The Offshore Voyaging Reference Site

Better Jackline Systems


In the last chapter we learned that the tether and jackline systems that most of us use are pretty close to useless, and may even be more dangerous than not wearing a tether at all because of the risk of being drowned or battered to death (probably a combination of both) while being dragged in the water at the end of our tethers.

In this chapter, after all that bad news, I’m finally going to get positive and look at the changes that Phyllis and I have made, and plan to make, to make our crew overboard prevention system on Morgan’s Cloud really do its job, rather than just lull us into a false sense of security, as it has been doing for decades.

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I am a long time climber and a much shorter-time sailor. I think you are overestimating the fall-force risks of anchoring to fixed points, like the mast or cabin-top hard points. A specific style of climbing, called aid-climbing, regularly exposes climbers to short falls of up to 3-4 ft onto static lines (webbing) with very little extension (i.e not opportunity for stretch to reduce fall forces). I can say it is not pleasant but not life threatening either. I know I am in a climbing harness while doing this so it would be worse in a set up usually used by sailors, but compared to the risk of dragging, or full-on POB, it is pretty manageable.

Feeling safe enough to rely on a cabin top hard point tether might allow you to reach your cabin top jack lines without needing the side deck lines. If you are still worried about the fall-forces on that hardtop attachment point, you could rig load reducers between the tether you leave attached to that point and the hard point. Climbers use load reducers called “screamers” – 1″ webbing folded together a few times and then sewn thru with sacrificial threading which will tear out successive bar tacks at set loads. So, if you end up going for a big flyer – which sounds very unlikely – the screamer will blow out a few bar tacks, and in doing so dramatically reduce the load. You would need to replace the screamer later (even if all the bar tacks release, the webbing itself retains full strength so it will never fail completely) but maybe a price worth paying. These units are perhaps 15cm long – the size of a “QuickDraw” – so should fit easily between a hard point and a tether.


In calculating your loads, have you assumed the human is a rigid body? This is the impression I got from reading the earlier posts — apologies if this is incorrect.

If this assumption has been made, this could certainly result in a significant overestimation….. Deformation of the body, and movement of the limbs during the arrest will significantly increase the deceleration period. This might explain why people get away with falls like this. Not ideal, relying on our bodies in this way though!


My comments were about the specific case of falling when attached to a fixed point as raised by Mike above, and his assertation that climbers do survive such falls withot major injury. So I am just talking about the impact load — I was not meaning to question any other aspect of your analysis.

The climbing calculator you’ve used appears to treat the harnessed person as a rigid mass. This is quite appropriate in cases where the overall deflection of the rest of the system is relativley large. However, it becomes less and less appropriate as the overall deflection decreases. For example, if you’re calculating the force on a sailor being arrested by a short, high stiffness line attached to a rigid point, the overall deflection will be very small — lets say a few mm — and the calculated force will be huge. However, the human body, with moveable arms/legs, a deformable attachemenet to harness, clothing, etc. may effectively add deflections of hundreds of mm. This will not make a small difference to your calculated force — it will make orders of magnitudes of difference.

Again — not meaning to question your overall analysis or conclusions at all. Just pointing out that you may be overestimating the dangers of tethering to a fixed point with a short line.


You state: “We also know that a fall of as little as 30cm on a static tether attached to a hard point can exert a force of 15kN”. I’m saying we don’t know this. It’s likely to be much, much lower than this.

The problem is, that for these short falls — especially once they’re down to 30cm — the deformability between the person’s centre of mass and the attachment point to the tether is going to be more significant than the deformability (stretch) in the tether/sling. So by neglecting that, they’re neglecting the most important variable in calculating the deceleration time and hence loads. So all they’ve really shown is what happens when you drop a rigidly attached mass, not a harnessed person. They may be expert climbers, but that doesn’t make them expert engineers.

A rough sanity check: A 75kg person falls 0.3m. Assuming freefall, energy to be arrested is ~220 Joules. Assuming constant acceleration (big oversimplification, but in the ballpark), for the arresting load to be 20kN, the total distance travelled by the mass over the arresting event must be ~11mm. Now, wearing your harness and while lying down on the deck, apply load to your harness with a halyard attached to a large fish scale. Measure the defleciton of the attachment point (relative to your body) as the load increases. Draw a curve, and extrapolate the load out to 20kN. I’m guessing it’ll be much larger than 11mm… And we still haven’t taken movable limbs etc. into account.


Fair enough John, But I’d like to reiterate that I’m not indicating ‘a bit of amelioration’ — I mean the actual loads measured for some of their data points would be ~an order of magnitude lower in reality. This is certainly not a best case assumption — just reality. The expert climbers would have found this had they not neglected the most important (in the case of short drops) half of the system. While what they have done does nicely demonstrate that low stretch=bad in terms of arresting loads, the actual numbers they’re reporting do not represent any real world case involving a person — in this respect what they have done is fundamentally flawed, and should not be relied upon.


Hi John, I must agree with Ross, if the “stretch” including tightening harness, compressing clothes, inducing a rotation, deflecting the body e.t.c. spreads the load out 10cm, instead of 10mm the load will be 1/10th (in the real world it would not be quite linear though). The arms and legs weigh about 40% of total body weight, these and the head will still be moving when torso stops.
On the other hand, when approving or classifying equipment there will be a need for a repetitive way of doing this.


Hello John/Ross D.

Although I have no formal training, with respect, I have to agree with Ross D. It seems to me that the video showing the forces encountered by a rigid mass could not represent the forces a human body would encounter. The human body represents a dynamic load with each part of the body decelerating in a different time frame based on its orientation to the direction of force applied. When the stored energy of the fall is arrested initially at the harness, that part of the body stops. Now depending on the orientation of any other body part (say above that point trying to “grab a hold”) wouldn’t that force arrest after the initial force has stopped thus lengthening time of deceleration of the given mass? Makes the forces encountered impossible to calculate because every fall will be different. A quick search on deceleration forces over time,(the video itself speaks to this) even minute increases in time make significant differences. Good information contained in the video but it’s apples to oranges as far as forces on the human body go. The video does a great job of proving dynamic line is better than static line for extending deceleration time. Again, extending the time of the force on body is a good thing. I’m sure that’s the take away message John had in mind!
Eagerly awaiting any A40 update.
Great site

Dick Stevenson

Hi John,
Nice analysis and suggestions and I look forward to playing on my deck to see how the layout could work on Alchemy. I have not good enough internet also for the video, so that will have to wait as well.
As for the mast to bow (or close to it) jackline,: I have used this system for a couple of decades on Alchemy (a 40 foot cutter) and our previous boat. A couple of comments:
I suspect it depends on windlass location, but this has never been an issue on my 2 boats. I stop my jackline well before the bow (I can just get to the bow to pass a line around the furled jib at full tether extension) which ends up being before the jackline gets to the windlass. I suspect this will be the case for many/most boats.
I do not leave my jacklines up at all times, so tripping is not an issue much of the time. That said, there are enough lines about to trip over that the few feet (cabin top to staysail) where it is elevated is just another area to be careful at a time where I am being careful anyway. In any case, tripping in the way you worry about has not been an issue in offshore foredeck work over the years.
When I stored a dinghy on the foredeck the jackline was an issue. (The dinghy is now under the boom.) The jl then needed to be longer and caught on the pontoons (or the tether did) in a way that was annoying. With a clearer foredeck now, the use of the jl is much simpler and safer as the jl can be set up more firmly and there is less “stuff” to work around.
On Alchemy, 90% or more of our out-of-cockpit work is between mast and bow, working the pole or working the asym. All reefing (except the third) is done from the cockpit so having the foredeck well set up for the jackline amidships is very important. We rarely have to leave the safety of the cockpit in boisterous conditions with the boat heeled over: times when I have felt most vulnerable to parting from the boat.
Doing a bit of a dance with the staysail can be an issue. Planning ahead and having a system worked out and repeated and practiced has made the being on the correct side of the staysail stay a non-issue. Rarely do I need to “unwind” myself. And I suspect that if you are doing foredeck work at 0300, that you are definitely not “a dope on a rope”.
I hope these comments are helpful.
Thanks for all the thought and work.
My best, Dick Stevenson, s/v Alchemy

Drew Frye

I like it. It feels familiar, watching how you get around the dodger.

Rigged full time. Like seatbelts, jacklines should be a part of the design, not an after thought. I’m stunned that boats don’t come factory with this sorted out. I can only imagine how many ways I could rig the lines wrong if I had to rig them fast.

My boat is different because it is a cat and because I have sugar scoops. Because falling down the steps while landing a big fish is possible, and because the dingy needs occasional attention while on the davits (Sail Mag recently published a story about a POB that occurred during emergency repairs to the tackle), I like to have specific jackline clipping points that get to to specific points around the transoms. These are ONLY used when accessing specific items. I have provided these using climbing bolt hangers.

I have also used bolt hangers other places on the boat for lashing points, tether anchors, and jackline anchors. Coincidentally, most of my guests are climbers and instantly recognize bolt hangers as what they are; safe clipping points.

I’ve done a lot of testing re. sewing and fabricating jacklines and tethers; I think your readers would like to hear how these were fabricated, since they are not off-the-rack. I would also like to here your thoughts on custom length tethers; just as you have a problem with side deck jacklines and stretch, I have a problem with one-size-fits-all tethers. I don’t favor home-made to save money, but if custom lengths can really reduce the fall risk, that is a different matter.


Great discussion. I’m usually in lurking mode. I recently had the opportunuty to return a transpac boat back to CA…..through the effects of 4 hurricanes this summer. Safe to say we were practically tethered in the entire trip. To me, it seems part of the problem is how low on deck the JL ‘s reside. Why not raise them higher off the deck and more inboard? Perhaps waist height to reduce tripping and minimize slack in tether. From my experience, it’s hard to imagine tripping in situations where we were outside the cockpit. You hold on for dear life, stay low, move slowly and watch every step.

It was an AWESOME experience. Learned tons.


I think roughly waste height gives you a couple options depending on rigging. Dodger to mast or granny bars? Dodger to stay if inboard? Mast to deck at the bow? Third setup for trips to the very stern? Shorter runs would also reduce stretch. W dual clip tethers, no issue moving from section to section. …just thinking out loud.

Drew Frye

I mount jacklines to the aft edge of a hard top. It solves a lot of problems for me.
Why not a reinforced aft frame on the dodger? Most never fold, so it could be built like a roll bar, and would also make dodger-mounted hand holds more valid. I’m not suggesting an ugly arch, just a fixed last frame with a good angle brace made from ~ 1 1/2″ tube. Engineering would be required, and ultra-low stretch jacklines materials are out because of the high forces they impose.

Rob Gill

Hi Paul,
I wouldn’t be in favour of anything attached to the mast or stays. Should you lose your rig, anything attached to it could well be lost overboard, possibly including anyone so attached. Then you would have lost your jack-line system at a time when you would need it most. Yikes!


I understand what all of you are saying, but most are focused on the engineering angle. Ie, why it can’t work as it is now (forces vs anchoring JL); not as it could be w some planning. If you agree a higher/more inboard JL improves the solution, then it becomes a more simple problem to solve… maybe granny bars are designed for expanded use w better anchoring to the boat.? If the mast is going over not sure there is a good location on deck. Everything is ugly at that point. Just happy to be clipped in. Plus most masts are lost higher up than waist height, wouldn’t you agree?

Rob Gill

Hi Paul, hmm… I can see where you are going with this but I just don’t agree “higher improves the solution” – to my mind it just complicates it. I can only see downsides to going up, specifically:
1. For the geometry to be much better than deck level jack-line in terms of freedom of tether vs safety from going OB, the jack lines would need to be at head height or higher, not waist height.
2. The only strong thing at head height (forward of the cockpit) on a yacht are the mast and the stays, and though I’m not qualified to comment on the engineering of it, as a racing yachtsman of many years I can say without fear of contradiction, rigs toppling in rough condition gets messy real quick.
3. Waist height jack-lines would create an obstacle course for anyone crossing the deck. You would have to climb over, or crawl under. Crew are used to avoiding deck level trips (jib sheets, halyard runs, sheet tracks, cleats etc). Will they be looking for waist high crew-stoppers at 0300hrs?
4. The worst case scenario I can think of where the life of our crew would rely on the strength and integrity of the tether system, is a complete “rogue wave/gust” knock down, involving free-fall across the deck. I am struggling to think of any situation where we would be better off dangling from a jack-line 2 metres in the air (or even waist height), than being tethered along the deck where we have a fighting chance to grab something solid.
5. If Bonnie Lass was knocked down beyond the horizontal (her point of diminishing righting moment is around 110 degrees from vertical), I believe I am right in saying having the tether above the deck, could result in the crew member being immersed in the water (geometry again).
6. In the event anything is carried away, our new inboard deck attached jack-lines would still be intact and the crew would be attached to the mother ship, inside the lifelines and probably clear of the water.

My last hypothesis in point 6. needs some more thought and measurement, not keen to test live.
Hope this helps,


John, If I were to put a center-line track on my boat (my preference), I would have to gap it for a hatch, a staysail track (and running rigging) a mast, a line locker, a hatch, a hatch slide hood, and that would just get me back to the dodger with 12 snaps/unsnaps. Given this reality which I suspect is recapitulated on most boats out there, I might as well put in hard points, but as you say, fetching up hard hurts and can seriously injure.

If I use jacklines to approximate the centerline track, dorade guards, life raft, mast, stay sail and running rigging impede smooth movement (5 snaps/unsnaps). If I fetch up hard at the ends of the jackline, it might as well be a hard point, if I fetch up hard in the middle it could be a bit dicey.

Since I now use two tethers to bridge gaps a few more snap/unsnaps in the form of hard points isn’t a lot of bother. Some trips only require 2 snaps/unsnaps, some the full 12 for an average of 7. . . Not much different than the centered jackline.

So for me the engineering challenge is a body-friendly dynamic response from a tether connected to a hard point on one end and an ergonomically designed vest at the other end. The strap vest design that is best for floating one is not the best design for arresting a fall with spine and ribs intact.

Where the force is coupled into the vest is a separate issue from where the force is then reacted into the body. It can still have a single attachment point. The desired outcome is spreading the loads to the body through a mesh of fibers which vary in diameter and elasticity according to the body’s ability to absorb the loads — like being grabbed in a snug hug rather than snared.

Such a vest**, considering materials advancement and finite element analysis, is not hard to do. Obviously this would be nice to fit into the world of XS,S,M,L,XL but even semi-custom, with a price point around USD 200. It would be part of cheapest life insurance one could buy.

**There are dog training vests on the market that are already walking this path. The idea is a comfortable load application across the chest and belly (rather than a jerked loop at the neck) gives better training results.

Drew Frye

Body friendly dynamic tethers are a personal interest of mine. I’ve separated ribs skiing.

Before digging too deeply into the jacket idea, start with what climbers do and what I have gone to; use dynamic rope. This has been proven over 100s of thousands of falls and is the simplest way to absorb most of the fall energy. You could try 3/8″ nylon 3-strand first, and then go to 7.7mm UIAA 1/2 rope once you work out the design. While not for everybody, they do take the sting out. Unlike a mesh vest, they also reduce neck-snap, since the stop is more gentle.

Not sayin’ you shouldn’t explore the vest idea. Some folks can handle the high impacts—and as John and others have pointed out, there have been very few serious injuries due to harness loads—but some don’t want to. And current PFDs certainly have drawbacks.


Drew, it sounds like you are making your own tethers. If so, check out the principle behind load limiters that climbers use, called “screamers.” Essentially, you add some extra length to your tether, fold that extra length up and sew it together with some bar tacks that are designed to rip out at pre-determined loads. It would take some trial and error to figure out how many stitches and thread count per bar tack, but then you are set – a tether which will lengthen (and not by much – just 5-10cm) under load and dramatically reduce fall force. It doesn’t weaken ultimate braking strength.

Drew Frye

Yes, I have climbed with Screamers, but I gave mine away many years ago. They are heavy (can carry 2x more plain draws), bulky, only reduce fall impact under certain circumstances, and even then the reduction is small. MUCH greater improvement is achieved with more running belays, better belay technique, low-impact rope, and paying attention to what you are doing. Although they do have a following, overall, they are not very popular with climbers.

I mentioned Screamers in the link I gave, and I did have a pair of tethers incorporating them for a year or so. For most of the same reasons, I did not like them and gave them away. Interestingly, though US OSHA loves the Screamer tech, European OSHA favors dynamic rope tethers. I also believe they are considerably more enlightened re. climbing and fall protection than their US counterparts (a dynamic tether can absorb about 30-80% more energy than an OSHA tether, depending on which tests you like).

That said, a modified version could be a good idea. Personally, I find the pop-up flags on the new generation of tethers rather silly; a sign to tell you you nearly died, instead of engineering a better product. Clearly a committee decision (we can’t solve the problems, so we’ll attach a warning label).

pat synge

I don’t like jacklines and we’ve solved the problem on Adjo Akama by simply not having any.

Most setups are usually (always) inadequate, as described by John in previous posts, and they simply give a false sense of security. Like most lifelines.

Our first three years of offshore cruising were aboard an 18 foot centreboarder and then four years on a 24 footer. Neither had lifelines or even much in the way of side decks. We had our fair share of bad weather (it starts at about 20 knots on an 18 foot boat) and I think this taught me a lot about how to move about safely on deck.

This seems to be a neglected topic and probably more relevant to not falling overboard than jacklines. Why stand up? As Paul says above – “hold on for dear life, stay low, move slowly and watch every step”. Crawl if conditions warrant it. If you’re really low and holding on firmly the chances of falling overboard are next to zero and getting swept overboard are minimal. Those who are entirely risk averse should stay at home and watch it all on TV and never, ever drive a car!

Don’t get me wrong I’m not saying harnesses are not valuable. I wear one whenever working on deck in anything but perfect conditions and always at night. The tether is short and I clip it on wherever I’m working. Round the mast or onto the inner forestay usually (both on the centreline). I try to avoid having to go on deck in bad conditions as much as possible and with slab reefing for the main from the cockpit and a furling headsail one is halfway there. Being able to drop and lift the anchor from the helm is another useful safety feature.

In my opinion (and experience) getting back and forth is not the dangerous time: it’s when you’re doing the required work and your hands and attention are otherwise occupied.


I don’t always wear a harness, but even when wearing one act as if I didn’t have one on. I have a military extraction harness that has a short built in tether as well as a ring for a longer tether and when at the mast I clip the short 18 inch tether to the mast itself.

This is an interesting discussion and it is obvious that jack lines only give you an impression of safety so I have decided to have long tethers connected to the mast under the spreaders that would not allow my crew to get more than their feet wet

Rob Gill

Hi Frank,
Coincidently, yesterday I was talking to a marina neighbour just back in NZ from the Pacific Islands. I commented on a nasty deep gash in his aluminium boom that isn’t there when they left in May! Apparently, he was on deck having just released the preventer on a moonless night, ready to gybe, and then got caught out as they crash gybed – breaking the end boom mainsheet attachment. Rounding up in the heavy conditions, the newly liberated boom was wildly crashing from side stay to side stay as they attempted to lower their mainsail.
In this type of incident where the mast could easily have joined the boom, I can’t see any good reasons to be attached to the mast, and especially not up high. I can think of plenty of reasons to have a centre deck rigged jack-line now, including having a secure webbing runway “the hell out of there”!

Drew Frye

Slightly off-topic, but…

My current boat came with polyester double braid traveler line. When it wore out I installed a piece of Spectra I had been given for testing. Pretty, but it was like jibing against a brick wall if there was even 1-foot of slack. That convinced me to try 8mm climbing rope. Much smoother, great in the hand, and the risk of real damage from a crash jibe is much less, since it will give 2-3 inches under impact in a 15-knot breeze (I try to avoid rough jibes!). This is NOT cheap line, and in my opinion, is the best material for the job at any price.

The best material for applications would make a good thread. Some folks are sold on Spectra for everything. I’m not.

Eric Klem

Good point on the danger of using low stretch line for all applications. I really like the strength and abrasion resistance of spectra/dyneema but I very rarely use it without having something stretchy in series with it. The great thing is that you can just cow hitch a loop of it with a loop of nylon or dacron and get a very good connection that doesn’t have major chafe or strength issues.


Rob Gill

Drew – nice thinking! At the risk of getting our comments moved, sorry John – does having the extra stretch in the mainsheet (multiplied by the number of parts) noticeably open up the whole leech (rather than just the tip) and de-power the rig in a gust? Nice in gale conditions, great idea on a multihull and my wife will love you for it. Not sure about the skipper though – does it affect sailing performance at other times – what have you found in practise?
Eric, I like the idea of pairing the two rope types in series for certain applications (we have this on our preventer set-up), but I read somewhere that the cow hitch as a knot is one of the very worst in reducing the resulting strength of the rope when tied. As I understand it, the way the cow hitch is formed with the tail and standing part pulling back against the formed loop at an oblique angle is the cause. The tighter the hitch gets the worse this angle, and the more the loop squeezes the rope turns underneath, weakening the rope fibres. More sympathetic knots like the figure of eight tied on the bight retain almost all the strength of the rope. I understand climbers are pretty expert on this, so Drew might comment further.

Rob Gill

Eric, I just posted then realised a) I was probably teaching you to suck eggs (apologies), b) rather than use a cow hitch to join your two loops, thread one loop through the other and pull through the other standing part, so forming a reef (square) knot which I understand is easier on the rope, and c) see a) above.

Eric Klem

Hi Rob,

I may be incorrectly using the term cow hitch. What you describe in your second comment is exactly what I do. I think of it as tying a cow hitch then dressing the knot differently by grabbing the loop that goes around the back and pulling it over the top until it resembles a square knot.

Drew can probably answer your mainsheet question better than I but I believe the key is that the mainsheet is not high stretch but the traveler line is. When sailing close hauled, the traveler lines are very lightly loaded in comparison to the mainsheet as they have much more mechanical advantage so they won’t stretch very much. When gybing, the traveler lines will carry much more load leading to the shock absorbing deflection. We don’t have anything nearly as stretchy as dynamic climbing rope on our traveler but the line we use is pretty low tech and it really helps in a gybe but has almost no slack on the wind. Of course, if you actually want the main to be let out by line stretch in a gust, then you could use a stretchy line for the mainsheet but it would likely be frustrating for trimming.


Rob Gill

Hi Eric,
Yes, a naming thing: the cow hitch would have the standing part(s) and tail through its own loop. I had a feeling after I posted that you probably knew this. And yes, I misread Drew’s post LOL – it makes much more sense now, I thought 8mm sounded skinny for a mainsheet but who knows with these climbers?

In truth, this was part wishful thinking from me. My dear crew, who loves sailing but is not a natural sailor, HATES the sudden lurching and heeling associated with strong gusts and gybing (even though we centre the boom for the latter, and she is in charge of reefing so we often sail around like we’re about to round the Horn) – I’m talking white knuckle stuff. So Drew’s post above grabbed my attention. I was thinking climbing rope may be a part solution (certainly for the traveller – great idea). The mainsheet as well? It would be cheaper than buying a carbon mast which is one alternative I have considered, or a catamaran which is another. The only other downside I can think of for us apart from trimming as you say is having to shorten the mainsheet so the boom doesn’t hit the side stays with the extra stretch, in the event of a crash gybe. Other thoughts anyone? John move my posts as you see fit – we are way, way off topic now.
Thanks, Rob

Drew Frye

The phrases “impression of safety” and “illusion of safety” are tossed around a lot, quite accurately if the sailor is naive enough to believe a little gear will make a storm “safe.” Many are. I’ve climbed rock and ice for over 30 years, often with less than perfect safety systems. Sometimes a lot less than perfect. As a friend of mine would often say, run-out on some adventure lead, “you have to know when not to fall.” A famous climber once said that “some times you must substitute time for gear,” and move deliberately. The first 2 fatalities I helped clean up were both festooned with more safety gear than I ever used. I think they misunderstood what it could do for them.

Yet would I toss the rope aside, claiming I was safer without it than using something that was not >95% safe? That would be rather silly. Instead, the emphasis (for climbers and sailors, and the point of the thread John is leading) should be to design the best we reasonably can without sacrificing movement, and then to understand what we have built.

I’m quite sure safety systems have helped me, and I have never let them lead me far astray.

Yup, I’ve climbed far and high without a rope on occasion, for the purpose of testing nerve and enjoying competence in technique and movement. But I never thought it was safer, and I’m too old for it now. It’s not fear that has caught up with me—rather a certain knowledge that my body can fail with less warning, combined with better knowledge of systems and no further interest in proving my nerve. I still climb ice.

Rob Gill

Hi John,
How is it that with all of the combined wisdom so many sailors, riggers, boat designers and builders over so many years, we still see most yachts (including Bonnie Lass) with side-deck rigged jack-lines? Forrest Gump’s “stupid is as stupid does” comes to mind! Perhaps because this centre-line system will only work on yachts with clear decks and cabin tops – having followed your advice on this to the letter, I am feeling slightly redeemed.
To my thinking, a real strength of your MOB prevention system John, is having the tethers already attached to the jack-lines so that crew use the right length of tether pre-measured for their position on the boat, as long as the jack-lines are limited so the crew can’t inadvertently wander into danger, particularly towards the bow or stern as the deck narrows.
Luckily for us, because the UV is so strong in NZ, we had our jack-lines made up with safety harness clips on each end to enable ease of rigging. So moving the jack-lines inboard should be a small change, but will require me to shorten our jack-lines and some of our existing off-the-shelf tethers, so looking forward to your next “how-to” instalment! My specific questions for the next instalment are: Do you colour code the tethers to ensure the right ones are used for the right situation? How do you stop crew taking the tethers with them by habit or mistake, leaving the next user stranded?

Thanks so much.

Pascal Cuttat

Hi John,
thanks for this thread. Great stuff. I believe that the centrally located jacklines contribute a lot to solving the problem.
I am also glad you mention your policy of never using the sidedeck jacklines unless another person is on deck. I have a simple and more radical rule: nobody EVER leaves the cockpit without somebody ready at the helm, autopilot or windsteering disengaged, ready to switch on the engine and drop the main if necessary. I know that this is unpractical for shorthanded crews and may have an impact on sleep and fatigue, as you mention. But I just can not stomach any other way, also meaning that I consider a crew of 3 the minimum for any passage over 200 miles or so.

Scott Fraser

A small point: in the video you exit the cockpit and move forward on the leeward side of the boat. I have always had crew go forward on the windward side as the heeling angle means the deck is closer to horizontal and if you get hit by a wave, you are likely to be washed into the deckhouse or down the windward scupper. Moving forward on the leeward side may provide more protection when you are in the lee of the dodger, but after that it’s a much shorter trip into the water.

I like the idea of centerline jack lines and will install them as soon as I workout the details of where to fix the hardware. Thanks for your diligent work on a problem we don’t want to have.


Rob Gill

Interesting observation Scott,
Inshore, I have always preferred crew (especially inexperienced ones) to use the leeward side off the wind – if we gybe, the boom or mainsheet hitting will hurt or maim them, but if they are to windward with the extra velocity built in the gybe, it will maim or kill them.
Secondly, a large wave coming from behind or out of the dark is a hidden threat – being to leeward means you are generally looking in the direction of the threat, and then the width of the deck gives you a split second longer to react and brace for impact. As a professional navigator on ships we always went on deck (if we absolutely had to) to leeward in storm conditions even though jack-lines were rigged both sides, and that habit dies hard.
The advantages of being to windward is a wave would first wash you on deck not off it (with our old side deck rigged jack-lines our maximum fall in this case would be say 2.5 metres with a 2m tether), you are more upright as you point out, and your feet stay drier. When racing, not many skippers would be happy with a crew member “mucking around” to leeward. A 50/50 call in my book.
But now with centred jack-lines, I am thinking that the leeward side is more advantaged still. A wave assisted fall from the windward side deck, until arrested by the centre jack-line for us could be 4.5 metres (given a 2 m tether length and 0.5 m stretch in the system). From the leeward side the fall would be less than one metre in most cases, and the crew would be inside the rail. I agree with John, there are times we will probably still use both sides, but this windward “further to fall” risk (compared with side deck rigged jack-lines) needs to be considered and understood.


Nothing like a nice bulwark to walk on when going forward on the low side when hard to weather. But I’m showing my age—– hard to find one on a boat designed later than 1950.


I have been following these posts and ensuing comments with great interest, thank you John and all commentators fur such an invaluable discussion. I have been thinking along a different path, and have no more than an untested idea to share; Stopping the boat as soon as possible after one has fallen overboard and is attached with a tether seems to me the big problem to tackle. The load on the jackline both from the fall, and from the dragging of the MOB will be significant, certainly enough to operate the steering system of the boat if that load was somehow applied to the tiller/wheel… something like a pre set breaking point embedded in the jackline, that once hit, frees it from its aft attachment point, bringing a connection with the tiller to life, so that the weight will swing it completely in the direction of the MOB… as I said just food for thought…


Dear John,
I look forward to reading your thoughts in the near future!
thanks as always for holding this interesting forum.


Last year, I installed a very similar jackline “system”:
1. Personal (“cockpit”) tether clips into cockpit padeye. This padeye is reachable before exiting companionway to cockpit.
2. Jacklines run down the center of the boat, from traveler to bow, along the sides of the mast. I fabricated a stainless steel bar, exactly the width of the traveler, with a U-shaped attachment. This bolts to the underside of the traveler (using traveler track bolts) and provides attachment for center-line jacklines.
3. Upon exiting the cockpit, a deck tether (permanently clipped to jackline) is attached to the harness and the harness-end of the cockpit tether is moved to the dodger frame. This process is reversed when moving back to the cockpit.
Admittedly cumbersome, it seems like a workable solution (ie. system deployed but fortunately not of proven efficacy). Tether lengths (with centerline jacklines) should not permit falling over or under the lifelines.

Eric Klem

Hi John,

This sounds like a good improvement. I had been toying with the idea of running my jacklines about a foot inside of the outer top edge of our cabintop but hadn’t yet figured out how to get around the dodger to get clipped in. Your system of tethers left in place should solve this although we would need to do a swap in the not so secure area outside the dodger. In our case, our beam is low enough that we could still go over the close set of lifelines with any reasonable length tether in a worst case scenario fall. I take this to mean that we still need to be very careful and pick which side we use carefully. Regardless, it will be a huge improvement over the current system where you can end up way over the lifelines on the close side and right up to them on the other side in a worst case fall.

One of the biggest issues that I have with our current jacklines is that they are not straight due to the cabintop shape so that if you manage to deflect them up onto the cabintop which doesn’t take that much force, they actually will go to centerline or beyond with not much force. The issue is that the jackline is longer than the straight line length because it has to conform to the cabinhouse side. The centered jacklines that you have set up and what I envision trying for our boat should eliminate this geometry issue which is present on a lot of boats that I have been on. I am looking forward to seeing what you come up with for a foredeck jackline. We use hank-on jibs so I do occasionally need to go forward in less than ideal conditions.


Drew Frye

The aft ends of my jacklines are actually secured to through-bolted stainless tube hand rails on the hard top. This puts them just the right place, for me; chest high, well in-board. The only problem is that the biner likes to snag on a gutter in the FRP at the front of the hardtop as I move forward. I keep meaning to make some manner of smooth transition to cover the groove. Now I will be motivated!

Yup, requires a strong hard top. On the other hand, even if the hard top supports cannot hold 5000 pounds horizontal (I’m sure mine cannot), most will bend and not sheer off, which is not terrible. I could brace them, if I though it was that critical.


Hi John,
Yet another argument for a hard dodger— especially on an aluminum boat where it is easy to make it very strong. See the Van de Stadt Samoa for the perfect example—–.


Hi John,

A most interesting discussion as I often had questions about this issue of staying onboard. I’m rather new to sailing, five years now – a juvenile you might say. And there are a few things about the “sailing world” I still don’t get, usual stanchion height being one of them. My post in no way criticizes the betterment of jack lines, I think it is an essential part of the whole safety system of a boat. However, the question still remains, why is stanchion height usually so low? It would be against any building code on a balcony, on a house on the ground, so why is it the norm on a moving vessel where lives are at stakes on a regular basis? Maybe this is some kind of traditional danger that has to be accepted, maybe there is an essential poetic stance in being so close to the sea, or maybe it is part of seasoned seamanship to survive with one’s own intelligence, physical stamina, awareness and experience.
I plead ignorance on those counts.

I’ve included links to two commented stills form your video. The first one is when you are getting out of the cockpit:

Here the yellow dot is your estimated center of gravity (60% of your height from the ground), the two black dots are the actual stanchion’s upper end with a black line going through them, and the other black line going from the stanchion nearest from you is the boat’s plane (you’re slightly healing). It’s obvious that your center of gravity is much higher than the stanchion, and it wouldn’t take much force to throw you overboard. In fact the stanchion line (lifeline), constituting a pivot, depending on how the force is applied, could even help in getting you into the water.

The second image is another commented still, taken moments later when you are fully standing:

The yellow dot marks your center of gravity, line 1 marks the boat’s horizontal plane going through the upper end of the stanchion near you. Line 2 marks the boat’s horizontal plane a little higher up and intersects a projection line of the stanchion itself. From this position, with lifelines and stanchions at this height, tremendous force would have to be applied, at least at shoulder’s height, for you to go overboard. And this is not to mention other benefits: you have now a third, maybe a forth lifeline to retain you, instead of only two; you have more to grab and hold on to; simply ducking, in lowering your center of gravity, becomes a very efficient move to stay on the boat (you just collapse on the lifelines for a moment – like a boxer in a ring). Of course there are other considerations: increasing the stanchion’s height increases the lever arm and calls for a much stronger type of stanchion; since we often have to be able to bend over lifelines there has to be some kind of system to facilitate that, same with getting on and off the boat. But the stakes are so high and the benefits so great, can’t that be worked out?

Leo Elwell

As you have correctly pointed out, the higher the stanchion, the more leverage, and then the forces transmitted into the boat deck/toe rail/hull area. Bigger base mounting. And the cost of more stainless, bigger cross sectional area of the tube, etc. Stronger and more expensive cable. But I have a feeling that the real answer is because “we’ve always done it this way”. Or the look. Ouch! The Emperor truly is missing some clothes.
Cheers, Leo
PS Great discussion gang!!

Dick Stevenson

Hi Drew,
Your move to have something to absorb gybes in the traveler system might be taken one step further. Even though there are substantial cushions at the end of the traveler, I sew a stopper that hits the line brake about 2-4 inches before the car gets to the end of the line. I could see the car, in a bad gybe, going right off the end. At full extension and with the sail loaded, the car sits about 2 inches from the cushions.
My best, Dick Stevenson

Drew Frye

I would do it exactly as you say; unfortunately, my line is continuous, meaning the most of the line from leeward is sucked to windward when I tack. I have marked the 2″-off spots with a whipping and use them as you suggest.

I also lock the windward control down so that the car cannot move; it does not mean I can’t have an accidental jibe, but unless I have a lot of sheet out (I don’t since I have a long track and back-swept shrouds), but it does limit the severity. I set a preventer when going very deep, generally wing-and-wing—it is one I can release from the cockpit.

Leo Elwell

Hi everyone,
I climb and sail. With respect to variable length tethers, I would respectfully submit that a “daisy chain” is a very elegant solution.
I am going to be getting rid of all my dyneema webbing after seeing the DMM fall videos, and going back to good old nylon. Dyneema is after all, light, thin, sexy and it’s approved, so of course it’s better (sarcasm). Anyway, I was wondering about combining the best of nylon and Dyneema. Dyneema has low deflection, so how about that for the jacklines, centre mounted. The tether/harness part being the shock absorber part, made out of nylon?
Cheers, Leo.

Leo Elwell

Hi John, thanks for pointing out that chapter again, I did read it! I guess what I was trying to wonder about was how to decrease the amount of deflection in the jackline, so as to keep us in the boat, yet at the same time having enough shock absorption. So maybe dyneema, with a screamer for the center jackline, and then the nylon for the tether/harness?
cheers, Leo

Ben Tucker

Looks like a good solution, It also allows getting to the lee side safely if needed.

One modification Iv’e used to your system of not uncliping the tether from the jacklines, is to remove the lower clip, and just sew the webbing tether into an eye that slides along the jackline. This gives less catch points, less banging and scratching as the clip scraps all the paint and is lighter and simpler all around.

Drew Frye

I have heard this described as a good soft shackle application. I tried it once, but I didn’t like it, since I like to be able to unclip fast. Who has not inadvertently clipped on the wrong side of a sheet and been glad to un-clip and re-clip swiftly? I have!

But I think the idea merits discussion for some purposes. It does slide better.


Having already commented on this thread, I’ve been watching the discussion through a few different sets of lenses. I think broader thinking may be required before things can be narrowed down. It seems to me there are three pieces that need some additional thought.

1. I’m not sure the objective is falling without damage from the fall/tether/hull/water — It seems to me the objective is “zero falls.” To me that means an overall system design that includes moving/working on ones butt or knees for tasks in the risk zone. It means multiple tethers of negligible length ( a foot or three) and just enough shock absorbtion too keep them from being uncomfortable.

2. It seems the discussion is narrowing in on hardware attributes before assessing how the sailor functions as part of an overall system which doesn’t fling parts of itself (us) into the sea. Having seen a lot of boats over the decades I am impressed with how they are laid out for plain sailing versus how poorly for Katie bar the door conditions. Even if the objective is safe and survivable falls, the boats aren’t laid out to promote that — too much of the problem solution has been allocated to the weakest links — the sailor and the tethering. To me the solution set should include moving some items (such as halyard winches) down to deck level. It may involve replacing winch handles with hand-wheels, It may involve changing vertical interactions with boat hardware to horizontal ones. etc, etc. Unfortunately, all these solutions are more expensive than tethers and their associated risks.

3. While I am in awe of rock climbers and what they do, what they do is climb rocks. It seems we need to broaden the search for analogues and solutions to persons who must do (non-climbing) work while in danger of falling. Arborists, linemen, and oil & gas rig workers come to mind. The reason I say this is the climber’s mind is absorbed with and attuned to climbing, and so is their equipment. These other categories are attuned to laboring while at risk of falling. A cursory review of their equipment shows three significant common features — falling loads are reacted into the wearer by parachute-like harness, suspensions points are high, and multiple tethers are often included for use in stabilizing the worker when the work point has been reached.

John, et al, as the tether/jackline discussion goes forward let’s nudge the target toward zero falls and more systems vs item solutions.

Drew Frye

Yes to the intent. In work and in certain facets of climbing, avoiding the fall is everything. As you say, it is both design and mindfulness.

The discussion centers on climbing equipment, in part, because the standards for OSHA derive from climber experience and standards that were originally developed for climbers (UIAA was the leader). Those are the companies and the people who had done the math. As for differences in intent, I’ve spent many hours doing refinery inspection way up in the air and designing work place systems. Really, they are not so different, except for a greater emphasis on not falling.

Regarding tethers, impacts, and all that, there is a simple test I always invite sailors to try, based upon what ever system they have in mind. Clip in, get the most slack you can, and run as fast (do this is stages—we don’t want real injuries) as you can at the end. Does it hurt? We haven’t even factored in a leaning deck or the wave that is chasing you. With your short work place tethers, you will be fine, not even annoyed. With a 6-foot Dyneema tether, please don’t even try, it will break you in half. It won’t be >5000 pounds, as a simplistic model would project, but I’m sure you can separate a rib. Then, based on this experience, do what you like with the system design you have. There is no one-size-fits all geometry. Even OSHA recognizes this, requiring jackline system design (yes, they are common, generally waist-high and often over-head—think roofing, construction, and rail car loading) be site-specific.

I have done these tests myself, with a tethers of varying materials tied to a hard point. You will get injured by a 6-foot run (not a fall) at any material other than nylon rope. Even nylon webbing is very painful and I never tried it at full steam—3 feet of slack was all my ribs could take. That led me to my design criteria, which I have said many times is boat-specific. I would simply like others to try this simple test-based approach to figuring impact. I’m an engineer and can calculate, but I really like simple tests.

As for the 5000-pounds general strength standard, it is based as much on basic durability and allowance for wear as actual forces. I wouldn’t muck with it if you want the gear to last.

Mike Sweeney

Hello everyone

As the owner of a new-to-us boat without an established jackline system, I have been following these posts, and the comments to them, with great interest. I filmed a short video of a Mod 1 jackline/tether system that may work on our boat. I posted the video clip on our Facebook page but I believe that you can view it with or without a FB account.

I’d welcome your feedback/critique/suggestions. A few points first though…

1. My key design consideration was to prevent the person that is clipped in from hitting the water, without tethering to a hard point. While I have yet to test this by actually hanging overboard, I believe that the objective is met in all but one or two spots.
2. The line being used for the jackline in the video is just an old sheet that I found on board. It is just for planning purposes. The actual material that we will use has yet to be determined.
3. The tethers shown are climbing slings. Again, used only for planning purposes to work out the length.
4. I chose to fix the jacklines at the mast to minimize overall stretch, and keep them close to the center line of the boat. While this does necessitate a tether change to go forward to the bow, I believe that the number of times this would be required on our boat with a roller furling headsail is much less than the times required to go to the mast to reef, etc.
5. I opted to use a remote tether at the mast instead of a two-part tether.
While it may have appeared to be cumbersome to make the switch at the mast, this was the first time that my wife had run through it, and the tethers being used were just improvised from climbing slings and carabiners. I’m confident that with practice, using the finished product, the result would be much more streamlined.

Thanks in advance.

Mike Sweeney

Thanks, John! While I believe the point that I have the aft end of the bow-to-cockpit jackline anchored to is pretty strong, I also thought that installing a dedicated pad eye on the hard top would be better. I don’t really like the stern attachment point either so installing a similar pad eye at the very back might be best too.


Hi John,

Really great article and discussion to further offshore safety. I’m putting a presentation together for the PSSA and have been doing some thinking on this topic. Anyone have experience being dragged while tethered? I’m trying to figure out the best courses of action if I find myself in this scenario, and what I should prepare, carry and mentally prepare for.

I’m aware of the drag testing done by Practical Boat Owner here: Their scenarios involve crewed vessels and illustrate the importance of slowing the boat down quickly for survival.

But let’s say it’s just you aboard, or you and one other off-watch, sleeping below. Somehow you end up over the side with the boat going over say, 4 knots. Let’s say you are a normal weight, above average middle aged man. Let’s further stipulate you have a state of the art Spinlock PFD & Harness & are wearing your crotch straps. Let’s further say you had some foresight and ran a line along the hull on both sides from the forward stanchion the the aft stantion. While being dragged, would you have an ability to:

1. Grab a line or emergency ladder & pull/ scissor yourself aboard on the low side? High side?
2. Grapple for an autopilot remote and turn the boat into the wind to slow the boat down so you don’t drown in 60 seconds?
3. Pull out your handheld VHF and call for help – and have someone hear you?
4. Activate your new AIS/DSC 1 watt Beacon to summon nearby aid?
5. Activate a PLB?
6. Move aft to the stern & gain access to a ladder or swim platform?

I suspect most think they would somehow move aft to their boarding line & pull themselves aboard, but I suspect they are unaware of the water forces on a body even at a moderate 4 knots. Maybe best to detach & float free to activate your PLB? Maybe there is an opportunity for a harness design so the tether attaches to the back so you would be dragged on your back & have an ability to do something with all the gear you carry. Or does the long tether make sense at all & just use short tethers at workstations? Don’t use a long tether while moving, but carry a PLB and VHF or AIS/DSC Beacon. At least, until someone designs a better harness? That way, when you go over maybe you are less likely to die?

Thanks for your thoughts. This whole thing feels like a mess. Someone should do some drag testing (carefully!) to figure out what is realistic to raise the odds of survival.

Rob Gill

Hi Whitall,
From my reading on this and other sites (including the PBO article), there is now no question in my mind that the best solution for us as John explained, is to always be clipped on to a tether system that in all cases prevents the wearer from going beyond the life-lines, whether the crew-member be on-deck, or in the cockpit.
The task for us then becomes one of rescue from the water, either because the jack-line, tether or safety-clip have failed, or a tired / careless skipper / crew member has taken a short-cut or made a mistake.
My supposition then is this enables us to focus on “staying aboard” systems, process and training including the absolute requirement for anyone out of the cockpit to be attached at all times to the tether system and actively watched by a delegated person who has acknowledged this responsibility (and requires the same level of awareness as if the other person has already gone overboard).
Then we can concern ourselves with our person-overboard rescue / recovery systems and training. Recovery from a tether is not a scenario that we need to consider and possibly Whitall, your list is then simplified since someone aboard will always know there is a person overboard and will be taking action. On the best “person-overboard and rapidly floating away” systems/equipment, your list has a number of valid options. I don’t have clarity on this, particularly since my previous long-held beliefs on tether systems were so flawed, but I am looking forward to future follow-up posts please John if possible.

Drew Frye

A wise old climbing partner of mine used to say “sometimes you just have to know when not to fall.” There’s no point in sugar coating it.

Robert Muir

Any recommendations for tether sources? I purchased some Wichard tethers, but they’re all six feet long! Six feet would be fine for the cockpit, but is too long for the side decks (unless I needed to climb up the forestay for some reason) and cabin top line. My beam is only 13ft, So a six foot tether has the wearer hanging over the lifelines even when attached to the center line if a bad wave were to hit at the wrong time.

Currently, I’m thinking the best choice would be to have a rigging shop cut and re-stitch a couple of the tethers that are to remain attached to the center line.

Robert Muir

I also wanted to add that when measuring for the jacklines, it’s better to be an inch or two short than to risk getting them too long. My side deck jacklines were the right length, but the one I measured around the mast ended up being a few inches too long and I’ll need to take it around the mast a second time to get it short enough.

Clive Arnold

Hi john,
I notice that on MC your jack stays are twisted along their length. Is there a reason for this I don’t see? I’ve always tried to keep mine untwisted…..
Regards, Clive

Ernest Vogelsinger

Hi John,
I just reread the parts covering your new jackline system and watched the video. Now I have two questions:
Part one – it seems to me that your tethers are still at a length where you night easily trip over the lifelines when hit by a sudden wave, e.g.. I am refering to the video where you step out of the cockpit and change from the (shorter) cockpit line to the (longer) deck line – at this position the line is slack enought to give you room for a Rittberger backwards 😉 Did you test this and this is only an invalid assumption?
Part two – the unused lines are positioned nicely in the video, but I doubt they will stay in place just like that in more than easy weather. How do you stop the hooks from annoyingly knocking the deck, or flying around to corners where you can’t reach them easily when needed?


I have read through these post in this book but I’m not sure I have seen any picture or description on how you attach in the cockpit. Do you have a center point padeye in the deck for a couple tethers or a jackline that runs longitudinally to the cockpit sole? I ask because the cockpit on our Kaufman 47 is quite large. I was going to put a padeye in the center behind the wheel for when driving, just long enough to sit comforably on the combing on either side. And forward of the pedestal I’m not sure yet because the engine hatch is the entirety of that deck. That will be secured from the engine room but I would rather attach somewhere else. Unless I try to put two padeyes on either side below the cockpit benches. It will be a tough one and will probably take some thorough thinking and experimenting. Just wanted to get an idea of how you do it on MC so I have an idea of what is a good way to do it.

s/v Redemption


Do you tension your cockpit jacklines with lashings?