Arcona 460 sinks

[phlip]

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.

Not quite right: the second moment of area is proportional to the _stiffness_ (for the usual "elastic plane-sections-remain-plane" scenario if we're going to be super precise - sorry!), not the strength.
The strength (resistance to bending moment) is proportional to r (or D) cubed - but there's more than one strength - are we looking for the bending moment that causes any of the section to yield, or the moment that is achieved when the whole section is plasticised - e.g. half of the circle is yielding in compression, and half of it in tension.

 

[phlip]

Thank you all for your replies!

@Neeves : Very thoughtful comments. Thank you. Given unlimited time, I agree entirely. Unfortunately, life is short, so just sitting idle while others figure this out isn’t really my style. Still I don’t want to do anything irresponsible, and I certainly don’t want to endanger others.

@Roberto : I don’t think a stainless steel rudder stock is a very appealing idea as these are performance-oriented boats which is no doubt the reason the original rudder stock was aluminum. Yet, I think using a different material might be the solution. There is a newer version of this boat, the Arcona 465, which is made entirely out of carbon fiber.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.

@vyv_cox : Thanks! This is the kind of info I was hoping for! I will definitely look into getting some kind of NDT examination done.

@B27 : I’ve been thinking along similar lines. We sailed through “Orca alley” two years ago sailing from southern Portugal to Northern Europe, and we will no doubt return through these waters again. There is a bulkhead just in front of the rudder and steering wheel. It is glassed to the walls of the hull, but it is not watertight. The whole area along the bottom is open, and there are a number of cables and hoses that pass through here. I don’t have access to the boat at the moment, so can’t say for sure that this is practical, but I will look very carefully into turning this into a watertight bulkhead. That would remove any issues around sinking due to damage to the rudder area, be it from metal fatigue, Orca attacks, or other calamities.

@yotter : Thanks! I don’t have any strong feelings when it comes to material choice, as long as everything is properly engineered. As we all know, sailboat design is an art of compromises and tradeoffs… I’d bet rudder failure dominates in the statistics you mention, since it is so much more prominently exposed to the environment. I too would be interested in these stats.

Thanks again! If anyone has further thoughts on this topic, I’m most interested!

Anders

I have an A435 which appears to have an identical steering system to the 465 (except perhaps for the actual rudder). While this is anecdotal, the carbon rudder option for the 435 has a smaller chord depth (the blade is more slender, apparently made possible by use of carbon instead of glass). While it's lighter, the main claimed benefit is drag reduction (from being more slender). There is apparently a price to pay beyond $$$: significantly less rudder authority at low speeds, leading to more "fun" in the marina. Others have remarked(?) that both the CFRP and GRP options use an alloy stock. Still others have pointed out that composites tend to be better in fatigue situations (weight for weight this is generally correct), but this isn't the whole story: Ductile materials (like many Al alloys) can absorb hugely more impact energy without fracturing. CFRP is definitely not so able to absorb energy in plastic deformation, so even if a CFRP stock were offered, it's not obviously a better choice.

 

[AngusMcDoon]

Not quite right: the second moment of area is proportional to the _stiffness_ (for the usual "elastic plane-sections-remain-plane" scenario if we're going to be super precise - sorry!), not the strength.
The strength (resistance to bending moment) is proportional to r (or D) cubed - but there's more than one strength - are we looking for the bending moment that causes any of the section to yield, or the moment that is achieved when the whole section is plasticised - e.g. half of the circle is yielding in compression, and half of it in tension.

It's a long time since I was a mechanical engineer. I left the dark side to go to the sunlit uplands of electronics and software decades ago.

 

[phlip]

It's a long time since I was a mechanical engineer. I left the dark side to go to the sunlit uplands of electronics and software decades ago.

Me too! - I just realised it's been 22 years since I got paid for doing structural engineering.

 

[Grace/Arcona-460]

I have an A435 which appears to have an identical steering system to the 465 (except perhaps for the actual rudder). While this is anecdotal, the carbon rudder option for the 435 has a smaller chord depth (the blade is more slender, apparently made possible by use of carbon instead of glass). While it's lighter, the main claimed benefit is drag reduction (from being more slender). There is apparently a price to pay beyond $$$: significantly less rudder authority at low speeds, leading to more "fun" in the marina. Others have remarked(?) that both the CFRP and GRP options use an alloy stock. Still others have pointed out that composites tend to be better in fatigue situations (weight for weight this is generally correct), but this isn't the whole story: Ductile materials (like many Al alloys) can absorb hugely more impact energy without fracturing. CFRP is definitely not so able to absorb energy in plastic deformation, so even if a CFRP stock were offered, it's not obviously a better choice.

Phlip!

You have a very nice boat! Interesting about the difference in rudder 'bite'! I know the 435 and 465 have a wider rear hull than the 460, so maybe things are different, but at least on the 460, the rudder has an exceedingly strong 'bite' - definitely among the strongest of all boats I've sailed. The pros are of course better low-speed maneuverability as you mention, but also less tendency to broach when overloaded. On the 460, you actually have to be careful especially when sailing with autopilot and not super-focused on tuning the boat - it is easy to ignore when you are overpowered and just let the AP tug along on the rudder when it would actually be faster and a lot less burdensome for the AP to de-power the main (flattening and/or twisting the sail, letting the traveler down a bit, or reefing). I could definitely survive with a bit less rudder 'authority' - within limits of course. Is this similar on the 435, or does the wider rear part of the hull require more rudder bite?

Your comment also makes me realize that I made an error in an earlier post here (^^^). I concluded that since there is a Jefa rudder with aluminum rudderstock for the 465, this is the rudder for the carbon 465. That is obviously not correct. The reason for this is that when the 465 first came on the market, it was offered in both GRP and CFRP versions. So it was obviously the GRP version's rudder I found on Jefa (see above). Later, they decided to make the 465 a pure CFRP offering - presumably since they had several models pretty close without huge differentiation.

Finally, on a happy note: I see that windy.sailing on Instagram report that they have arrived safe and sound at Hiva Oa. This is the boat that picked up the shipwrecked owners of IdaLina. A happy ending indeed!

Anders

 

[Grace/Arcona-460]

Just thinking out loud here, but if one were to swim under the boat with, say, a bunched up duvet and force it up into the 15cm hole, would that slow the ingress of water enough for the bilge pump to keep up?

I wouldn't go swimming in this situation unless the conditions were exceptionally calm - and I have done a fair share of swimming in rough water - it is exceedingly exhausting (as others also point out above). But perhaps a duvet could be stuffed into the hole from inside?

Another thought I had was perhaps one could line the rough edges with towels or similar, and then inflate a PFD or two to fill the hole?

Note: I'm in no way trying to second-guess what was done on IdaLina - I'm very impressed that they managed to survive this at all in what must have been an incredibly stressful situation - just thinking about possible routes of action from the comfort of my armchair.

Thanks,

Anders

 

[Roberto]

Another thought I had was perhaps one could line the rough edges with towels or similar, and then inflate a PFD or two to fill the hole?

This system was presented at the boat show a few years ago

 

[Grace/Arcona-460]

This system was presented at the boat show a few years ago

Very cool! Sadly, they seem to be better inventors than salespeople… On their webpage there is a link “Contact us for a quote”, but the script doesn’t work on my iPad… Not exactly 1-click shopping. :-(. And if they want to be found on the web, they should consider selecting a name that isn’t one letter off from AirBnB…

Sad to see a good idea monetized so poorly.

Anders

 

[Sea Change]

This system was presented at the boat show a few years ago

I'm sure that works fine when it's a hole, rather than crack, and you can get your hands through it, on a nice flat part of the hull, and it isn't next to a bulkhead or under an interior moulding.

For small or hard to reach cracks, I carry a can of expanding foam. Might prove totally useless, might not.

 

[phlip]

Phlip!

You have a very nice boat! Interesting about the difference in rudder 'bite'! I know the 435 and 465 have a wider rear hull than the 460, so maybe things are different, but at least on the 460, the rudder has an exceedingly strong 'bite' - definitely among the strongest of all boats I've sailed. The pros are of course better low-speed maneuverability as you mention, but also less tendency to broach when overloaded. On the 460, you actually have to be careful especially when sailing with autopilot and not super-focused on tuning the boat - it is easy to ignore when you are overpowered and just let the AP tug along on the rudder when it would actually be faster and a lot less burdensome for the AP to de-power the main (flattening and/or twisting the sail, letting the traveler down a bit, or reefing). I could definitely survive with a bit less rudder 'authority' - within limits of course. Is this similar on the 435, or does the wider rear part of the hull require more rudder bite?

Your comment also makes me realize that I made an error in an earlier post here (^^^). I concluded that since there is a Jefa rudder with aluminum rudderstock for the 465, this is the rudder for the carbon 465. That is obviously not correct. The reason for this is that when the 465 first came on the market, it was offered in both GRP and CFRP versions. So it was obviously the GRP version's rudder I found on Jefa (see above). Later, they decided to make the 465 a pure CFRP offering - presumably since they had several models pretty close without huge differentiation.

Finally, on a happy note: I see that windy.sailing on Instagram report that they have arrived safe and sound at Hiva Oa. This is the boat that picked up the shipwrecked owners of IdaLina. A happy ending indeed!

Anders

We have the vanilla rudder, and like yours it is amazingly powerful: We have had a few moments where I thought a broach was inevitable but it hung on, so I completely agree about getting sail trim right to minimize rudder drag.
It is a big relief that no/one was hurt.