The Offshore Voyaging Reference Site

Flawed Jackline Systems, Part 1


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overboard from the bow (SW of Key West), was the brother of one of my wife’s life-long best friends…She has shared the unpublished details with us…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.

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Rob Withers

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!


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.


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?

Eric Klem

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.


Richard Dykiel

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.

Richard Dykiel

Yes it must be like you said. Can’t get to the boat but they look like this one below.–orc-specification-single-safety-tether–11878709?recordNum=1

Rob Gill

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.

Steve Stucko

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

Bruce Savage

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.

Steve Stucko

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.

Dick Stevenson

Hi John and everyone,
With respect to shock absorbing tethers, I have used handmade tethers for decades using good quality 1/2 inch 3 strand nylon. It rates as 7,500 pounds average breaking strength degraded some by the splices I put into the ends for the attachment fittings. I am not sure of its stretch stats, and it may be that climbers have better alternatives, but among common marine lines, I think 3 strand nylon has a lot to offer in this regard.
My best, Dick Stevenson, s/v Alchemy


Another point of reference, beyond climbing, that may be of interest in this discussion would be industrial construction. The number one cause of fatalities in industrial accidents (in the US at least) is falls. As a result, there are extensive regulations regarding fall protection requirements, as well as a very broad base of highly engineered and tested restraint systems available. In the US the general industry fall restraint requirements are codified at 29.CFR.1926.502(d):

Scroll part way down from the link until you get to 1926.502(d) paragraphs. Some interesting points. 1) A fall restraint system must be design to be loaded to 5000 lbs (comparable to the ISAF requirements). This assumes a safety factor of 2 above calculated loads, and is considered sufficient for persons up to 310lbs (including body weight and any carried tools) in a dead vertical fall. 2) The restraint system must be designed to limit arresting force on the body to 1800lbs or less. 3) The arresting system must limit the free fall to less than 6 feet or less and arrest/decelerate the fall in an additional 3.5 feet or less, 4) The attachment point on the body harness (full body harnesses must be used, belt systems are not allowed as fall arrest devices) is on the back between the shoulder blades. This will distribute the loads across the entire torso, concentrating loads on the hips and shoulders and limiting loading on the spine and abdomen. And 5) the tie off point (the jack line in sailing terms) is almost always structural steel beams or steel cables, so all shock absorption is incorporated into the tether.

As for shock absorbing lanyards, they are numerous with various different sizes and styles, but they are standard equipment on any industrial job site. And while not cheap, they are comparable in cost to standard marine lanyards.

And there are even options for inertial restraint devices (think seat belts) that are pricey, but may in theory work in certain fixed locations like a cockpit or at the mast that should limit fall distance to 2 feet or less while allowing movement around greater distances.

There are millions of these systems made and sold every year, and are engineered and manufactured to extremely tight tolerances with strict regulatory oversight for use in extreme environments and use profiles. Think of the steel workers building your local skyscraper walking on 8” wide beam 700 feet in the air with sharp tools, torches, and in all weather conditions. Fall protection takes a whole new meaning to these guys, and that is what these systems are designed for.

These solutions may not be fully practical in a marine environment, and are contrary to many common marine practices like attachment points high on the back and using full body harnesses. But for a different perspective I think they add value to the discussion around actual loading and restraint system design, and I see no reason why at the very least the shock absorbing lanyards would not be directly applicable to marine application.


One additional thought about the industrial systems. These systems require either locking beeners or two finger locking cam shackles on both ends of the tether to prevent any chance of an accidental disengagement. Most certifying agencies in the marine world require the shackle at the harness to be quick release to allow easy disengagement if you are being dragged. So if you need to meet safety requirements for offshore racing as an example, these tethers likely would not qualify. Though this may be an acceptable risk to some, or there may be alternative options for releasing in the event of being dragged in a cruising application. This would also be true with the harness connection high on the back, since you couldn’t reach the shackle even if it was quick release. As I said above, these devices may not be directly applicable to the marine application, but they hopefully provide some food for thought none the less.


As a climber, I dislike fumbling with locking carabiners. I can’t imagine doing this while being dragged, while choking on seawater and possibly after a bang on the way overboard. You can use two carabiners with opposing gates to deal with accidental opening. Also I just looked at a locking carabiner; I can’t imagine a salt encrusted lock would slide very easily. Without taking it apart, I can’t tell what the spring is made from but I doubt very much it is 316 SS.



does your post mean, that one of the 20′ lifeline roller strategically placed close to the hatch will allow carefree movement in the cockpit and up to the mast while offering more comfortable and better protection?

Dick Stevenson

Hi John,
No, I remember no concerns with getting a round tether underfoot. I tend to hold excess in my hand when moving on the side decks. Dragging the fitting on a fg deck would wake Ginger and this keeps me from getting it between my legs and twisted around). And when I get to the “work” area, say the fore-deck putting out the pole, I maintain a sense of where the tether is: not so much a worry about rolling underfoot as a dislike of getting tangled and needing to “unwind” myself.
I think also that the ½ inch/12mm 3 strand nylon is better behaved than my Lirakis and Wichard tethers as it is a bit stiffer.
My best, Dick Stevenson, s/v Alchemy


I would be hesitant to use a 20′ reel system as you describe for a couple of reasons. First, if we consider the idea of connecting in cockpit and walking up to the mast, and the reel is at full extension, then you fall, you are going to tumble and roll all the way back to the cockpit (or end up in the water 20 feet away) while the lanyard retracts then have to fall past the lanyard until it gets loaded up and the lock mechanism activates. second, walking around your lanyard will get wrapped around everything which would just be annoying, third the cables are generally steel and dragging steel cable over your deck probably would lead to some cosmetic issues, and forth the retractable lanyard is always pulling on your back which would be uncomfortable. The longer ones, in industry, are typically used in applications like being lowered into a tank. You repel off of standard climbing gear, and the fall restraint slowly extends as you repel but doesn’t take any weight. If your climbing system fails your fall restraint will engage and save you.

My thought for an application for these would be 1) if you have a strong stern arch you could attach one to that arch and use it as your tie off point while at the helm, 2) at the companion way. If you have a strong hard dodger you could hang one above the companion way. Extend the lanyard to the base of the steps and then tie off when exiting cabin. As you climb stairs it retracts and if you fall you are limited to 2 feet which would be important since the distance to the floor is so short. Then once in the cockpit transfer to a traditional system. As I understand it, the companion way stairs are one of the highest percentages of falls and injuries occur and these systems my help there. And third, in theory if you had a strong point on the mast you could connect one there (say a pole ring or an added padeye) and use them there. However at the mast I would picture the reel swinging and banging around all day, chaffing the halyards, damanging the mast, and the like. So, though in theory it may work, it may not be useful there in practice. Also worth noting if the restraint system stops the fall in 2 feet or less, it is only required to support 3000 lbs, not 5000 lbs, so your stern arch or hard dodger would not have to be quite so beefy.

Martha Hunt

Re: Steve Stucko’s and Robert’s post there are also shock absorbing tethers made to climb Via Ferrata routes in Europe.
These attach to a reg hip harness , you could probably add the quick release attachment to the harness end instead of tying through the harness loop. You also might use Steve’s idea of the energy absorption device in your pad eyes, if you want to clip in at the mast of at the helm?

Ed Finn

Jacklines, gee, am I the only person who thinks jacklines add a false sense of security? In the ” ’60’s” people argued that the health risk from smoking tobacco was minimized by chosing a cigarette brand with a filter – as apposed to a straight Camel, or Lucky Strike cigarette, these had no filter. Looking back at that time its easy to see how flawed and self dilusional the filter versus no filter arguement was.
Apart from being strapped in in the cockpit, I can’t think of any way to design or rig jacklines where they would be truly effective. They don’t prevent a person from falling overboard, they just drag and drown you quickly, rather then die from hypothermia. Thats a small comfort I guess.
They do keep your body attached to the boat making retreival easier. My family and loved ones would have my body, and be able to reach closure more easily. That’s something positive.

Personnally I think clipping in to a spare halyard before leaving the cockpit, gives a person a better chance of not going overboard in the first place, minimizes impact , and makes getting the live sailor or dead body back on board a lot easier- since they are already attached to a halyard, just put the halyard on a winch, and grind them in-
but even that is easier said then done.
I’m for clipping in to a halyard…

The realistic, but not intentionally morbid,
Ed Finn

Matt Chauvel

Does Drew Frye, climber and sailor who writes Sail-delmarva, contribute to this site? I thought his discussions of tether arrangements and the underlying physics, as well as his practical solutions, were very informative as well, and I’ve adopted some of his tools (appropriate for a mono-hull). I would love to find out what conflicts with his experimentation arise, if any.
Sorry John for the inherent ‘anticipation’ included in some of the posts…

Hi Richard,

The corithmatic about load factors and attachment points are all great. But how about the reality of going overboard while tethered and your shorthanded crew getting you back on board. I have never experienced the situation short handed. I have experienced the situation on a fully crewed boat racing offshore twice and getting the man overboard back on the boat took no less than 4 big men and both times the crew member overboard was almost drowned.

Ed Finn

Re Halyards
Yes I tried it offshore in lumpy seas.
I did have someone “tail” the halyard from the cockpit.
Yes it rattled, and slapped. No it did not snag in the mast.
As my boat has chainplates at the gunnnel I went inside the shrouds and the halyard passed beneath the spreaders, but every boat is different.
No I did not fall overboard , so I didn’t give it the ultimate test thank God.
Regardless of noise or degree of difficulty clipping on to a Halyard will keep you on the boat, not allowing you to go overboard in the first place and/or facilitate MOB retrieval readily.
Regarding your experience in racing boats, I have no such experience, and really , we are talking about
Chronologically challenged and shorthanded cruising not racing , but I agree its the same ocean.
I personally don’t believe jack lines running along the deck, will keep you on board, nor aid in your retrieval
And will likely drown the victim… That is my openion, and I am entitled to it.
Moving on, The matheletes (math atheletes) are right – the forces generated in fall restraint, harnesses, attachment points, and jacklines are enormous, and potentially fatal in themselves. So again that makes me question the classic wisdom of jacklines.
Lets all (including myself ) keep an open mind about it,
and allow for a realistic review of this important topic.
As Anderson Cooper would say, ” just trying to stay on the boat here”
Lastly, John please keep up this valuable and interesting work, I’m sure it must be a real pain on times,
But the website and topics discussed have real value. Thank you for your effort.
Ed Finn

Marc Dacey

While I would tend to agree with you, John (as is itself a marked tendency), on this, simple observation of the ‘behaviour’ of halyards at particular points of wind would suggest that the possibility of a halyard binding, wrapping or going round the shrouds/stays is a function of tension and apparent wind.

So my reluctance to use one would not necessarily be absolute. If I have someone to tail around a winch, even if I have jacklines out, in a worst-case scenario of going over the side, my odds are better with the means of retrieval (that isn’t a boarding ladder) already on me. This assumes, of course, that my PFD has an integrated harness and the sort of thigh straps that would help me keep my head above water.

I’m not advocating tended halyard over jacklines of sound, proven design, but I’m not ruling it out as a rational choice.

Marc Dacey

I’m not proud. In rough stuff, I have dropped to hands and knees in order to do some things necessary, wet, ill-lit and unpleasant on the foredeck. So we agree on what I suspect constitutes “mindful”!

Perhaps in the context of this discussion, however, we may broach the linked topics of rail or stanchion heights (too low on most production boats in my view, the second set of lifelines that generally form part of that setup, and the use of netting, shock cords or cloths to keep the tethered but plummeting sailor on deck? It strikes me as a missing piece of the solution to just assume that one would fly over the toerail/gunwhale and under (perhaps snapping in the process) the lifeline halfway up the stanchion.

Henrik Johnsen

The force we are talking about her make me wonder, what about the life vest we use? Any chance it will stand up to such forces? Hardly.
Some places it is referred to fall factor 1 and 2. Don´t forget that mountain climbers or constructions workers always use harnesses made especially for absorbing the force of a fall, and that the main force will be put where we are most solidly built, the pelvis. A slim life vest, whit out thigh straps, will be a sure looser in that matter.

Erik Snel

Reading through this chapter, I can only come to one conclusion: use a jackline-tether combination that is short enough to keep you from falling overboard. Also, if the tether is short enough, using fixed points to attach to also becomes possible again: the force generated on you through the tether/harness by a fall on deck cannot be higher than the counterforce of the deck, which you would experience anyway even without a tether.

Marc Dacey

By that logic, there is no escaping the conclusion that a short tether, one that allows you to reach the toerail only with arms out, and an even shorter tether (for working at the bow) are the best choices, and that there is no shortcut that safely avoids a lot of clipping on and unclipping.

I have personally experienced a knockdown that would have thrown me into the Atlantic at night in a squall had I not been short-tethered (three feet) to a hard point on the binnacle. The same boat had backed eye straps either side of the companionway, and very strict standing orders on their correct and unvarying use. The three-foot “drop” (more of a slide or lurch, really) had no effect on me physically, save for the adrenaline rush and the scramble back to ease the main sheet before the skipper appeared to “find out what that thump was”.

David B. Zaharik

My thoughts too Marc… I have a friend who uses a short tether and it saved his life also. He was at the bow when his helmsman accidentally backed the jib which snapped into his face and almost knocked him overboard if it weren’t for his short tether. He told me he hung in space, as it were, the back of his legs touching the lifelines but his body hanging over the ocean until the helmsman reversed the accidentally quasi-tack and he was able to pull himself upright.

In his words, if he did not have the short tether, he would be dead. He has no delusions of being able to be saved if dragged… even less if he goes over with just a PFD.

Dick Stevenson

Hi John and everyone,
I will mention one suggestion that goes some ways toward having the jackline amidships that we have used on Alchemy for decades. That is to have the jackline in two parts. Once we get to the mast there is a fairly taught jackline going from the mast to the bow. This (on our 40 foot boat) does not need a long tether. I have not executed the following, but a short tether, adequate for fore-deck work, could be waiting fixed to the fore-deck jackline. Not only does this keep one nearer the center of the boat in a mishap, but you will not slide far if you slide the length of the tether.
The next idea takes me back to when I sailed with my children. I feel I am likely to go under the lifelines if a mishap occurs (not over the top). The older I get the more likely I am to crawl or work on my butt and I definitely keep a low center of gravity as I move as a given. I am thinking that some sort of large hole netting executed with some high modulus line would catch and tangle up a good percentage of the person-overboards that occur. Much easier to untangle from a netting than to get someone aboard from the water.
Anyone have any idea what the stats are for those that go overboard being swept through the lower lifelines vs other ways? My reflection is that many are swept from deck overboard under the lifelines.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

I swear I didn’t read your post before I made mine…great minds, etc.


Hi John and thank you for the great article. I have a question for you or anyone who might know. Of the recorded sailors that have gone over the side with their tethers on did they go under the safety lines or over the top of the safety lines? I have only one experience with all most going over and it was when working forward and green water came over the bow and knocking me to the deck. As the green water evacuated the deck I was going with it feet forward but luckily was able to straight arm the lower safety line and that was enough just barely to keep me on board.

What do you or others think of safety net along the sides attached to the stanchions and fastened to the tow rails. I have been thinking about doing this but still up in the air about it. Not going to help if you go over the top but may just save you in a case like I mentioned above.


Drew Frye

Just joined!

I wrote the Practical Sailor article on jacklines. Unfortunately, they are always cut for length, leaving out precious detail. There were really only 2 key points that motivated me to do the testing and write that article:
* Yes, a jackline made of Amsteel or similar meeting the minimum ISAF stadards is not strong enough. Same with SS, and I actually ripped one off a deck during testing. If you are going to use low stretch materials, you must go up in strength for line and anchors. The standard was written many years ago based on polyester webbing, and was never corrected.
* Amsteel ONLY makes sense when the line is over 45 feet long (50′ boat). In fact, under 15-20 feet, nylon webbing may be the best material, since stretch is not important and low anchor loads are.

So long as the low-stretch materials were ONLY used in lines over 35 feet, the impact loads were not that bad. My personal opinion, and why I don’t mind low stretch lines, is that I use dynamic, 8mm climbing rope tethers. No matter what I am clipped to, the impact is never too great. For catamarans (I have a 34′ cat) with wide decks, long stumbles are likely, tethers are longer (2’/8′), and impact matters.

My main habits are actually rock and ice climbing. I’ve taken thousands of falls on rope.

Rusty Gesner

Drew’s mention of dynamic, 8mm climbing rope tethers is well worth a trip to his web site to read more. I’m convinced, and will replace my webbing tethers with that, as well as adding a parking loop as he suggests.

Drew Frye

As per the comment guidelines, there are many areas where taking a hard line ventures from fact into opinion. That Volvo boats run Dyneema jacklines and tethers restrains me from expressing certain opinions. There are also opinions of other writers stirred in the mixed, often with old-school ideas. The result is a mixture. I’m satisfied that the data table was printed—readers are free to interpret according to their needs and experience. The truth is that I agree with nearly all you say; the math is the math.

Yes, the failure of the SS line was because it was low stretch. There is also the question of a weak shackle, since that was the actual failure point.

One thing to remember when calculating impact loads (and there are so many variables) is that there is some tether stretch, and there is deformation of the sailor (your clothes shift and you ribs bend). The peak loads are below theory. Even so, I really wish that more shock absorbent tethers would become commercially available.

Drew Frye

Practical Sailor has received a number of questions on that article, and I am in the process of writing a response explaining both our reasoning and posting some information that was cut for length.
– Energy units. ft-pounds, not pounds.
– Fall assumption. Since there is no generally accepted basis, I chose a 220 pounds sailor (ISO standard) and slides ranging from 8-16 feet/second (faster on bigger boats). It was a guess that seemed to square with the fact that jacklines do not fail in actual use. With these assumptions, however, some of the safety factors were pretty low.

Many are concerned that you can still fall off. Yup, staying on board is complicated, and that was not my focus. My focus was to show that nylon webbing can be fine and perhaps the best choice for small boats, that well-proven polyester is in fact the best choice for most of us, and that if you are going low-stretch you MUST go way up in strength. I wanted to kick the door open, for others to continue that work, since the ISAF standard contains an error resulting from the introduction of new materials that did not exist when it it was written.

Why did we accept stainless, for example? I like jacklines that are permanently rigged. The racer concept of rigging them when needed makes no sense to the single hander or short handed sailor, since they are need every day (alone on deck, night, squall, cold water). I don’t like the idea of re-rigging something safety-critical, and perhaps doing it wrong in a rush. Thus, I’m not convinced webbing is always the best answer, since it will loose strength in the sun. I use rope, since my jacklines are not on the deck (along cabin chine). I have a catamaran, so my jacklines stop well short of the bow (mono hull sailors worry about sliding off a narrow bow, cat sailors worry about stuffing a wave and being thrown forward). The point is that we need engineering flexibility and that people need to THINK about what fits. With all of the variations in design and rigging, there is no one-size-fits-all, and you will always need to think about how you move.

Rusty Gesner

Drew, in google searching 8mm dynamic seems to be scarce, and being ignorant of climbing rope, I’m not sure if what I do find is the right stuff. Do you have one to recommend?

Drew Frye

Do NOT use accessory cord. That is basically non-stretch polyester double braid, like StaSet. Avoid anything that has “static” in the name. Climbing ropes will carry either a UIAA single, half, or double rating. You probably want either half or double.

Climbing ropes are available from (no connection) by the foot.

The ideal material might be a very stretchy webbing (does not roll underfoot), but I have not identified such. I am personally OK with 8mm climbing rope (5/16″), but webbing would be better. On the other hand, I like the feel of 8mm line in the hand.

I think this is an interesting area for development and would love others to work on it. I may be missing something.


Regarding dynamic climbing rope. Can you please provide a pointer to information on making the necessary eyes in the ends using splicing or tying? This is a complex, cored rope. How does the stretch affect the ability of a knot to hold? (taken up the limit, bungee is impossible to knot) Are new dyneema-focused knots like Estar appropriate, or do they slip because of stretch? There is a whole hidden area here….

David B. Zaharik

Fascinating discussion! Wow was I ignorant and delusional in my acceptance of just tie on and stay on. I didn’t even contemplate the math and physics involved on the jacklines, attachment points and even more specifically the human frame!

Its like reading a thriller… can’t wait for the next chapter! Good work!

Robert Patterson

If you clip onto a jackline, then fall, won’t your clip slide to the attachment point? If so, you may as well have clipped to a padeye for all the energy absorbtion you will get.

Vesa Ikonen

Hi John,
small typo?
1% stretch will drop our lowest impact force to around 600 lbs (225 kgs),…

600 lbs ~ 270 kg

John McCrary

Back in a past life, I used to be an FAA-certified Senior Rigger, so I’ve learned a thing or two about harnesses. So, a couple of thoughts.

  1. Dragging of a conscious MOB could be almost totally eliminated using a three ring release system on the harness. It’s what we jumpers use to connect our main canopy risers to our harnesses. It’s designed to offer a 1,000:1 mechanical advantage for release, and since most of our gear is comprised of fabric and webbing, it is usually constructed to be highly corrosion resistant so we don’t get rust abrading or sawing through our gear.
  2. Way back in the day, I had a brand new canopy that had consistently, brutually fast openings to the point that it was physically damaging both the canopy and myself. At one point, I had a “fuse” concept in mind that was a piece of webbing that had attachment loops at each end and the body S-folded linearly like old time ribbon candy. The folds were stiched together with multiple lines of break cord (thread with a consistent, known breaking strength), and the idea was that the process of breaking each line of stitches would attenuate the shock just a little bit. I think something like that would allow for a less dynamic line to be used.
John McCrary

Hi John,

I find it reassuring to know I reivented the wheel with the screamer, means I must have been on the right track!

As for the Spindrift HRS, I justed watched a video Spindrift put out about it. With all due respect to Spindrift, and freely admitting I have zero experience in boat harnesses, I do not like it. It has multiple potential failure points, most of which look like they would cause a jam which means they are not fail safe.

Both the way the release webbing larksheads into itself and the circular hub look exactly like techniques that parachuting has tried and discarded in the past because their failures generated multiple fatalities. In particular, that larkshead makes my skin crawl. Additionly, it appears that there is no mechanical advantage in the HRS, and one thing jumpers discovered is that when you load up a release mechanism, it dramatically increases the amount of force required to actuate it. It was also found that due to body mechanics, the amount of force you can exert on a release system is dramatically less than you would expect. Bill Booth designed the system as a three ring rather than two ring system because a mechanical advantage of 100 to 1 was found to be too low to overcome these issues. With the third ring, the OSHA max loading of 5,000 pounds can be release with a mere 5 pounds of force. Finally, the three ring system has one realistic failure point, and in this application, it would fail safe.

Again nothing against Spindrift, but I am willing to bet the number of real emergency HRS activations is miniscule when compared to the number of real life use of the 3 ring, especially when you consider that virtually every tandem skydive (millions annually) includes a 3-ring activation as it serves as the release mechanism that allows the drouge chute to also serve as the pilot chute that initiates main canopy deployment at the bottom of the free fall.

If you are interested about the 3 ring system, the wikipedia entry for it has an excellent animation of its function and a decent summary of its pros and cons.