely not getting a carbon rudderstock.


Anders

 

[RunAgroundHard]

There are too many unknowns to make a judgement. From the video the stock has sheared just below the clamp, and the top of the clamp looks to have a gap between the clamp and shaft. That could be as a result of the failure, or was the clamp not fitted correctly such that there was a stress concentration. Who knows? Aluminium is relatively malleable and if a tapered clamp was fitted off taper, or a parallel clamp was partially fitted onto a taper, it could deform the metal. I would certainly question criticality of clamping with the OEM of the rudder, especially if there was taper involved. While one could expect a key to maintain correct positioning, maybe there was an issue with the key. Who knows, certainly a difficult one to make an analysis on. It is certainly worth understanding exactly how the rudder stock should be assembled if making additions, adjustments, modifications or even just removing for inspection and inspecting ones own boat if the situation is similar.

 

[jlavery]

A tube does not have a higher bending strength than a solid cylinder of the same diameter. The strength of each depends on the second moment of inertia along the Z axis. The second moment of inertia of a solid cylinder is greater than the tube of the same diameter because the cylinder includes all the central part of the solid that is not there in the tube, apart from an infinitely thin strip at the centre of the cylinder that contributes nothing to the bending strength.

You calculate the second moment of inertia of a cylinder in the Z direction like this...

View attachment 175509
For a tube you do the calculation twice - for the outer radius and then the inner radius and subtract the inner result from the outer result, or combine the subtraction into the equation like this...

View attachment 175511

As you can see it depends on the 4th powers of the radius, so for an identical mass of material a tube gets much stronger than a solid cylinder quickly. For a given mass per length the strongest you could make it would be a tube with the mathematical limit of an infinitely big radius and a zero thickness wall, but fitting it in your hull may be tricksome.

Thank you for the reminder of the theory. Takes me back to my aeronautical engineering degree - coincidentally, I was discussing the complexity of the maths involved today with the marine engineer working on Arpeggio.

Pedant alert - I think it's either second moment of area or area moment of inertia. I remember liking the idea of an infinitely deep zero thickness I-beam.

 

[AngusMcDoon]

Pedant alert - I think it's either second moment of area or area moment of inertia. I remember liking the idea of an infinitely deep zero thickness I-beam.

You are right - my terminology was an incorrect combination of terms. It's 35 years since my degree. Never used this stuff in anger.

 

[Sea Change]

I agree with Tranona, stupidity, collisions and Orcas apart, the incidence of rudder and sail drive failure is simply not significant

Without looking up the statistics I would hazard a guess that rudder loss is the biggest cause of boat loss (whether by sinking or abandonment) in offshore sailing.
'Not significant' is no comfort when you're a thousand miles offshore.
Call me a luddite but I think a high aspect ratio aluminium stock spade rudder is a racing design, not a cruising one. No thanks.

It does seem perverse not to have watertight and structural bulkheads sealing the rudder from the rest of the vessel, excepting the bow - which should have its own sealed bulkhead (in both cases a spin off hatch could be used to allow access). This is easier on a multihull which has too much storage space anyway .

Jonathan

Agree. I've crudely blocked off the space around my own rudder, just a few layers of glass here and there. An afternoon's work which could save the boat.

 

[Neeves]

The 6082 aluminium alloy has a very high tensile strength (much higher, like 2 or 3 times the tensile strength of 316 stainless (which, 316, has impressive yield) and its major usage is in aeronautics. I believe its also used by Apple for their lap top cases, because of its tensile strength. Its yield strength is very close to its break strength, it is not malleable but has good abrasion resistance.

I tried using it for some of my components and when it broke it broke under test 'like a bullet shot'. Its said to have poor resistance to sea water. I did find corrosion an issue but was never sure it corroded because I did not keep the material clean. Much rock climbing kit and LFRs are 6082 and it does not corrode but it is anodised - which I tried, anodising, to good effect. For my application galvanised HT steel or Duplex stainless was the best result. I recall reading that mixing 6082 with other metals is a real issue, maybe including other aluminium alloys.

The steel for the threaded rods, studs, to hold up wind farm towers are galvanised HT steel (100mm diameter, about 2.5m long 120 of which are set in concrete).

Jonathan

 

[B27]

Seems odd to break the stock a long way above the lower bearing, which is the most stressed area.
That suggests the quadrant must somehow have damaged the stock?

I don't think the buoyancy of the rudder will make much difference, the leverage of the blade against the single lower bearing would probably jam the stock in the bearing with even the slightest motion of the boat. Getting a simple dinghy rudder on its pintles afloat can be difficult enough.

 

[AngusMcDoon]

The 6082 aluminium alloy has a very high tensile strength (much higher, like 2 or 3 times the tensile strength of 316 stainless ...

316 Ultimate tensile strength 700 MPa
6082-T6 Ultimate tensile strength 330 MPa

316 Young's modulus 200 GPa
6082-T6 Young's modulus 69 GPa

You have your strength ratio the wrong way round.

 

[davidej]

I have just looked at the diagram of the rudder. I noticed that there is a keyway, presumably to locate the quadrant. The end of a slot like this is ideal for propagation of a fatigue crack and, indeed, the fracture is right next to the quadrant.

 

[robmcg]

Call me a luddite but I think a high aspect ratio aluminium stock spade rudder is a racing design, not a cruising one. No thanks.

As a fellow luddite, I am inclined to agree.

 

[GHA]

As a fellow luddite, I am inclined to agree.

And another. Unsupported spade rudders for crossing oceans is just too scary for me though plenty do it. So much garbage out there to hit.

 

[Grace/Arcona-460]

Call me a luddite but I think a high aspect ratio aluminium stock spade rudder is a racing design, not a cruising one. No thanks.

As a fellow luddite, I am inclined to agree.

And another. Unsupported spade rudders for crossing oceans is just too scary for me though plenty do it. So much garbage out there to hit.

Well, luckily boats come in all shapes and sizes!

Personally, having worked for 35 years in high-tech engineering, I think I'm permanently banned from any Luddite community. From the rafts I used to build with logs and sticks as a kid, through all sorts of floating and not-so-floating contraptions, including a large number of kayaks, a multitude of cruising and racing sailboats up through IACC Americas Cup yachts, I have always enjoyed playing on and in water. All I can says is that a well-tuned high-performance sailboat brings me a joy that few other things can compare to.

Now, as we all know, the ocean can also be a cruel and horribly unforgiving place. When disaster strikes, everyone still alive leaves with dread and regret. I've seen boaters just through inattention get themselves into deadly trouble, seen drowned bodies being dragged up on the beach right in front of me by rescuers, and witnessed other near drownings. I have also had the privilege to lend a helping hand to someone who otherwise no doubt would have perished. We all have a responsibility to ourselves and all the people around us, to play responsibly, and use whatever information available to increase safety. Learning from IdaLina's unfortunate disaster is one way I think we can do that.

Thanks,

Anders

 

[RunAgroundHard]

... The end of a slot like this is ideal for propagation of a fatigue crack ...

Design of keyways to locate devices on shafts is well understood and if there was an issue with the design, it would likely be known by now. I would be very surprised if the keyway was a stress concentrating feature of significance. It is just not possible to know what happened with current information.