Nearly three years ago I started an Online Book on crew overboard prevention based on the system that Phyllis and I have used for some 20 years. All went pretty well for the first three chapters. In fact, I believe we publicized a major improvement to the state of the art for short-handed crews: the multi-tether system.
But then I got to the subject of jacklines and attachment points and that’s where it all came unraveled, when several engineers and climbers pointed out that my assumptions were, let’s see if I can put this politely…absolute crap.
In fact, it’s at least arguable that Phyllis and I might have been better off with no jackline and tether system at all, because at least then we would have been fully aware of the dangers as we moved around the deck, rather than wrapping ourselves in a false sense of security conferred by a fundamentally flawed system.
And here’s the real scary part. I’m pretty sure that many (probably most) offshore sailors out there are at least as deluded as we were.
Since that revelation Phyllis and I have put a bunch of time and effort, including some real world experimentation, into improving our system. But before we get into that, we need to expose those scary delusions and understand the fundamental mechanics at work here. That’s what this chapter is about.
Much Ado About Nothing?
The realization that this rethink was required, and the huge effort it cost me (more than any other subject I have ever tackled), did not make me happy, I assure you. However, every time I stalled, a comment by Chris W drove me back to work.
…Randy Hooton, who died [by dragging] after going overboard from the bow (SW of Key West), was the brother of one of my wife’s life-long best friends…She has shared the unpublished details with us…
…while it is obvious Randy died, a dream died, a marriage died, and a family was brought to its knees…
You may wish to keep Chris’s comment in mind as you wade into what is not going to be an easy read, particularly since we have a bunch of explanation to get through before we get to the good stuff: fixes to flawed systems.
This month’s Yachting Monthly has a mediocre article on the same subject. It’s poor in the way it bandies around numbers with seemingly little understanding of what they mean (measuring momentum in Kn and then ‘converting’ to lb/feet, for example)
However, it does open with an interesting statistic that the author had gleaned from official accident figures, namely that “71% of those who went overboard while clipped-on and wearing a harness died, as opposed to 41%who fell overboard while not clipped on”. There are reasons why we cannnot rely on the significance of these figures such small sample size and the increased probability of being clipped on in bad conditions – but there is a clear question to be answered, namely “If you do go over the side, do you want dragged behind the boat or left bobbing in its wake?” The answer is not clear cut – but there is an argument that rather than strengthening all aspects of equipment until the weakest link is one’s own ribcage, it might be better to have equipment with a ‘fuse’ so that if you go over the side, you might land in the water capable or breathing and moving.
I personally do not agree with the argument not to use padeyes. I understand the shock-loading numbers but I cannot see how working at the mast or standing at the helm can be likened to a 2m freefall. Clipping to a point reduces potential falling distance and so reduces the energy that needs to be absorbed in a fall. If anything, it suggests that tethers made from energy-absorbing, stretchy fabric might be helpful.
Speaking to a veteran of numerous single-handed ocean voyages, including 4 OSTARs, he said that when working on deck he clipped on for one reason only – to make it easier for his wife to get probate if he went over the side! He’s nothing if not a realist!
Hi Rob,
Wow, that’s a disturbing statistic. As you say, too small a sample to be definitive, but still important, I think. Thanks for sharing it.
As to not clipping onto hard points. You are, of course, right that there are plenty of situations, like the ones you mention, where clipping to a hard point is probably safe. One could even have a rule that said: “don’t clip to a hard point that is closer that one tether length to the rail”.
But, to my way of thinking, that’s just too complicated. If we allow crew members to clip to hardpoints, sooner or later they will clip to a point closer to the rail.
Also, what about a wave strike in which person is washed to the end of the tether by green water? In this case without any give in the system the arrest load will still be high.
Better to just be clear and simple: no hard points, particularly since, as we shall see in future posts, there are plenty of viable alternatives.
From an engineering design standpoint it makes sense to incorporate the desired energy absorption capability into the tether rather than the jack-line. As John points out the degree of resiliency in a typical system is highly dependent upon the length of the jack-line, ranging from zero when attached directly to a pad eye to x for a jack-line that reaches from the cockpit to the bow. Whereas the tether is a fixed length so one can actually calculate the flexibility and absorption characteristics, and even fine tune it to body weight.
And for the next article, is not the ideal location of a jack-line on the centerline of the boat, thus allowing a length of jack-line short enough that it is nearly impossible to be swept overboard? (except possibly at the very bow?) I do recall seeing such an arrangement with a high mounted shroud-sized cable running from the mast to the front of a pilothouse and then another running forward.
Hi Richard,
You are right that incorporating stretch into the tether makes a lot of sense. However, even if we were to use dynamic climbing rope for tethers, we could still be subjected to some pretty nasty loads that any sane climber would try to avoid.
The basic problem is that in our case the ratio of fall distance to rope length—what climbers call the fall factor—is always between 1 and 2. Good climbers try to rig protection and belays to avoid any fall that has a factor over 1 and regard a factor 2 fall as potentially life threatening, and all of that assumes dynamic rope.
I can easily envision a way to inexpensively manufacturer a tether that incorporates progressive energy absorption up to the point of zero give rather than the more linear characteristics of climbing rope. Not a elastic tubular one like the one I have for sure, and not a bulky, one shot device that ends up putting the victim in the water like the Petzl would if used as a sailing tether.
I don’t think you can isolate a discussion of proper tether design from one of jack-line design. They are two dynamic components of the same system.
Perhaps I have missed it along the way, but are there statistics available for actual injury rates and severity from being arrested at the end of a tether vrs. engineering calculations that attempt to predict injury?
Hi Richard,
I think you are absolutely right that a shock absorbing tether might be doable. Although there is still the problem of fall length against tether length (the fall ratio). Dynamic climbing rope stretches 20-30% (max 40%) in a bad fall, but that is still not enough to reduce shock loads to acceptable levels. To really solve the problem we need to do as prudent climbers do and make sure that the shock absorber is longer than the fall. Give in the jackline can go a long way to doing that.
Hi Richard,
Good question, no, no solid stats at all—check out Rob Withers comment above for a bit of information—that’s certainly a problem, in that we are all guessing.
Bottom line, POB just don’t happen very often and I think actually the risks are very low on a well run boat. The problem is that if you go to sea long enough… See this post for more of my thinking aon the actual level of risk:
https://www.morganscloud.com/2013/02/17/staying-aboardpart-iirisk-analysis/
Hi Richard,
I agree that incorporating energy absorption into the tether is a good idea. There are times that being clipped to a fix point makes sense and many people don’t differentiate between tethers clipped to jacklines versus ones clipped to hard points so your standard tether needs to incorporate this feature. Regardless of what assumptions you make about fall distance, the loads are going to be high even with a good tether and they are being placed on a very scary part of the body.
However, I also think that it is necessary to incorporate some absorption into the jackline although for different reasons. With the jackline, you can’t load one sideways that is still in a straight line without having major issues with breaking stuff, you need it to deflect. The two ways to deal with this are to go with something that has some stretch and also preload or to go with something that has slack in it but is non-stretchy such as wire. Strechy with preload by definition is an energy absorber. The non-stretchy option has a major flaw which is that there is extra travel in the system which will increase the fall distance. Therefore, if we do traditional jacklines, I think that you need to have some energy absorption even with an ideal tether that doesn’t require additional absorption. To your point, having the energy absorption in the tether allows us to be less careful about designing for energy absorption and simply work on managing loads (the two are inseparable but this does simplify the problem as you can solve for loads with no jackline stretch). Of course, the way around all of this is to go with something that is truly rigid and designed for bending loads such as a strong track system but I have never seen a boat with something like this.
Eric
I saw a picture of John showing he’s using a tether that seems to have no stretch. I purchased for myself a tether that can stretch (I don’t recall the brand, italian made). Are there drawbacks to this kind of product?
With respect to jacklines or attachment points, would it be better to have them on the centerline instead of on the sides, with the tether length preventing your going overboard? Granted with a jackline on the centerline you cannot have an unobstructed run to the bow, but I would rather use several prepositioned tethers.
Hi Richard,
Yes, you have seen photos of me using a tether with no stretch, and clipping to hard points with it, and doing a whole bunch of other dumb stuff. That’s the whole point of the post, I’m trying to do better.
It would be very useful to know what tether you found that incorporates stretch. However, be aware that if it’s one of the ones that incorporates some elastic to take up the tether so it does not get in the way, that will do almost nothing to reduce arrest load.
Yes it must be like you said. Can’t get to the boat but they look like this one below.
https://www.westmarine.com/buy/west-marine–orc-specification-single-safety-tether–11878709?recordNum=1
Hi Richard,
Yes, that’s what I thought. There is no shock absorption in that tether. See Steve Stucko’s excellent explanation of why below.
Hi All,
I specifically asked everyone to stay on the topic of this post and not anticipate the next post on locations of jacklines and related geometry.
Please respect that request.
This is important stuff. The only way we are going to get it right is to come at it in an organized fashion.
Also, if you anticipate, your input and wisdom will be lost to the discussion on the next post.
Hi John,
As a newbie to the offshore cruising world, and still reading AAC back-chapters as part of our offshore preparations for Bonnie Lass, having the content in “bite-size” definable chapters is really helpful – first in learning and then applying that learning. So I fully endorse this request to stay on topic, especially on these really important safety related posts.
On the other hand, I understand many of us also appreciate the free-form exchange of ideas that often occurs on AAC in responses and I am sure some posts benefit from topic drift! I am not sure you could be clearer John on “how” and “why” this is important. Maybe some hint in other chapters/posts of “when” you would be happy for us to be more free-flowing might help us be more disciplined when needed.
cheers
Rob
Hi Rob,
Good point.
I guess my thinking is that we have such incredible expertise among those that comment that it is pretty much always better to concentrate that wisdom on a single topic.
Otherwise we just end up with a huge free form mess of comments that, after the initial flurry, no one will ever read and benefit from. TLDR (Too Long, Didn’t Read) will kick in.
Thank you for diving into the important details of this subject. As a sailor and climber I can confirm much of what you have written here and at least comprehend the rest.
One point that I think needs clarification is what constitutes an energy absorption device verses a “stretchy” tether. The teachers that noticeably stretch do so to help keep the tether from looping down and becoming a trip or entanglement hazard. This is usually accomplished by using tubular webbing (which provides the strength) and incorporating a piece of elastic inside to bunch it up and remove much of the slack as you move around. I use this type and believe they are beneficial in the designed function of reducing trips and entanglements. This stretch however IS NOT an energy absorption system. If you actually fell against it you would never notice the difference between the stretchy tether and a tether made of the same length of webbing without elastic inside.
In climbing energy absorption devices are used when an anchor point’s integrity is questionable, such as when using screw in ice, as they substantial reduce the shock loads imparted on the anchor. These are generally carefully engineered zigzags of webbing sewn with very specific thread and stitching which progressively breaks when loaded. These are by design, disposed of once they absorb a fall that breaks any amount of the sacrificial stitching. An example of this type of devise is the Petzl Nitro 3 http://www.petzl.com/en/Sport/Anchors/NITRO-3?l=INT#.VkzYgYRsxVo
Hi Steve,
I tried to make that point above, but you did a way better job explaining that issue than I did. Thanks.
Nicely made point Steve. I too am a climber and sailor, but am not that experienced a climber that I have ever used an energy absorption device. Are they small and neat enough that they could be incorporated into the tether? If so this could be a really good component of the load absorption solution, if the load is high it will give, so no injury from load shock. One drawback is that if the load does trigger the device you will end up on a longer tether, but at least you will be alive and kicking.
Yes Bruce, you are exactly right. Most are small enough to be easily incorporated, and as you pointed out they will add feet to the effective length of your tether if they open up, so it would require some additional compensation in your system design.