The Offshore Voyaging Reference Site

Better Jackline Systems

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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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Mike

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.

Rossd

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!

RossD

Hi,
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.

RossD

Hi,
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.

RossD

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.

Johan

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.

Sandy

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
Sandy

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.

paul

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.

Paul

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!
Rob

Paul

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,
Rob

ChrisW

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.