A few weeks ago I spent a huge amount of time digging into the tether tension we will be subjected to if we fall overboard and are dragged by the boat, in an effort to understand why this often, but by no means always, results in a fatality and how to reduce the possibility of a tragedy.
As I hoped, others, particularly Luc, an engineer from Switzerland with deep hydrodynamic experience, corrected my mistakes to the point that we now, for the first time ever (as far as I know), have a real understanding of the tether tensions a dragging crew overboard (COB) will experience.
A vital process given that most offshore boats I see out there are still using sidedeck jacklines.
And this time, given that my numbers have been both audited and corrected, I will draw conclusions, including answering the critical question: are sidedeck jacklines worse than nothing?
This chapter will replace the original, so even though there is a bit of repetition it is worth reading carefully.
I think these are solid conclusions John.
I reluctantly fitted side jacklines to my centre cockpit boat after spending considerable time trying to come up with alternatives, and finally coming to the conclusion that nothing else was reasonably practicable without serious modification, or installing systems that could cause other problems, and/or make the tethering process riskier from too much stuffing around (IE I neglect to use it because it’s such a pain).
One benefit I have found is that I can use the tether to pull and steady myself to the deck, which may not help getting swept off by a wave, but does help with keeping your balance.
Hi Dan,
Sure that’s a valid approach as long as we have a boat that does not exceed say 7 knots very often. The other thing to think about (and practice) is the actual mechanics of slowing the boat very quickly and getting the dragging person back aboard. I think both are way harder than many people realize.
Also I will be covering our jackline system on the J/109 which may give you some ideas for reducing dependance on the sidedeck jacklines, which is always the best option given all the other risks of being dragged, other than tether tension.
I’d love to be doing 7 knots on a regular basis! That said if I am doing 7 knots it’s probably pretty gnarly and hence reinforces all the points made.
I did mean to mention one solution that I looked into that is fitted to most pilot boats: https://www.ronstan.com/au/work-boat-safety-rail-systems
It is pretty expensive, and complex to install, but allows a solid attachment point and short tether to prevent COB. On a boat like mine with a raised cabin top and centre cockpit it would allow you to to circumnavigate the entire boat and only change tether to reach the bow.
Hi Dan,
Sure, that would be the ultimate. However we would want to be very sure that the tether was short enough to insure we can’t go over the side since so doing could be a bad climbing type fall with pretty much no stretch in the system, and that in turn leads to concern that with a tether that short we would not be able to reach things like the boom without another change. But if those problems can be solved, sure it’s an interesting option.
This really useful; thank you.
I want to get the balance between “perfect” and “practical” practice and this article allows me to do that.
My absolute max speed is 7 knots (4.5 to 5 is more typical) so I will persist with my current jackstays… but knowing the risks I run.
In addition to having the right set up it important to have the crew well trained. My rules are don’t leave the cockpit unless someone else is on deck (might not work for many but it does for me) and everyone should know how to stop the boat fast.
Hi Clive,
I hear you on stopping the boat fast, but based on some testing Phyllis and I did I think you may be underestimating the difficulties and dangers of doing that. Just one example, if the COB is overboard on the lee side aft (quite likely) and we turn up hard into the wind, the skid of the stern will quite possibly hit the COB or push them underwater while increasing tether tention. And that’s just one problem I have thought of, there are a bunch more.
Excellent article and obviously the little details matter a lot. A different arrangement that I don’t believe was considered in the analysis (sorry if I missed it) – does the attachment point to the crew member have to be on the chest? Could it be on your upper back so that you were dragged through the water in a more streamlined form? Perhaps someone could come up with a system where under shock load the attachment point moves to your upper back?
Hi Dean,
That’s sort of what the TeamO backtow does. More on that coming. That said, I don’t think it would be practical to use a harness with a back attachment. Too much trip and tangle risk and also that would preclude our multi tether system:https://www.morganscloud.com/2014/10/02/staying-aboardpart-iiiour-gear/
People need a fancy study (and lots of data) to convince them of the serious flaws in side deck jacklines? Really? These would be the same people that don’t recognize that a 10′ leash will keep one’s dog closer than a 20′ leash. Am I breaking community guidelines with my sarcasm?
Good common sense point Mr. Toews. However, I am grateful for the data from this heavy lifting research. It is really nice to understand it more conceptually and what is really happening. I have sat out staring at the dark water on moonlit night passages and generally contemplated just the sort of scientific nightmares that John et al have conjured up here for our benefit. All the best.
Hi Gregory,
Yes, you are close to a comment deletion with that one. Sarcasm seems to turn discussion hostile very quickly, so please ease up on that.
For example, as someone who used sidedeck jacklines for some 30 years before I got right, I could easily take offence! Point being that these things are easy to see after the work is done but the fact that most boats still use sidedeck jacklines shows how hard it is to make change, even when the numbers are compelling.
Thank you for your continuing work around this important safety issue. I believe it was when I saw an earlier version of your discussion about the dangers of side jacklines and your, in retrospect obvious, recommendations for moving jacklines toward the centerline of the boat, if possible, that I chose to become a subscriber. The best thing is to never go over the side at all.
That said, it is not always practical to arrange things the way one would like, and your and Lucs work have resulted in some more solid and nuanced numbers and conclusions taking several different variables into account. So even if you can’t arrange things to entirely avoid the risk of coming outside the boat, you might be able to make more informed choices that can at least reduce the consequences when the worst happens.
You mention briefly the very real dangers of striking the sides of the boat or double rudders, and the effect of the “firehose in the face”, both of which are related to the subject of falling overboard while tethered but are separate from the focus on tether loads.
I think you should consider adding something about the repeated non-static tether load effect that waves will have, accelerating and decelerating the COB over and over while the boat moves (and even at rest if the waves are large). While the initial shock and assumed static loading is important for both incapacitation or injury early on, I feel the repeated hard tugging, and from different directions, while being dragged can be well as important as the other aspects. Basically, as long as the boat is moving with any speed the forces on the COB will be far from static. The repeated tugging can easily be stronger than what the COB can control and will tire the person quickly to incapacity even at moderate levels. This underlines again the importance of stopping the boat as quickly as possible.
Finally, your graphs (including caveats) are enlightening. However, many people are not used to reading graphs critically, so your changes in scale may be confusing even though they make the graphs more appealing. This is an effect that is sometimes cynically used to mislead, though this is obviously not the case here. The best solution might be to just point out in the text that scales are changed to make a more readable presentation.
Thank you again for your informative articles.
I rig high lifelines (strong but stretchy rope) running through rock-solid heavy-duty stainless fittings attached chin-high to my side-stays. Nothing to trip over on the deck and easier to move fore and aft.
Me too. Though the jacklines are dyneema, and they run through a d ring, lashed to the outermost wire shroud. I was a Rope Access technician for a living and prefer to think of “jacklines” as fall prevention, and not only that which tethers oneself to the boat. Of course that is another goal achieved here, along with a short fall factor and higher drag position to the deck should the worst occur. The large diameter of the dyneema is and excellent hand hold and is more likely to stop you from cantilevering over the lifelines as well. The high windward jack lines are useful for supporting your weight through the harness during reefing and any other maneuver. Maybe an engineer here can point out any dangers associated with loading a shroud in this kind of dynamic situation. This has been my primary concern with this approach but much prefer it’s prevention approach.
I wouldn’t connect anything to my rigging. I want my jackstays to be independent and not rely on other items for integrity beyond the deck padeye, eyelet or other foundation device. Standing rigging is an extra failure point compared to jackstays secured at deck level. I use short safety lines so that any fall is limited. When working on deck, at the boom or mast, I rely on my stance for stability and not leaning on anything, which if it fails would set me up for falling. Should the conditions be wild enough that I can’t use a stance for stability, I would lean on boom or mast, but with my short safety line limiting length of fall. I try and fit independent systems without single point failure, wherever possible.
I appreciate your choices and that they may address your risk analysis.
Hi Justin,
See my comment to Gerald: https://www.morganscloud.com/2025/02/11/the-risks-of-sidedeck-jacklines-quantified/#comment-313375
Also, you are right to worry about the potential loading on the rig. You will find the math here, although it needs updating, but that will not change it materially: https://www.morganscloud.com/2015/11/25/person-overboard-flawed-jackline-systems-part-2/
Hi Gerald and Justin,
I just want to throw in my thoughts on the loads on overhead jacklines ignoring whether or not they are a good idea for other reasons.
For any relatively tight line, exerting a side load towards the middle of the line causes very high tension in the line. If your load F is in the middle of the line, then the formula is F*1/2/sin(theta) where theta is the angle of deflection of the line at the end. For example, if theta is only 5°, then the tension in the line is nearly 6 times the disturbing load. We get away with it if we use line that has a lot of stretch as the angle gets big quickly and at 30°, the tension in the line is equal to the disturbing load. John has written about this in other chapters where he examines jacklines finding that either they have very little stretch and have enormous loads or have a lot of stretch allowing the person to go overboard.
If you terminate a jackline on a shroud, the shroud now becomes that tight line mentioned above and shrouds are pretty stiff. In the recent chapters on preventers, Kurt calculated for 1 scenario that the side load on the shroud could be 1300lbs. I did a rough estimation of the tension in the shroud and found that it likely didn’t break but it was about the highest force the shroud would ever see and this is for a 47′ boat. My conclusion from this was that significant side loading is indeed a concern. Note that I am assuming that you terminate at the shroud and not just have a fairlead there which is low friction with termination elsewhere as that will apply a much lower load to the shroud.
Since the jackline itself is also a tight line, there will also be load multiplication there. Hitting 1300lbs would not be hard. The angles on the jackline are going to be higher than you might expect at first as the shroud will have deflected but you could still have a load multiplier if all the gear is stiff.
To me this is a matter of size and design of the system. On a smaller cruising boat, a hard fall at the right angle to a jackline attached at one end high up on a shroud could probably cause a dismasting. However, if you own M5 with the rig preload measured in millions of lbs, we don’t need to do any calculations, you won’t break anything.
Eric
Hi Eric,
Thanks for tackling that and confirming that danger.
For others, M5 refers to Mirabella V: https://en.wikipedia.org/wiki/Mirabella_V
Hi Gerald,
I know that high-lines seem comforting, but I have never been a fan. But based on the study, I’m now reasonably sure high-lines are one of the most, possibly the most, dangerous alternative. The problem is that if we go in the clip will slide back until hits the end of the high line resulting in a lot of slack (if we are any way forward) and very little stretch. This is the scenario that Luc’s graph shows can kill from peak tether tension at as little as 4 knots boat speed.
I explain more about my reservations about high-lines here (will be updated): https://www.morganscloud.com/2015/11/25/person-overboard-flawed-jackline-systems-part-2/
David,
Good point on the scale changes, I will do just as you suggest. I have been so immersed in this for 2 months that, what seems obvious to me, won’t to others, so that sort of suggestion is very useful.
As to waves, I hear you, but I’m not sure, although I could easily be wrong, that waves will make a bit difference. The other issue is I just can’t see any way to bring any numerical clarity to that, just way too many variables. All that said, I will think about how to add a note pointing out that waves could add other dangers.
Hi David,
I have added two paragraphs based on your suggestions, thanks.
Hi John, greatly enjoyed your focus, detail, and risk assessments here. A really valuable addition to the sailing risks library!
A terminology Q… You used ” terminal velocity” in relation to a short fall from a yacht deck, but as an ex skydiver the time to get to terminal velocity when falling is around 9 or 10 seconds. Might need some rewording there for clarity/accuracy?
Top article, thanks!
Hi David,
That assessment came from Eric Klem who is usually accurate. Perhaps the difference is that I think I’m right in saying you skydivers deliberately orient yourselves to maximize air resistance? Anyway, I will look into it.
Eric, any thoughts?
Hi David and John,
I took a quick look back at this and think I may see where this came from. In response to the previous article, I made a comment that if you want to use a climbing fall calculator to look at dynamic loads of a tether coming up tight after you fall in the water, you can trick it to mostly apply (there is still gravity acting so not quite accurate and it doesn’t give you the right static load). The suggestion was to use a fall in air of 0.49m before the rope becomes tight as that will equate to a speed of 6 knots which we are using as boatspeed when the rope starts to apply force. I don’t think I would consider the 6 knots reached to be a terminal velocity when looking at a fall in the air as terminal implies no more acceleration and a person in air will stop at closer to 100 knots. My comment was trying to explain how to crudely use a fall calculator to look at loads when someone is in the water and the tether comes up tight. I believe that where it is mentioned in this article it is in relation to a fall in air where the boatspeed of 6 knots has no bearing as you are in the same reference frame. The good news there is that a climbing fall calculator applies directly and you don’t have to trick it. Does this seem right?
It looks to me like Luc has done the work to make my comment on the previous article irrelevant as he has modeled both static and dynamic loads for people to examine and tricking a calculator made for another application is not necessary.
I haven’t gotten a chance to go through this whole article yet but it looks very interesting so hopefully I am not misunderstanding.
Eric
Hi Erik,
That makes sense, thanks. Sorry I misinterpreted. I will remove it.
Hi David,
You are right. I misinterpreted Eric’s comment. I have removed the para. Thanks for the catch.
Great analysis, thank you! I’m confident that it will move the needle more broadly. I’d love to see someone rerun the dummy-overboard test with a load cell on the tether to get empirical evidence.
You mention the increased drag from an inflated pfd increasing the load, but I think that load is transferred directly from the pfd to the tether rather than going through the unfortunate soul overboard. This might reduce the upper end load on the person, although it doesn’t change the takeaway for me at all.
Hi Dave,
We discussed testing on the last article, but the problem is that there are so many variables that it would take a huge amount of time and effort to get anything useful, and there is real risk of damage to the boat or a tester getting hurt. Also the gear alone to do this right is well over $5000 (dummy and wireless recording load cell). And finally, Luc’s modelling is so good that I really don’t think testing would add a lot.
As to the tension increase of an inflated lifejacket not effecting the COB, I’m pretty sure, but not certain, that’s not the case. Engineers?
Hi John and Dave,
My guess is that the extra load of an inflated pfd is split between being reacted directly to the tether by the pfd and being reacted through the person first. If you think about the bladder being pushed back, around the shoulders I would expect it to be pushing on the person. However, near the tether attachment point, I believe that the harness will be pulled away from the person’s body and it may well just be reacting to the harness only. I don’t actually know the breakdown of how much to each category.
Eric
I’d agree with Eric, it will be a complex set of reactions acting in multiple vectors that would both mitigate and exacerbate the various forces. It may also act as ‘scoop’ that forces the upper body back and places more of the load on the waist strap. It may also provide enough buoyancy to cause a lighter person to plane to some degree. It would probably need testing in a tank to come to any meaningful conclusions.
Hi Eric,
Thanks for tackling that. I was pretty sure that some forces would be transferred to the body, but no idea how much. I know a lot of people hate these unknown variables when we are just not sure, and have no way to be sure, but I for one find that identifying them gives me a better idea of how to manage these risks. Knowing what we don’t know is way better, in my view, that the getting clobbered by the unknown unknowns, to mangle a Rumsfeld quote.
Thank you John for – once more – an article that makes us think. I find it by now remarkable that a whole industry (including ISO standards, trainings, suppliers etc) is basing safety on assumptions, habits and (as I have seen often in my consultancy days) copying others in the assumption/ hope that they did figure it out and got it right so we don’t have to.
We are actively using your articles to improve upon our boat and seamanship skills in general. Not by copying you or someone (what seems a root case of the problems in this context) or by trusting that some manufacturer has thought it through (we got that wake-up call by now). But by being better informed with arguments and being critical in our own thinking. Thank you for helping us becoming better sailors!
I pickup on your findings that slack is one major factor in the risk of injuries during the initial phase of a COB emergency.
One idea I now will look into on our boat is to have our jacklines mounted to several points on deck using D-shackles or so. E.g. multiple shackles at cockpit, handrail near sprayhood, mast, anchor locker with the line moving freely through it. That should reduce slack as the tether has only a small length to slide before it hits a shackle. Yet, the jackline can stretch over its full length to reduce the shock load and/or reduce risk of the jackline breaking due to lateral forces. It should also help in routing the jacklines over the deck as much as possible towards the centerline. Downside is of course that we might have to rebuckle more often. But that might improve safety in itself. Curious what you and others think of that.
Hi Maarten and Marga,
I can’t say that really appeals to me. Clipping and unclipping have proved (by fatalities) to be a big danger time. Rather, if you want to make real change, get rid of the side deck jacklines altogether, which is by far the best alternative: This online book provideds complete instructions on how Phyllis and I did that: https://www.morganscloud.com/category/safety/book-crew-overboard/
Hi Maarten and Marga,
One other point on this. Even with a bow to stern jackline, as long as the boat has lifelines and reasonably robust stanchions I don’t think there is a high risk of a lot of slack because either we go under and the tether probably comes up against a stanchion, or we go over and a lot of the slack gets taken out by the tether sliding aft along the top of the lifelines and being arrested by stanchion tops, at least partially.
The two really dangerous situations for slack are a boat with no lifelines or highlines strung from arch to shrouds because both provide a long and unimpeded slide back for the clip before it comes up hard against a hard point.
Anyway, all of this is academic if we do by far the best thing and get rid of side deck jacklines.
Thnx!
Hi John,
This is great. I read through Luc’s work and it makes sense and really helps me understanding the magnitude of what is likely to happen.
Doing a quick bit of math on our own tethers which are made from 7/16″ 3 strand nylon from New England Ropes with most being 4.5′, I came up with a stiffness of the tether of ~54000N/m. Of course this is on the tether and would represent falling off clipped to a hard point with no angle in the tether. It looks like Luc’s work uses a rigid mass and then assigns all the stiffness to the tether. If I understand that right, then the stiffness of the harness and all make the conclusions make sense. But it also shows that even on a jackline, you want a tether that isn’t too stiff.
Eric
Hi Eric,
I think Luc stiffness includes the entire system. So in cases where clipped to a hard point, or coming up hard at the end of a jackline, he is assuming high or moderate stiffness and is providing both numbers to account for uncertainty about body deflection and different tether materials.
The only way I can see his low stiffness number (1000 N/m) is if we get a lot of added stretch from jackline and lifeline deflection.
Anyway, good to hear that your tethers result in close to his moderate stiffness and probably would be less stiff with body deflection and harness stretch.
That said, I would caution others not to assume that a nylon tether makes clipping to a hard point near the rail OK. Would you agree?
Of course the trade off here is that rope tethers can roll underfoot, which is the reason I have never used them in high traffic areas. A hard one to reconcile with the very real benefits of rope particularly DCR in added stretch and ease of making.
Hi John,
Note that the number I got for our tether stiffness is an order of magnitude stiffer than Luc’s medium number. Hopefully I didn’t mess up a unit conversion but it seems about right. I think that with a tether like this and the added deflection of the person and the harness, you might end up at his high stiffness number when clipped to a hard point with that relatively short tether. The scenarios you have outlined for the various stiffness seem about right to me.
Falling off onto this is an interesting question and makes me compare it to a fall across the deck on this sort of setup. When you fall in the water, your max energy is capped by the boat speed regardless of how far you fall. However, if you fall in air, you can fall far enough to be going a lot faster (a 10′ fall puts you at about 15 knots). The advantage of being in the air is that you don’t have added mass and if the boat is going fast, the gravity effect in air is less than the static drag in the water. I guess the dangers are different but maybe on the same order of magnitude. Assuming we are not wearing climbing harnesses, worst case scenarios with both can be quite bad. In the world of probability, if you use a stretchy tether, you have probably mitigated this for the vast majority of situations where all of the worst case factors do not line up but situations do exist where the arrest related to either can be fatal.
To me the reason not to clip to a hard point near the rail is that you can end up overboard while clipped in. As you do a good job of pointing out in these chapters, the real key is not to end up in the water at all.
We have been using the 3 strand nylon tethers for close to a decade now. I tried DCR and found the knots bulky and they would catch on stuff. The next closest thing in terms of performance that I know is 3 strand nylon and I can bang out a splice in under 5 minutes which makes it easy to implement. Rolling underfoot is a real issue but it also is not as bad as I had feared. I have always been someone who does my best to pull on the tether to keep the off-watch crew from listening to a biner scrape along the deck and that also keeps it from being underfoot if they are short. I have heard of people putting webbing loosely over line to mitigate rolling but I don’t know if it actually works. As Drew mentioned, I wish there was something like nylon webbing that had appropriate stiffness but there is not that I am aware of.
Eric
Sorry, Eric,
I mis-read the stiffness number. Thanks for the correction and fill. Given the stiffness of your tethers, even though the are nylon I draw a rather different conclusion: that changing to high stretch rope does not make much difference.
My reasoning is that the impact will still be horrible if we clip to a hard point close to the rail, and if we clip to a Dacron jackline, in most cases tether stiffness won’t make a lot of difference compared to jackline deflection, and therefore I would not add the roll under foot danger in a high traffic area for what I see as a marginal benefit.
That said, I would be the first to say this is a personal risk management call that depends a lot on how our boats are set up.
I also use DCR for tethers that clip to hard points inboard.
Good stuff. Good analysis, smart conclusions. But let me add something from a multihull sailor and climbing perspective. Also some history.
To my knowledge, there have been no instances of tether failure when clipped to a jackline (because even a Dyneema jackline gives some sideways stretch and slide), but at least 3 that I know of when clipped to a hardpoint (no stretch). As you noted, hardpoints are where stretch is valuable and impact loads soar. Additionally, it is considered good practice to clip (short) to a hard point when working. Finally, on multihulls with 18- to 25-foot wide foredecks, it’s pretty hard to avoid substantial slack at times, no mater the jackline routing, unless you want to clip and unclip a lot. The larger the boat, the more true.
The solution, obvious to any climber, is a tether with more stretch. Dyneema and even polyester tethers are well known to kill in climbing circles; the impact force if you climb even a few feet above the anchor point breaks gear and climbers.
The problem is several fold:
So it is an interesting puzzle.
I made some climbing rope tethers when I had my cat and I still use them occasionally. Additionally, one leg was slightly longer and one was shorter, to better suit that specific boat. They worked very well. I will also remind the readers that at the time I was testing sewing methods, so I had the expereince and testing back-up to know that my eyes were full strength (nylon rope is hard to sew properly). The stretch was perfect, but the under-foot problem was … a problem. They were still the best thing available for that boat.
And yes. Don’t fall off. I will always believe, that like climbing, your immediate belay is your hands and feet, you should have a plan, and mindfulness and practice are everything. Focus on the movement.
Hi Drew,
Good analysis, thanks. I particularly agree that we in the sailing community have our heads firmly in the sand, when compared to the climbing and industrial safety community. I postulated as to why here: https://www.morganscloud.com/2024/12/17/8-offshore-cruising-boat-gear-evaluation-tips/
As to the roll underfoot to stretch trade off, it’s a hard one, but my approach on both our boats has been to use a system where I can’t go over the side while tethered, which is, I think, still the best option: https://www.morganscloud.com/2024/09/16/person-overboard-better-jackline-systems/
That said I use DCR when I must clip to a hard point, thanks to your writing at PS: https://www.morganscloud.com/2017/02/20/the-right-tethers-to-keep-us-aboard-part-1-a-mix/
Just came across this this by John Kretschmer on his Instagram account.
https://www.instagram.com/reel/DFDGmigt80Q/?igsh=dzBhcHM3eXB5bncw
He drags someone with a Harness, the post is sobering especially for shorthanded crews