The Case for Softer Tethers

Rev.1-2-2014
(Since the original post we have moved to dynamic rope tethers (softer catch), but I have left this as there is still good information and it is still valid. However, in reviewing a number of accidents, where multiple impacts would have been poorly served by a single use energy absorber, I now recommend dynamic materials [elastic, like climbing rope].)

As an experiment, hang in your safety harness for 1 minute. Go on. We'll wait for you.

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Hurt pretty good after a few minutes, didn't it? Imagine falling a few feet on a 6-foot tether with no real shock absorption. The impact would over 10 Gs, or over 10 times what you  experienced just hanging there. The maximum, in lab testing, is over 20 Gs. Can you feel you ribs cracking and spine shattering? Of course, this is an exaggeration of anything likely to happen on a boat. How far can you fall?

Over the railing, of course. But there will also be some attenuation as the lifeline stretches and stanchions bend. The length of the fall will be lengthened about 2 feet (1 foot of give). YOu probably didn't really fall 6 feet; the jackline is inboard and if you fall more than 5 feet you'll hit water, depending on whether you went head or feet first. It.s going to hurt, but the actuall fall distance is only 4 feet and the was ~ 2 feet of breaking. The harness impact should be perhaps 3 Gs. Survivable with bruising, and probably no the worst case scenario.

Thrown from the cockpit. There have been a few cases where a sailor was washed out of the cockpit, the tether broke, and the sailor lost. How could the impact force have reached 4000-5000 pounds? Lets assume the sailor was thrown at 19 feet per second-- 11 knots, a good wave strike, only a fast jogging pace. He is clipped to a hard point--no give-- so the only attenuation will be the tether stretch, plus the wave is still pushing. In the case of the above 6-foot fall the time is about 0.61 seconds and the final velocity is 19 ft/second. Ouch.

It will save me a lot of typing if you visit this post about fall energy first. The long and the short of it is that falls on jacklines are less severe because the jackline stretches and the sailor also slides along during the line. Falls against a fixed anchor point are brutal. This is where we take up the thread.

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I've made some effort to research sailing chest harnesses and injuries, with very little luck. Clearly, if a sailor takes a 4,000 to 5,000-pound hit in the ribs there will be serious injuries; OSHA and military studies support this. Yet, because of the circumstances of these accidents, the actual injuries are usually unknown. The sailor is lost. Other times there are injuries, but the assumption--right or wrong--is that the sailor hit something, other than the end of his leash.

The harness or tether is typically blamed, though there are 2 falsies in this logic: the harness should never have seen a 5,000-pound load because there should have been shock absorption in the system; the sailor would have very likely died from his injuries anyway.

• "A lesson in harness and tether construction can be learned from Tami Ashcraft, who along with a sailor friend was making a sailboat delivery to Hawaii when they ran into a powerful storm. Ashcraft was knocked unconscious. When she came to more than 24 hours later, she climbed to the cockpit where a single tether dangled over the side. The D-ring had snapped where it was connected to her friend's PFD. He was gone. Ashcraft was convinced a round ring might have held, like those on Mustang vests." Ashcroft, in my opinion, was wrong in her conclusions.
• 1998 Sydney-Hobart Race Accident. Glyn Charles was never found, after his tether parted. But the force of dragging his body through the water could not have generated that kind of force, only a sudden impact. The inquest stated that he was attached to a "fixed point" and thus would not have benefited from jack line shock absorption. The lanyard may have also been defective, though I can't locate the testing information. http://www.parliament.nsw.gov.au/Prod/parlment/hansart.nsf/V3Key/LA20010307027.  http://www.telegraph.co.uk/sport/othersports/snooker/2994528/Yachting-Charles-died-after-safety-harness-failed.html.

(I now believe dynamic tethers make more sense, in terms of energy absorption and multiple impacts. It seems that several or even most of the fatal MOBs that were related to tether failure involved multiple high impacts, and this need would be poorly served by a single use devise such as a Screamer. I have left this article as food for thought, but please refer to this more resent post for more up-to-date information.)

And this leads to a very interesting conclusion: the shock absorption device (Screamer) doesn't really need to be on the the tether, it could be on the u-bolt. It is the fixed-point anchor that is the hazard. If the sailor is on the jackline, the system will stretch and slide and nothing will break. Some sailors have expressed concern about the elongation of the Screamer on-deck, and this removes that concern. No one wants more clutter on the tether. Up front, the real hazard is being dragged in the water, as many have said. Only if the sailor is attached to a u-bolt in the cockpit is the impact force extreme! Thus, if we clip Screamers only to the cockpit u-bolts, the force is absorbed where it needs to be and no new hardware needs to be invented! While this adds a little cockpit clutter, it could save lives very cheaply. In fact, the mere presence of shock absorbers on construction lanyards has been credited with safer work practices, as it reminds the workers of the incredible forces involved. The presence of Screamers in the cockpit might even encourage sailors to shorten their tethers.

Well, at least that's one possible answer, something very simple that would only need be deployed in extreme conditions. This is much how climbers originally use Screamers; other applications came later. 

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So I made up some new tethers a few weeks ago, for my own use on a catamaran. These are my best yet, and I like them both, though for reasons I will explain below--multihull reasons--I like the one on the left better. They both have features I like, though they will not be for everyone. (I have since switched to dynamic tethers.)

• Knots instead of sewn loops. I'm still playing with the lengths and years of climbing experience tell us that the knots are not a real risk. But years of climbing, industrial and marine experience also tell us that sewn joints are stronger, reliable, and durable. They would be sewn if I were not still experimenting. (These knots weaken the webbing about 30%). Tails must be 3 inches and knots must be tightened by bouncing on them with body weight to prevent loosening or slippage--standard climbing practice.)
• Attaching the snap to the center of a 2-leg tether with a larks head is quite convenient. It is a simple matter to adjust the legs without re-sewing or re-tying the ends. The larks head will not slip appreciably during a fall and could also be secured with a lashing, though that is not needed. This also simplifies the construction if sewn and probably results in very little net loss over time; several fewer joins, it can be moved if worn, and there is no UV vulnerable stitching. It also takes the load equally well from either leg or from end-to-end.
• We like screw gate biners on the jackline because we do not often unhook from the jacklines. They are light and snag-free. We unhook from the harness end. This is because we have a deep center cockpit and never harness in the cockpit. If you clip on-and-off the jackline, the Kong Tango is a better choice.
•  Rope jacklines. Rope is stronger and withstands UV better than webbing. It offers controlled stretch, absorbing fall energy. It gives a better hand grip and clips more easily. On the down side, it can be mistaken for running rigging; on our boat this is no problem because of location and because we leave the jackline end attached. It is worse underfoot, rolling and making for poor footing; ours are not under foot because they are run on the cabin roof--cats are wide. If I had a monohull I would still strive to get the jacklines up against the cabin trunck and out from under foot. I might be forced to use webbing, which I would replace frequently (6-months?) and not like. I believe jacklines and harnesses are for everyday use, that practice with equipment breeds competence, and that rigging jacklines only when "needed" is a big mistake. Even more than reefing too late, who wants to be rigging jacklines too late, in a thunderstorm or at dusk? On a narrower boat, non-stretch rope is required.
• Quick release on harness end. We don't, since getting washed off a cruising cat by a wave is very unlikely and capsize even less so. We prefer the security of a locking biner and the Tango is a quick-open compromise. However, there have been a number of drownings of sailors dragged by tethers or on boats that capsized; monohull, sport boat, and performance catamaran sailors should have quick release snaps at the harness end. However, be warned that most tether quick releases have failed in testing and in the real world to release under load . Test by hanging before you buy! Practical Sailor is investigating this issue. If your boat can capsize or is narrow and washing off a possibility, use a quick-release shackle.
• One leg vs. two. Vital, as most boats need a short leg on the side decks and at the bow, and a longer leg around the mast. Though I played with 2 versions for some years (the conventional 2-leg version; a 1-leg version with an intermediate clipping eye at about 65% length) we have settled on the 2-leg version as handier. Do NOT clip the unused leg to you harness; clip it to the eye below the harness shackle. In the event of a capsize you will find that releasing the harness tether has left you clipped to the boat via the unused leg!
• Shock absorption: Screamer. Notice the Screamer at the harness end. A simple $18.00 gadget that ensures the impact force cannot pass 600 pounds. I've broken ribs snowboarding and don't fancy doing it again. Climbers have been using these for many years, and ALL industrial and military tethers use them. Because they are easily replaceable (larks head to the 2-leg tether and overhand loop on the end of the 1-leg tether), triggering one is a minor expense (they are still full strength after triggering but will no longer absorb impact). I thought about only using the Screamers on the fixed-point anchors, but decided a fall over the side could be rough for me and that they were not in the way. This feature is NOT yet commercially available, though prototypes are being field tested.
• Shock absorption: Dynamic Rope.  8-10 mm dynamic rope is another excellent option. Read this later post. The advantage of dynamic rope is that it can withstand multiple impacts.
  
• Fit to boat. I think it is a mistake that all tethers are 6 feet or 3-6 feet; there are sound engineering reasons why falls over 6-feet are very serious, but honestly, boats are not one size. Most boats should have a short leg less than 3 feet. Multihulls need more than 6 feet to work effectively on the tramp. This is one reason I make my own. If you feel the need to modify your tether lengths, just be careful. This post may help.

There  does seem to be one potential weakness to having shock absorption installed in the tether; when it triggers the entire tether must be replaced. With a commercial tether, this will be about $200.00. There are 2 things I've asked the manufactures to consider:

• The Screamer section should be replaceable. Then it is only a $20.00 fuse.
• If the Screamer is non-replaceable, it must be protected from wear with a single bar tack. I have already learned that repeated non-critical impacts tend to loosen the stitching. This is not a safety concern, but in heavy duty use (Volvo?) it will reduce the effective life.

If this is something you would like to see on the market, call Yates and ask for them! They've built prototypes, but demand is required for an idea to make it to market. 

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 Another option is making the tether from a more dynamic material, such as nylon dock line or climbing rope. Let's consider climbing rope. A single rope meeting UIAA standards cannot have an impact force of greater than 1650 pounds (187-pound climber), while a typical modern rope gives a fall factor 1 impact force of more like 1000 pounds. These ropes are rated for twice this fall distance. How does this compare to the Screamer? The elongation is similar; about 2 -2.5 feet. The peak impact force is about double because the Screamer gives steady braking instead of graduated braking. The climbing rope gives lighter weight and greater simplicity.