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.

Storm Tactics Part 2 Before the blow

Yes this is a lame picture,about 35 knots so get some sleep now.

The answer to the Blizzard question from part 1, We were kind of reverse hove to with just a backed storm jib and the helm down, this steadied us up somewhat but we were still lying more or less beam on, or maybe the bow was abit down, perhaps 110 degrees relative. The wind was touching 50 in the gusts so probably about 35-40 knots of real wind, and the seas were still pretty low, no more than 3 meters? We were really just taking a break, it not being worth bashing into for the miserable progress we would have made. The wave that came over was definitely a rogue wave, and much bigger than the normal. I sensed it just in time to swing the camera around.

But to move on, I made a mistake in Part 1 Did any of you notice? See I knew you weren't paying attention.... (mind you I didn't notice it either) The caption on the first picture reads "Careless is the ass, who sleeps with a low and falling glass" when in fact this is probably exactly what you should be doing.

Rolf and Deborah from "Northern Light" drummed this into me as a kid when I spent hours reading my grandfather's copy of their excellent book "Northern light". They have a term called the sleep bank, you put hours into it and you take hours out. simple, and you do have an overdraft facility, but go to far into debt and you are in serious strife... It is scary how quickly your decision making part of the brain gets defective with lack of sleep. You start doing really stupid things... Like writing blogs..

So Number 1, get some sleep. Also:

• If you haven't already (and you really should have been..), get the best weather info you can and work out if you can set a course to clear the worst areas.
• Study the chart, make sure you are no where near a seamount, edge of a steep continental shelf, or fractured zone these may cause upwellings or somesuch that can make the sea state horrendous. Two deep sea fishermen I have talked to like to get 30 or 40 miles clear of even very deep seamounts if there is any bad weather forecast. They also report 3+ knots of subsurface current at times near these areas. These fishermen are the only good source of info I can find on this, so don't take it as gospel. Here is an interesting link about the schooner "Orbit" capsizing, make sure you also check out the technical notes at the bottom of his page.
• Start logging weather info as often as possible. Then you can possibly track the low center. If the storm force winds aren't forecast you have an obligation to let the nearest met office know, and broadcast it to other vessels.
• Cook up a feed, eat and drink lots of water, organize some snacks.
• Tidy up all that mess that always accumulates (on my boat anyway, maybe you are a tidy person)
• Secure boat (ie lash down that heavy sextant box)
• Get storm gear ready
• Make sure batteries are topped up
• Lash spinnaker pole down well (it is your emergency mast). I must set mine up to store below.
• Put clothes, computers and stuff in dry bags
• Get crap off the deck (fold and stow sails) 
• Top up any day tanks, empty all bilges, and make sure they are clean and the pumps work.
• Put the small vane on the wind vane
• maybe consider seasick tablet if you are so inclined
• I put extra lashings on my liferaft, to disable the hydrostatic release. You can make up your own mind about this.. BUT definitely make sure a sharp knife is handy...

But also SLEEP.

I'm sure there are many points I've missed. This list is not definitive, let me know if you think of any others and I'll include them.

You are allowed a moment of panic, abit of fear, and quite alot of worry,(unless you really are a robot) but try not to let it stop any of the above. And if you have crew, don't let it show to much...

I will try to write something on sleep patterns latter, but for now I will just note that kind of meditating and relaxing in my bunk seems to work almost as well as sleeping, so now I just get my head down and relax, rather than trying to force myself to sleep. Even 20 minutes of not quite asleep refreshes me alot. You shouldn't be in nasty stuff in the first few days (if you are you need to learn to read a forecast..). So at least  you should be in sea sleeping mode (It takes me a few days).

At this stage the boat should be making its way out of the path of the worst weather and hopefully towards your destination. You may have some great sailing, as the winds and sea build, slowly reducing canvas. I don't push the boat hard at this point unless I really need to get out of the way of something very nasty.

I like to study the sea, Especially the swell directions - there will be two or even three swell patterns overlaid, and of course the wind-driven sea on top of this. Getting this info may prove vital later, because this will give you some idea of the direction of any rogue waves, and may give you some idea which is the best tack or gybe be on latter if things start to get really nasty. Rogue waves tend to come in the average direction of the swells and seas - ie a NW swell with a SW sea will likely give a rogue wave from the west, It also may not, but on probability it is more likely, so if running it would be better to be on the Starboard gybe heading east rather than on the port gybe heading north. Or if hove to, be on the port tack heading west, rather than south. You really want to minimise the chance of a rogue hitting you on the beam... So pay attention to the waves.

I think rogue waves are the big killer, and ultimately the best defence is a strong boat and a bit of luck -  the specific tactics have less to do with it, but certainly can help. That's why I like to prepare for the worst before leaving the wharf.

I have run with drogues in the past, With mixed results, and I would like to talk about this at some later point. But for now need to leave this topic a bit. There will be a part 3, or more, but for now this will have to do, I have already spent too much time deleting and rewriting sentences, and I am still not really happy with it...

All the best, and I hope you never need to use any of this stuff.

Ben

Storm Tactics Part 1 Preparation

After last nights boring battery discourse I thought I'd better give you something abit more salty, Although truth be told electrics will probably give you more grief than storms...

Careless is the ass, who sleeps with a low and falling glass.

I'm sure what you want to hear is lots of salty sea dog tales of waves washing the ship, while lightning flashes around our stout ship and the low clouds race overhead.. But I can't be bothered tonight (maybe I'll do some as case studies latter). What I can do is give you a glimpse into my thoughts on dealing with very bad weather.

I have been through some nasty weather in many different yachts and ships, There are plenty of people with more experience than me, and plenty with less, and lots of perfectly valid ideas and concepts. My best advice is to read widely, including opposite opinions to what you might think, Ie the Pardeys have a very different ideas to what a round the world ocean racer might prefer. Both are valid for different vessels and sailing styles and also listen to that quiet old sailor in the corner of the bar, his words might make all the difference in some blow somewhere.

My thoughts are that it really comes down to luck, good preparation, a strong boat and a good mental attitude, ultimately it has less to do with the specific tactics involved. I think to many people get tied up in the one magic solution, Hanging all your hopes on one favored tactic is rather dangerous, The sea is too dynamic, each storm will have very different wave characteristics, and each boat can behave unpredictably in extreme winds. what worked well in one case might not work at all in others.

Ok I will give you a salty story to illustrate this point. My friend Dave Pryce is one of the most experienced sailors I know. He took his 20 meter alloy yacht Blizzard down to Commonwealth bay and got caught in a very serious blow. In the past he has always run off at speed, steering down the waves, but this time the wind was to strong, the boat kept surfing down the waves, broaching uncontrollably, He described it as as if he had a spinnaker up in 40 knots. And he was under bare poles. He ended up lying a hull, apparently the wind lay her over so far she just slid down the waves with no rolling, like being hove too. Now this is interesting because if Dave said the storm was something else, it was, and the vibrations from the harmonics in the rig undid one bottle-screw, and the prop shaft bolts (he lost his shaft..). Dave agrees that lying ahull is not normally a good tactic, but he had no other options, and it worked for him. I wouldn't recommend this, beam on is usually the worst way to lie, but it shows that you need an open mind.

I start my storm tactics at the dock before I leave. Mentally I expect to be rolled, that way I am not surprised if it actually happens. And at least some of the lockers will stay closed (because I will have made sure they all have some sort of hold down). I make sure I have the basic gear and an idea how to make some sort of jury rig, because in a rollover the odds are not good that the rig will stay in the boat. I make sure I have confidence in the boat actually staying together. Any specific niggles are dealt with as far as possible, either physically or mentally (I tell myself that cars are much more dangerous..). 

There are always some things that will worry you, That is normal see The wimp, fear at sea . I suppose that's where experience comes in, and the luck. Fix what you can and try not to worry to much about what you can't fix. Remember, the fundamental prioritys, don't do any other nonessential work until these priority's are sorted out.

1 Keep the boat afloat and in one piece (staying afloat also means keeping off the rocks)

2 Keep yourself attached to the boat and uninjured.

3 Keep the rudder attached.

4 Keep the rig up.

If you can maintain at least 1 & 2 from this list you will almost certainly survive any storm. if you are not sure of these maybe reconsider your plans? Not sure who to credit these rules to, I first saw something like this written by John from www.morganscloud.com  many years ago in a magazine and they certainly are spot on.

I know when the boat and crew are ready enough. The key word here is enough, It doesn't mean all the jobs are complete (they never are...), just that the boat is safe and the crew are ready. The crew are familiar with the boat, this is best gained by working on the boat.

On Number 2  Keeping uninjured, I like to have helmets for extreme weather, and some sort of restraining device for my bunk, so even inverted I will not fall out. Quarter berths are good for this, but stick your head up the cockpit end. Lee cloths need to be strong (the only nasty injury onboard was due to leecloth coming undone) , and I like to have another flap that holds me in and protects me from flying sextant boxes and the like.

Enough for today, I have run out of steam, and you are probably bored, I will continue this latter, if my fickle mind doesn't forget and give you more electrical drivel instead.. 

But if you have got this far I have a reward for you, a Video,  I hope it loads OK, I took it from Blizzard on the way to South America somewhere halfway across the southern ocean.  See if you can work out how we were lying and what the wind speed might have been... Its about 3 megs or so, and has a big wave hitting us.

By the way, I think I lost the game (see the video to understand this cryptic remark..)

Cheers

Ben

The wimp and I... Fear at sea

Saw a very interesting  blog on the excellent site Attainable Adventure Cruising. See here for the article. John mentions dealing with fear and nerves at sea. It got me to thinking and I even decided to reply, I think this is a key topic and I hope to add more later. Here is my reply (after abit of tweaking and spellcheck!), but I recommend reading johns piece first.

About to enter the pack ice for the first time..Me and my Father, I hope you can tell I'm the one on the right.. Photo Matt Tucker

Hmmm..

Interesting, and thanks for being so frank about the wimp within.
I can be one of the wimpiest sailors I know, fretting and worrying over small and big things.

But then I have also managed to safely sail a small engineless 26 footer singlehanded across the Tasman in winter (Nelson to Sydney), Sail my 34 footer from Hobart to Commonwealth Bay, Antarctica and run a 60 foot charter boat in antarctic peninsula plus manage numerous dodgy deliveries.

Still the wimp within remains with me, nagging at me and keeping me up to scratch, and so far safe (touch wood).

I am a great believer in John Vigors Black Box theory, a kind of safety Karma thing, if you haven't already heard about it go and look at http://johnvigor.blogspot.com/ and click on the black box theory link at the top.

Thanks to all of you for making me feel not so unusual in my wimpyness. It does makes me wonder if maybe some of the great sailors of the past may also have a similar degree of wimpiness, and if in fact the very presence of Mr whimp is what somehow drives us to test our selfs.

When I first got my Ticket and became Third Mate on a 40000 tonne container ship I started having nightmares about running aground (the real wake up sweating type). When I mentioned this to the chief mate he just laughed and said they were normal “mates mares” and would soon pass as I got used to the responsibility.

By building up my skills and comfort level slowly I have gained an understanding and ability to analyse some of my feelings and know which ones are normal jitters and reactions and can be ignored and which ones signify some subconscious and important concerns that need to be actively dealt with.

However I still envy those happy go lucky sailors that seem to get by with not a care in the world, maybe there is 4 stages,

1 Ignorance is bliss, not even being aware of the dangers
2 Knowing the dangers and fearing them, or fear of the unknown
3 Confidence that you can deal with the dangers as they arise and any unknowns
4 Overconfidence and a fright can kick you back to stage 2

And those fearless sailors are at either at stages 1 or 3.5, or maybe they just lack any imagination.

Cheers

Ben

If you have any comments to add, please consider chucking them up on Johns site, This post is really an extension of his.. Ohhh and a big hello to the three people who have been over here for a look at my amaturish efforts..

Ben

Cyclone Yasi.. and old car tires

Katabatics Starting to ease off. Photo Matt Tucker

My thoughts go out to all those in the path of cyclone Yasi.

It's got me thinking about what I would do if Snow Petrel was up there, Having seen the strength of the Katabatics which I estimated gave us 80 or so knots for 3 or four days really makes me aware of what you guys must be going through. Fortunately we had no storm surge to deal with and lots of solid boulders to tie our 5 shorelines into.

My only slightly original thought (actually think I have seen it in a book somewhere.. like most of my `original` thoughts). I'd head to the nearest garage and get all the old tires possible, to use as shock absorbers on all lines, and fenders around the boat.

The humble tire makes a great if somewhat dirty and heavy fender (actually the weight is good, light fenders blow out of position). Tied into the line they absorb shock loads better than any snubber, and if the worst were to happen they make a good pad on the bilge if you end up on a beach. Tires are used by yachts during panama canal transits see here for some good photos.

Yes they can even be useful at sea, making a great emergency drogue, even better tie a couple into the line to make a crude series drogue.This has been called "de-spare tire..."  GROAN..

By the way I have never actually used tires as a drogue, it has been done before by others. I have only used conventional drogues and warps. Maybe I need to test the theory.... 

I am sure I don't need to remind you to make sure the rope is well protected from chafe, ie with chain or plastic tube covering, and if you plan to keep them choose small mini tires, paint them a nice topside matching color and cut some drainage holes in them with a hole saw. My parents Gaff ketch is painted black, just as well because we couldn't afford proper fenders so tires were used instead, and the black didn't mark the hull.

All the best

Ben