Arcona 460 sinks

[yotter]

How d
How do you work out that a tube is stronger than rod, assuming you are talking about the same diameter?
You don't hear of saildrives sinking boats. The issues with saildrives are more associated with corrosion. Rudder lose or steering lose is far more prevalent.

I understand that is is well accepted that tube is stronger than solid. Please read my previous post re sail drives.

 

[geem]

I understand that is is well accepted that tube is stronger than solid. Please read my previous post re sail drives.

Tube isn't stronger than solid bar. That's crazy!

 

[Grace/Arcona-460]

If I were you I'd be also thinking of how to make the bulkhead forward of the rudder watertight. It is on mine and on many other boats. It should be easy to achieve this on most boats, so I find it really puzzling that firstly manufacturers design their boats with this vulnerability and secondly that boats are not routinely modified to rectify it. This is far from the first boat to sink from damage in this area.

Zing,

Thanks! Yes, this was already on my list of ideas for future projects (triggered by Orca attacks, see my comments above). Give these recent developments, this is at or near the top of my list now.

Thanks,

Anders

 

[yotter]

I hate to create drift, but I recall rudder failure during the the 1979 Fasnet with OOD34 carbon fibre rudder stocks,. Standing by to be shot down on that. (I am trying to add to the debate and trying not to be be negative).

 

[yotter]

Tube isn't stronger than solid bar. That's crazy!

That's my perception (looks to be wrong), I am probably crazy as you say! Perhaps its weight for weight for weight. We need Viv Cox to sort us out. I just want to add that I feel for tor the Ancona 460 loss of their home, this is not forgotten and I wish them well.

 

[Grace/Arcona-460]

I hate to create drift, but I recall rudder failure during the the 1979 Fasnet with OOD34 carbon fibre rudder stocks,. Standing by to be shot down on that. (I am trying to add to the debate and trying not to be be negative).

As far as I remember, back in those days, there was still a lack of understanding of how to correctly calculate loads on stand-alone rudders, and also how to calculate the strength of carbon constructions (especially when combined with other materials). The way I remember it, since carbon fiber is so stiff, many early calculations missed that the carbon component essentially takes all the load early on, while other materials bend to accommodate an increased load, lending very little of their strength to help the carbon component before it breaks.

I’d be more worried if modern designs had a tendency to break regularly, but as far as I can tell, these early issues still stick in people’s minds… But maybe I’m missing something?

Anders

 

[Neeves]

As far as I remember, back in those days, there was still a lack of understanding of how to correctly calculate loads on stand-alone rudders, and also how to calculate the strength of carbon constructions (especially when combined with other materials). The way I remember it, since carbon fiber is so stiff, many early calculations missed that the carbon component essentially takes all the load early on, while other materials bend to accommodate an increased load, lending very little of their strength to help the carbon component before it breaks.

I’d be more worried if modern designs had a tendency to break regularly, but as far as I can tell, these early issues still stick in people’s minds… But maybe I’m missing something?

Anders

I agree

Extrapolating from almost 50 years ago is completely daft. Failures in motor vehicle engineering has moved forward since then as has use of carbon (which was in its infancy then) and yacht engineering.

If the rudder was supplied by Jefa then there should be two teams of engineers looking at this failure (and a lot of worried owners of Jefa kit). Surely Jefa approved and recommended the rudder assembly. This lets Arcona have some wriggle room but does not remove the issue from their responsibility.

Jonathan



Jonathan

 

[Tranona]

I hate to create drift, but I recall rudder failure during the the 1979 Fasnet with OOD34 carbon fibre rudder stocks,. Standing by to be shot down on that. (I am trying to add to the debate and trying not to be be negative).

Not sure there are any "statistics" on failures of rudder stocks or saildrives although many reports over the years of individual failures, or small numbers of the same type such as carbon stocks on Beneteaus and GRP stocks on American Hunters. In both cases those resulted in redesign/replacements. The only stock failure that has resulted in a public examination was that on Megawatt a Hanse 371. However despite rigorous independent examination the results were inconclusive and no other failures have been reported of the same type of stock.

As for saildrives, not sure there have been any failures that have resulted in sinkings apart maybe from collision. Like aluminium stocks, literally tens of thousands of saildrives in use over the last 40 years or so with no evidence of systemic failures. On the other hand isolated incidents over the years of boats sinking because of failure of shaft drive sufficient to le enough water in to sink the boat!

 

[Neeves]

I agree with Tranona, stupidity, collisions and Orcas apart, the incidence of rudder and sail drive failure is simply not significant (and this despite many gaskets being used well beyond their use by dates). If rudder or sail drive failure were common place, they would be reported, by someone (bad news travels fast) - we would know (and so would insurers).

One suggestion might be that in general yacht designers and boat builders have got it about right (and probably over specify) - so the current Arcona incidence is the exception.


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

 

[AngusMcDoon]

That's my perception (looks to be wrong), I am probably crazy as you say! Perhaps its weight for weight for weight. We need Angus McDoon to sort us out.

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...


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...



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.

 

[Stemar]

All that is why I didn't become an engineer...

Sympathy to the shipwrecked crew - glad they're OK

 

[doug748]

Good for them getting a clear image of the problem:





.

 

[Concerto]

Looking at the video, what is holding the broken rudder within the hull? I would have thought once the shaft had broken, the rudder would have dropped out. This screen shot from the Arcona 460 brochure (https://arconayachts.se/wp-content/uploads/2018/10/arcona-460-information-new-sept.pdf) suggests this could be likely as there is no rudder bearing on a skeg..

 

[Grace/Arcona-460]

Good for them getting a clear image of the problem:

Yes! I was thinking the same thing. Imagine our confusion now if they hadn’t had the presence of mind to take that movie snippet, while fighting for their own survival! This was a gift to all of us!

Anders

 

[geem]

Looking at the video, what is holding the broken rudder within the hull? I would have thought once the shaft had broken, the rudder would have dropped out. This screen shot from the Arcona 460 brochure (https://arconayachts.se/wp-content/uploads/2018/10/arcona-460-information-new-sept.pdf) suggests this could be likely as there is no rudder bearing on a skeg..

View attachment 175522

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

 

[doug748]

" I would have thought once the shaft had broken, the rudder would have dropped out."

I am sure that my old waterlogged rudder would go right down, however, It's possible that a pretty new rudder may be quite buoyant?

 

[Grace/Arcona-460]

Looking at the video, what is holding the broken rudder within the hull? I would have thought once the shaft had broken, the rudder would have dropped out. This screen shot from the Arcona 460 brochure (https://arconayachts.se/wp-content/uploads/2018/10/arcona-460-information-new-sept.pdf) suggests this could be likely as there is no rudder bearing on a skeg.

The boat has a deep spade rudder - no skeg. The text (in Swedish) on Instagram said they had to remove the quadrant to jettison the rudder. They did this as the broken rudder stock was tearing up the rear end of the boat due to the motion in the (relatively benign) waves.

Anders

 

[AngusMcDoon]

The way I remember it, since carbon fiber is so stiff, many early calculations missed that the carbon component essentially takes all the load early on, while other materials bend to accommodate an increased load, lending very little of their strength to help the carbon component before it breaks.

The difficulties with CFRP are brittleness meaning poor ability to stand up to shock loading (which is basically what you are saying), cracking from stress raisers like holes & sharp corners, and difficulty in manufacture. CFRP requires careful heat control to cure & get its maximum strength. Get that wrong & a failure is likely as happened to Team Philips when the front fell off after hitting a wave, even though it was designed to very rigorous maritime engineering standards (no cardboard, no rubber). Chance in a million hitting a wave, apparently.

 

[AngusMcDoon]

" I would have thought once the shaft had broken, the rudder would have dropped out."

I am sure that my old waterlogged rudder would go right down, however, It's possible that a pretty new rudder may be quite buoyant?

My trimeringue's GRP centreboard and rudder are both buoyant. Release a downhaul on either and they bang up pronto. It's a huff and a puff to get them down.

 

[Concerto]

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 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.