Tang Bolt and mast strength

Jean-Luc Van Den Heede's recent problem with his lower tang pulling down through the mast after a severe knockdown during the GGR got me thinking about the loads the lower tangs see. I've heard of example's of the tang bolt or tang breaking but this failure mode is less common and it acts as a useful data point for the loads in a rollover, Since the rest of the rig survived we can narrow down the loads and failure points.

This is a rough drawing of the setup he had, plenty of unknowns but I have made some educated guesses

8mm rigging wire
16mm tang bolt
4.8mm Monel rivets

8mm wire fails at around 4.6 tonnes . Since there are two shrouds the maximum load would be unlikely to have exceeded 9.2 tonnes and was probably a lot less than this. But the wire yields and would begin to show permanent elongation at approximately half the breaking strain, or in this case a total load of 4.6 tonnes, though it is unlikely the load is evenly shared between both shrouds.

The failure mode was the tang bolt pulling down through the mast 50mm on one side, presumably the rivets must have sheared off and all the load went onto the aluminium wall.

Here is the results from a quick spreadsheet I knocked up.

So if we assume a 16mm tang bolt we see that its Shear strength is about 5.7 tonnes. But since it is in Double shear due to the external strap it should be able to take about twice that load, more than enough to cope with the maximum shroud load to failure.

Look now at the bearing strength of a 4mm wall section mast, It yields at 2.5 tonnes and fails at 3.9 tonnes. The other side attached by 7 rivets fails at between 2.3 and 3.8 tonnes depending on the size of the rivet. (rivet breaking loads from Stanley). There is also likely to be some clamping force helping to secure the plate to the mast, but this is very hard to calculate.

My guess is that the mast wall yielded slightly, putting more of the load on the rivets. this may have happened earlier, as the mast yields at a reasonably low load. When she got knocked down the Pop rivets sheared placing the full load onto the mast wall which tore down, exceeding its bearing stress. the load may also have permanently stretched the wire, and in his latest phone call, he mentions the D2 (Diagonal 2's, discontinuous intermediate shrouds) needed tightening indicating they also may have yielded slightly. I am guessing they are 7mm wire so 3.5 tonne break, 1.8 tonne yield.

We can also see the required wall thickness to support the full load of the bolt alone, in the case of a 16mm bolt we would need a 9.1mm thick bearing surface, but if it's in single shear you would need to upsize to a 19mm tang bolt and make the wall 11mm thick to match the strength of the shrouds.

This mast was absolutely bulletproof for all normal sailing loads, but the incredible shock loads placed on the mast when it hit the water found the weakest parts.

Of course, the failure of the mast wall may have relived the loads enough to prevent the shrouds from failing. Perhaps if the tang had been stronger the whole rig might have been lost?

I am not an engineer, I do have some basic tech school training in these types of calculations (1.5 years at Polytechnic studying a diploma in mechanical engineering and drafting) but not enough to be happy if you use these figures to design anything without first having them checked by a Qualified and competent engineer. Also working with figures past yield or maximum design stress is not good practice. To really nut out what happened the mast needs to be analysed and the calculations made with much more advanced non-linear techniques like FEA.

I'd be interested to hear any real engineers comments and observations.

Fatigue... of the metal variety

This was the chainplate from an old aluminium open 60 I skippered. This happened before I joined on the voyage back from South Georgia to the Falklands. The boat got badly knocked down and the cap shroud hole in the chainplate pulled out, resulting in the mast getting a rather nasty bend just above the 1st spreader (it was amazing that they didn't loose it!). It took the resourceful Falkland islanders 10 days or so to pull the mast out, straighten it, sleeve the distorted area and re-install it (an amazing job...) While they were at it they welded in a set of massive new chainplates. All done with very basic facilitys. As you can imagine we had our hands full cleaning up the boat and fixing the million and one other broken things from the knockdown.

Also broken was the staysail stay chainplate, you can clearly see another crack underneath the point of failure. This thing was just waiting for an excuse to let go, and it failed before the knockdown... 

A full inspection revealed a crack starting in the stainless steel solent stay fitting (right side, at the top of the weld), This is a common place for failure, right near the heat affected zone, just outside a weld. Once the crack starts it concentrates loads at this point creating a stress riser and accelerating failure, also nasty things like crevice corrosion can start inside the crack, eating further into the metal....

The forestay chainplate wasn't too crash hot either... lets not push her too hard...

The point of all this is not to bag out the boat, just to say that on older boats don't assume that just because things have not broken, doesn't mean to say they won't. Metal fatigue and corrosion can invisibly weaken parts that otherwise look solid. The Open 60 had a complete new rig 7 years before this, but the chainplates were over 20 years old, and possibly slightly underspecified to start with. She had had a hard life, alot of it in Antarctica, (cold may accelerate fatigue - any engineers out there?). Nobody had thought the solid looking welded in chainplates could give any problems....

The most highly loaded parts of any boat are normally the rigging and what they are attached to (and maybe the keel bolts...). Sailors are well aware of the need to replace stainless steel shrouds every 7-10 years or so, but many fail to realise that the rest of the load path is also highly stressed and subject to fatigue. Quite commonly people replace the wire religiously (to keep insurance company's happy..) but never change the highly loaded through mast tang bolts, or the tangs themselves... The tang bolts particularly seem to fail quite often, then dropping both port and starboard shrouds, meaning you can't even tack to stabilise the mast.... Turnbuckles also need replacing every so often, as can the chainplates.

To be really safe I think it makes sense to consider replacing all the components every 2nd rigging change (20 years or so) and very carefully inspect all components at every rigging change. Chainplates should either be massively oversize, or easily replaceable. Be very wary of any stainless that passes through a damp deck, stainless can corrode very quickly in a damp low oxygen environment.

I replaced some U Bolt chainplates as a precaution before a delivery, and was horrified to find two of the ones I removed had lost 90% of the metal due to corrosion, they had looked perfect.... Lucky I had been told to watch out for this by a friend who had one fail on his boat. Stainless is treacherous stuff....

My chainplates are mild steel (with SS edging and inserts) to reduce the problems with fatigue. Mild steel has some pretty forgiving properties, one being that if you keep the stress below a certain point (the endurance limit) it doesn't really fatigue, unlike most other materials which will always get slowly weaker with each load cycle (that year of mechanical engineering was useful after all!). My bottlescrews are also galvanised steel, as is my rigging wire - both come with industrial test certificates and a SWL (unavailable for yachting stuff...). When I next re-rig my boat I will replace my stainless tangs with galvanised mild steel mast fittings as well.

There are some non destructive ways to test for cracks, but they are not 100% and could give you a false sense of security. I normally use a magnifying glass and look carefully, but I really should invest in some dye penetrant and developer, this supposedly highlights any cracks, has anyone used it?

Anyway I guess I have just given you yet another thing to worry about... Thats good if it means you might be less likely to loose your mast. If anyone has had a rigging failure I would be very interested to hear all about it....

Cheers

Ben

Roller reefing, and misguided mascochism

When I was young and full of misguided masochism I used to sneer at Roller Reefers - They ruin your sail shape I would say, and they can fail, and anyway I like to go forward and change sails, makes me feel like I'm alive... The truth of it was Reiger had a nice big sail locker and a heap of hank on sails, and to change to a roller reefing system would have meant a heap of redundant expensive stuff. I learnt to live with the sail changes - even got quite good at it - and to be honest sometimes I even enjoyed the process. 

But I remember one dark stormy night coming across Bass Straight, with a quickly rising head wind, wishing I didn't have to go forward and change down to the spitfire jib... Wishing I could pull on a magic piece of rope and magically make that sail smaller... wishing I hadn't been such a masochist..

All that wishing didn't work and I eventually had to go forward and change down , and all that salt water down my neck sure did make me feel alive - also made me feel bloody cold, wet and miserable, The old no 3 tucked under my arm was quickly stuffed down the cockpit locker, and the little spitfire jib had settled Reiger back into the groove.... Until the wind (as quickly as it had appeared) died away to nothing.. where's that magic rope? 

The next boat definitely had to have a Roller Reefer, I had also realised that carrying 5 or so soggy wet mildewy lumps of poorly folded sailcloth around was a bit dumb, tramping all that salt into the boat was bad for everything. Rinsing them with fresh water, drying and folding them up properly took the best part of a day, and the amount of space dedicated to them could be much better used.

So Snow Petrel has a "furler" (yes I know it's really a "reefer" but from now on I will call it a furler) and I am happy, whenever I wish I can pull on that magic rope and the sail magically disappears, Its all rather wonderful, and I wonder what ever possessed me to sneer at such a device... However some of my scepticism remains, so my furler is a simple time tested reefurl with a halyard return, and absolutely no roller bearings, swivels or anything to that can readily fail. It is a marvel of minimalist engineering, the only bearings are about $18 worth of orange PVC electrical conduit. I also put a really heavy 10mm 1X19 SS forestay inside it (I don't much like SS but needed something smooth) , with toggles top and bottom. For another similar unit see Alado

Mine has a massive welded aluminium drum that can fit a huge amount of 10mm double braid (big enough to hold and pull on, and strong enough to keep the wimp happy). I like to know I have enough rope on there to never run out of turns, even when I have rolled the sail real tight. I have had this happen on delivery's, Its a real problem, the corner of the sail is hanging out and there's no more rope to pull... the only option is to lash it up and try to dismantle the complex guards and then add a few more wraps, or run off and blanket it behind the main and quickly unroll and then re-roll it abit looser before the wind flogs it to pieces (neither option is even slightly fun... unless you really are a masochist)

I was lucky enough to get a really good furling headsail with Snow Petrel, It is radial cut, with a high enough clew to enable deep reefing without having to change the sheet lead. The high clew also keeps the sail driving well on a reach (look at a blast reacher on a racing boat), gives visibility and keeps it out of the bow wave. Low cut roller reefing sails are terrible things..

This sail also has a foam pad in the luff and still sets and drives quite well to windward in 30+ knots with the sail reefed well down to spitfire size, I have a solent stay that can be rigged just behind the furler, with a nice hank on spitfire jib but I don't use it, the furler works fine...  And when the wind eases I can quickly ease that magic rope and keep her moving at her best. I can set a small storm jib of the baby stay to help in stronger winds, but this sail is not essential. In a real blow I am quite happy to use just a corner of my furled genoa.

I suppose there are tradeoff's. I make sure the sail is in good condition, and fit a strong spectra leach cord to hold the sail together if It ever rips, at least until I can get it unrolled and down. A shredded sail streaming off a furler in a blow is not a good look.... Once that leach line goes you are abit stuffed.

Also the sail adds alot of windage forward that needs to be considered, and alot of weight aloft. The windage forward is useful in a blow with a drogue out, holding the bow downwind, but can be a pain at anchor, or when manoeuvring under motor. I wonder how some of these boats with twin furlers manage in strong winds, although I can see the advantages.

I use my furler like a throttle, tweaking it frequently to keep the boat sailing at her happiest speed. To reef the sail I now prefer to run off for a split second and blanket the sail behind the main, it means I can pull it in by hand easily and safely. I just have to make sure the sail is rolled evenly and tightly. If I can't do this I luff the sail just enough to take the weight out of it and winch it in. I don't like to let the sail flog to roll it up unless the wind is light, and then I normally need some weight on the sheet to get a nice tidy stow.

I am abit paranoid about the furling line being let go by mistake, and always lock it in the clutch and on the self tailing winch. When I leave the boat I tie the furler up with a separate piece of line around the drum (which I normally then forget to untie when I go sailing..). The only other problem I have had is the turns being thrown off the drum if I let it go to quickly, or let the sail collapse and fill with a slack furling line. At least with my open drum it is easy to fix.

I Now take some more care, and always keep half a turn on the sail, with a tight furling line. I also check it well before I need to roll the sail up. I have had turns get jambed inside fancy expensive units for the same reasons, and it can be hard to see, and even harder to fix, so if you have a covered drum I suggest you follow the same procedures, and also keep the fiddly cover guard removal tools handy...

So as you can see I am a pretty happy about it all, although  do have some ideas for improvements to the unit (I always have ideas for improvements....) and for the next boat I would consider a complete change of tack by going for a junk rig... But more on that one later.

Cheers

Ben

PS I had to buy my roller reefing unit so the praise is earned, I don't think I'd have a fancy one onboard even if they gave it to me... But saying that the good quality modern units do seem to be very well made and reliable. I just prefer my simple system.

Galvanised Rigging Vs Stainless Steel

Galvanised rigging doing its work

I hate stainless steel rigging. There I said it.. Yes it's shiny and looks nice, BUT... I think it is dangerous. Please realise that this is my opinion and many successful voyages have been made with stainless steel rigging, but I feel that galvanised rigging is a cheaper and better alternative that is often overlooked.

Right first some background , Back in the days when I was an obnoxious 16 year old I remember visiting a yacht that was about to head to the US with a very experienced delivery skipper. 

I  commented on the stranded stainless rigging on the mizzen (At least one strand was broken, and taped up with electrical tape).  Said something like "If I was you I'd replace that here in Picton rather than Auckland, the Waiarapa coast can be pretty nasty..." Needless to say he ignored my pimply advice and off he went. He Ignored the BLACK BOX and it was empty (see http://www.johnvigor.com/Black_Box_Theory.html ). Out of luck or bad Kama but anyway the Mast came down and landed on his head. 

I now always do a full rig check before any delivery trip, and normally with stainless steel will find at least one stranded wire, if the rig is over 10 years old or so. Here's some stainless failures I have picked up.

Chubasco- chico 30, Stranded lower after Napier-Auckland run (middle of shroud)

2041- A Challange 68, stranded intermediate, (Inside swage)

Wyuna 11- 42 foot steel ketch, Stranded intermediate (middle of shroud)

An adams 35, Stranded stb cap shroud (inside swage)

Interesting that they seem to fail either at the middle of the wire or inside the swage. Maybe harmonics in the wire cause it to fatigue in the middle? Getting the rig tension right is very important, as is correct alignment and toggling. Often the wire inside the swage that breaks is on the inside closest to the mast, so it may be due to the slack rigging on the lee side loading up the swage unfairly?

Admittedly one strand on 1x 19 wire is only 5% or so lost in  breaking strain, but it creates a stress raiser that can accelerate the breakages of the other strands in the same spot, and also can indicate that the wire is fatigued.  It needs replacing, (orthough we did gingerly sail 2041 across the Tasman after finding the broken strand).

Finding a broken strand can be tricky when its inside the swage, check for an uneven bump on the wire where it exits the swage, (I use feel as much as my eyes). If you find a bumpy strand see if it moves, using fingers, pliers or a screwdriver carefully, any movement is bad, it is broken inside the swage. When you slack of the tension in the wire the strand will often pop back into its spot and look fine, but the break is there, conversly if you crank the crap out of the wire the broken strand may pop out of the swage for all to see.

My folks gaff ketch has galvanised rigging. It's rusty now but it is at least 20 years old. Snow Petrel has mostly galvanised rigging (except under the furler). It is over 7 years old and is mostly still fine, or although the lowers under the ratline seizing's are rusty. (I should have used Linseed oil). The Square rigger I worked on had 30 year old galvanised rigging that was still as good as the day it was put in thanks to worming, parcelling and serving with lots of linseed and stockholm tar protecting it.

This is why I use 10mm 7x7 Galvanised wire, Its cheap, reliable, easily inspected (look for bad rusting, a few small spots are no problem!) and strong (as strong or stronger than most stainless steels). I have a Stainless forestay under my roller reefing head sail as much because it is smoother so the foil rolls better on it, but it is Massively overkill 10mm 1x19 wire with staylocks at both ends rather than swages, and is fully toggled. It is about due for replacement before the next big trip, so I will buy a roll of 7x7 and use Flemish eyes (much stronger than a splice) to terminate it.

The only problems with Galvanised rigging is that it chafes rope and sails more than stainless. And doesn't look quite as "yachty". I use split black poly tubing to stop chafe, and to be frank I don't want my boat to look like a yacht, I prefer a tidy work boat look.

I get 7x7 aircraft grade with a galvanising certificate, as used in Aussie by farm irrigators. Most of the galvanised 1x19 seems to rust to quickly.  Also 8mm is about the smallest practical size to get good galvanising.

However  I am looking into using dyneema for the lowers, backstay and solent stay to lighten the rig..have had a good run with spectra running backstays so...  maybe I will discuss this latter.

Cheers

Ben