Dynamic Tethers

 rev. 4-19-2014, rev 8-8-2024

Stitched Splice Warning. Based on limited testing we have found that nylon rope is much different to sew than polyester rope. Because of the extreme elongation prior to break, the stitching at the tail must carry nearly all of the load, while the stitching near the throat carries no load. It is rather like tug-of-war with a bungee cord; only the 2 men nearest the center can do any work.

Because of this difficulty, I strongly suggest all dynamic tethers be knotted rather than sewn. Climbing ropes have very high knot strength and are drop-tested with a figure-8 to attach the mass. A double overhand noose has also been drop tested. 

[Since this article I switched to a webbing retracting Proline tether by Wichard. I don't like the clips quite as well as the  Kong Tangos, but once you get used to them they are very good. The webbing has a bit more stretch than the prior polyester webbing and can pass the ISO drop test, but with a terrible jolt. After 5 years, the elastic is about shot, so the long tether droops dangerously. It has a quick release on the harness end, but I have mixed feeling about something that can release when I don't want it to. This winter I will either buy a new tether or build a new dynamic tether with Kong Tangos on for all three clips (On my F-24 we detatch the rope end as well, rather than leaving the tethers on the jacklines.]

This subject is a little too esoteric--for most sailors--and so magazines aren't much interested.   Fatalities due
to tethers breaking are rare. On the other hand, bruised ribs and back injuries are more common, but wrongly accepted as a part of rough weather sailing. However, if  we simply apply what we know from climbing about falling--lots or real world experience and lots of lab testing--it just doesn't need to be that way. Let's see if I can sweep away a few bits of conventional wisdom that are just plain wrong. Though you may never sail the Sydney/Hobart, capsize your boat and test the limits of  the human body, routine bumps against tethers don't need to hurt either. We can make our sailing experience more comfortable and safer at the same time. My tethers don't hurt.

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Dynamic tethers spliced from 8mm ice climbing rope. Knots would serve just as well. 10mm climbing rope works, but it is heavier and a little rolly if you step on it.

I started my investigation into soft tethers and impact forces years ago, but honestly, my boat is not the ideal
test bed; catamarans don't heel, I don't sail in big waves, and I'm not as willing to risk physical trauma as I used to be. I hold on and I don't fall. However, I've let myself slam into tethers intentionally a few times, just for testing, and I'm not happy with the current state of design. My back is too old.

Let's step back for a moment, before we charge forward, and state the limits of our investigation. We're not discussing jackline and energy absorption. I've covered that before and common experience indicates that a jackline will absorb enough energy to safely slow the sailor. I am discussing only the case of a sailor clipped to a rigid point.

While the Screamer/webbing combination is a huge safety improvement over the typical non-elastic tether, it is really engineered for a different purpose; to catch severe impacts in a survivable way for climbers and workers wearing hip or full body harnesses. Sailors, on the other hand, have a chest harness, which is lousy at distributing force. The screamer is just too hard.


A little backyard testing

I do NOT suggest this as you can easily hurt your spine. I've climbed enough to know what I was getting into.

• Anchor one end of the tether to an immovable object, in this case a large tree wrapped with non-stretch line.
• Create several tethers, 6 feet long from test materials: 1-inch nylon webbing,  1-inch webbing with Yates screamer, 10.4 mm dynamic rope (UIAA single), 8.2 mm dynamic rope (UIAA double).
• With increasing amounts of slack in the line, run at the end of the tether as hard as you can back first.

Results:

1-nylon webbing with Screamer. A very tough stop. With about 3 feet of slack I was able to just get the Screamer to trigger and it hurt. My back will complain for a few days.
10.4 mm rope. Firm stop. I didn't go further than 4 feet, not wanting injury. I'm sure I could have run from 6 feet with no harm, though it would be a jolt.
8.2 mm rope. Even at 6 feet and with a sore back, the stop was practically fun. It might take you off your feet, but was no harder than hitting a well padded couch. At 2-3 feet it was only amusing, with about 1-foot of stretch.
1/4-inch Amsteel. I was smart enough not to try. It would be worse than webbing without a Screamer.

General Observation. I wear my harness high, nearly in the armpits and definitely on my lats. If the harness had been worn lower, where catalogs show them or where combo PFDs fit, I would have injured my ribs.  Whiplash is also a problem. Wearing a harness down near the solar plexus and lower ribs, where so many are located, is plainly dangerous.

In the end, only the 8.2 mm rope felt good, thought the 10.4 mm was livable.


Data and Calculation

Is it strong enough? 8.2 mm line is not 5000-pound rated, but as we will see, it is far tougher (can absorb more energy) than 1-inch webbing.

What do we know from laboratory and real-world fall data? For new products....

• Webbing tethers are just adequate: They occasionally fail at forces near 4000 pounds. Rare, but we can use that as a design point. 
• 1-inch webbing can just manage a 6-foot fall with a 185-pound rigid mass; this is generally considered as equivalent to a 200-pound person, since the harness and body absorb some energy by deforming (data from DMM, a climbing equipment manufacturer).
• 10.4 mm UIAA single rope can manage more than 10 falls from nearly twice this distance.
• 8.2 mm UIAA double rope can manage more than 10 falls from nearly twice this distance with a 242 pound rigid mass (261-pound person) using 2 strands.
• 8.2 mm UIAA double rope can manage more than 10 falls from 6 feet with a 242 pound rigid mass (261-pound person) using 1 strand. Thus, it is more than 50% tougher than 1-inch webbing.
• 8.2 mm UIAA double rope can manage one fall from nearly twice this distance feet with a 242 pound rigid mass (261-pound person) using 1 strand. Again, it is more than 50% tougher than 1-inch webbing.
• 1/4-inch Amsteel is the worst material that could be used. While strong, its energy absorption capability is negligible. Its only advantage over chain or cable is light weight.

I created data using well-used products. Why used? Because they will become so, obviously. Less documentable and less repeatable by others--science is all about reproducible results, be they real-world or not--but ultimately more practical.
 

 Percent Elongation (left) vs. Pounds Stress, pounds (bottom)

Webbing is 3-5 times stiffer than dynamic rope. Amsteel is very stiff.

I took the samples and pull tested them up to 2500 pounds; I didn't feel like destroying my tether just yet. I used on-line data  to extrapolate out to the breaking point of each material. In dynamic situation, the breaking strength and the elongation to break is typically 30% less and energy absorption about 3 times less, but for blog post purposes, the above data at least allows comparison on an equal basis.

What matters is the area under the curve.

Static Energy Absorption, 6-foot tether, ft-pounds (left) vs. Stress, pounds (bottom) 

Static calculated energy values are greater than fall values for 3 reasons:

• Ropes are tested with figure-8 knot, weakening rope by ~ 40%.
• Dynamic testing does not allow for heat dissipation and reorientation of fibers.
• Dynamic ropes survive 8-12 falls, the webbing only 1 fall in our test data. Dyneema survived zero test falls.

Dynamic Energy Absorption, 6-tether, ft-pounds (left) vs. Stress, pounds (bottom) 

 At lower stress, 8.2 mm rope absorbs more energy for a given impact force, but eventually 10.4 mm rope has greater capacity. Both dynamic ropes have greater energy storage capacity than webbing. 8.2 mm rope is more than twice as tough as 1-inch webbing.

Both this data and UIAA drop data thus confirm the following:

• 11mm rope can absorb many fall factor 2 drops.
• 8.2 mm rope can absorb 1 fall factor 2 drop.
• 1" webbing can absorb 1 fall factor 1 drop.

What happens in a 6-foot test fall?

• 1-inch webbing: At 4000 pounds impact, 1100 ft-pounds energy. We know from fall testing, however, that 1-inch webbing can only absorb about 1100 ft-pounds (sometimes passes, sometimes fails), so we will use 1100 ft-pounds as the design point.
• 10.4 mm rope:  If we accept the design point only 1100 ft-pounds of energy, impact force is about 1400 pounds. New rope fall data indicated significantly less.
• 8.2 mm rope: If we accept the design point only 1100 ft-pounds of energy, impact force is about 1100 pounds. New rope fall data indicated significantly less.
• 1/4-inch Amsteel: Only 236 ft-pounds of energy were absorbed by the point 5000 pounds of stress were reached, and only 677 ft-pounds of energy were absorbed at the break strength of 8600 pounds. Amsteel will kill the sailor and pull the harness apart before surviving a 6-foot fall. This has been duplicated in real-world fall testing with spectra products; Dyneema is worse.

If we limit the fall to something more reasonable--a 2-foot fall is similar to wave strike and 6 knot stumble, and more severe than my backyard testing--the energy for me is only165# x 2' = 330 ft-pounds, which can be absorbed by a 500-pound impact; you'll feel it but not get bruises. With a webbing lanyard it would be 3 times that pressure and you will be injured. A Screamer will trigger but not be fully spent (though they are single-use).

How much energy can a Screamer absorb? About 270 foot-pounds according to several testing sources, or about 25% of a 6-foot fall; in combination with the stretch of the webbing, enough to reduce the impact force below 2000 pounds. And then you bottom out and the force goes up, though less so.

Other Factors. Does the extra stretch bother you? Probably personal. Is stepping on rope a greater hazard than webbing? For 8.2 mm rope, not much, but on a monohull (sloping decks) that may be more of an issue. Construction? Either knots (testing assumed figure-8 knots) or sewn eyes are practical.

A critical biometric factor, not easily relayed by a machine or simple data, is that a force that comes on gradually (the dynamic rope) allows the muscles time to tighten and resist. A sudden 500-pound kick before a screamer gives is painful and damaging, like a blindside blow, even at the same force level. You truly feel the difference. 

Dynamic rope or a screamer--The best solution? For smaller, more routine falls on the deck, the rope is
kinder. For that once in 100 sailors' lifetimes, torn from the cockpit by a rogue wave impact, the screamer/webbing combination is competitive, though the climbing rope tethers are still survivable without serious harness impact injury; both could have impact forces of 1000-1500 pounds depending on the force of the wave, and it would hurt, but not break ribs or vertebrate. A climbing rope, even 8.2 mm, has greater energy absorption capacity than a webbing/Screamer combination. Thus few climbers use Screamers, but they ALL use dynamic ropes.

The only disadvantage of rope over webbing is the  potential for rolling under foot. A dynamic webbing would be nice, but I could not find any. On a catamaran this is a limited problem (level decks), and the roll-under-foot potential is reduced by the small size of 8.2 mm line (5/16-inch).

What have I done? I've switched to 8.2 mm dynamic tethers. It is kinder on the back for minor falls and even over the edge. It is lighter than the webbing/Screamer combination and easier to hold on to.

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Where to get limited quantities of dynamic (climbing) rope?
MEC sells both single and half rope off spools, by the foot.
Half Rope,
Single Rope.

Hang Em' High

7-16-2012, rev. 7-31-2024

I always liked Clint....

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Nothing secures a dinghy, specifically a non-RIB inflatable, like lines cris-crossed underneath. No more sway. No more worries about loosing air pressure and strain on the attachment points.

How high does a tender need to be above the water? Surely this depends on the waters sailed, how far it extends behind the boat, and the motion of the boat. It depends on how well secured the tender is to the davits, in case it does take some minor hits.

Often, I see towering, high davits, with tenders swinging far below them. Sometimes it's coastal convenience; they hoist her up when off-shore. Too often, the tackle or attachment bridle were poorly conceived, pointlessly consuming critical space between the tender and the davits. Hoisting bridles waste space and are thus, well... dumb.

In my case, the tender is nestled between the hulls, only extending a bit beyond the sterns. This is typical on cats and makes carrying a tender safely easier. I've also taken steps to shorten the lifting tackle and added a bow spreader bar, such that the tackle is contained almost entirely within the tender, thus consuming no lift space.

An so I can hang her high without towering davits. Triced up, with cris-crossing lines underneath, she snugs up tight in the gap between transoms with no need to remove the engine or hoist her up into the wind. The aft edge is about 4 feet off the water, and top edge just above the deck, convenient for loading.

Very simple.

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The spreader bar is made from 1-inch x 1/8-inch aluminum square tube that clips the tender floor and D-rings on both tubes, and is clipped to a lifting tackle only 8 inches above the tender floor; you can't just clip the floor of a sport boat, because if the tubes go flat the floor comes out. And tricing lines, of course.

Fall Maintance and Halyard OCD. Reeving Splice.

22016, Rev. 7-30-2024

In the spring I enjoyed posting a rant about the 7 sins of spring boat work. However, either not all sailors read this blog and follow my sage advise, or perhaps I simply need to these few items to my  growing list of obvious time wasters.

1. Pulling halyards and replacing with messenger lines. Let's look at the case of a typical 45-foot mast:

Capital. 4 halyards x 90 feet x $1.30/ft = $468. Of course, these will last 8 years anyway, so perhaps hiding them for 5 months will stretching that to 10 years. We'll save $117.00 over 8 years. We'll need messenger lines; I'll give you a discount and assume that most are old halyards, so only $150.00, or a $33.00 loss for our efforts. 

Labor. If we are very efficient, it takes us 1 hour each way for all 4 halyards, or 16 hours over 8 years. -$33.00/16 = -$2.06/hour loss.  If all the messenger lines were free (and I have watched people buy them) you would have earned $7.31/hour. Whoopee. And I bet that won't seem like much if just once in 16 x 4 = 64 cycles, your stitching breaks free and the line goes to the mast head and back down to the base.

Those messengers need stored and to be hauled to home and back. More work.

Perhaps the most obvious flaw in the practice is that the lines need not wear at the same points in the winter that they wear in the summer. They can be shifted by simple means: the roller furling jib will be lowered, the topping lift can be raised 1-foot, and the main and spinnaker halyards can be extended 1-foot to hide the normal wear points; pull them out a little further and tie them to the rail instead of clipping them on. That will take only minutes in the fall and seconds in the spring. With simple practice, I suspect the lanes will fail first at one of their normal wear spots and all of the effort of storing the lines will have been for naught. Good exercise, I suppose.

Perhaps they are high-tech lines on a tricked-out "racing" boat. Though she may have Kevlar sails, she's an over weight monohull and pretending to race is rather like a whispering contest. Real race boats can go 20 knots. They have 2 or 3 hulls or or flat bottoms. One designs race too. Handicap racing, particularly hauling lead and cookware around sheltered waters is plain silliness, in my not altogether humble opinion. I feel the same about bowling and golf too, by the way; really, you either win or you don't. Age and weight classes I get.

On my last boat I got 10 years out of my Kevlar halyards, and they are anything but durable. I simply trimmed them a few feet every few years. 

See also Reeving Eye, below.

2.  Hauling for the winter. My last boat was seriously damaged by improper blocking. Really, how many boats are better supported on stands than by water?

Freeze damage is a lesser risk in boats in the water, since the temperature very seldom dips below freezing; the water keeps them (relatively) warm. Every spring I see a few rudders and keels that have been split by ice while stored on the hard; they've seen temperatures they would never see in the water. Yes, you need to be sure of your through hulls and drains.

Off-season sailing. Anyone who would willingly miss the Chesapeake in October shouldn't call themselves a sailor; it's the finest season. Additionally, the cost-per-sail is half when you sail twice as much, or nearly so. Spring commissioning largely goes away. Engines and electronics prefer regular use.

Expense. About $325 to haul/block/launch. Another $1,000 in storage. A total waste. Yes, hauling for painting is unavoidable, but there are plenty of 2-year paints, and that only takes a week.

Insurance. No, hauling is not required and staying in increases rates only about $50/year (they know yards aren't safe places).  You are typically required to "lay-up" for a few weeks (the specific weeks are designated in my policy), but that can be in the water. The only requirement is that "the boat not be available for immediate use", which is simple to document in the log. Don't be dumb enough to have an accident sailing during the designated week.

3.    Taking everything home, for safe keeping. Thanks, but no thanks. I've got enough stuff at house. My marina is safe--in over 20 years nothing has moved on anyone's boat.

4.   Extra lines, but no chafe protection for those they already have. Would you rather have one good climbing rope or 2 ropes that were pulled up from the ooze under the marina?

5. Tarps. Yes, they can help, if well thought out. I've also seen gelcoat rubbed to bare glass, stanchions pulled in, biminis crushed flat, and stands pulled out (the high-wind domino is always a fright to see). Invariably, they blame the damage on the winter, not the tarp. I'm very judicious in my use of tarps.

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I simply feel better knowing that my toy, my alter ego, my escape, is in a continuous state of readiness, to go where I want and when I want. Any illusion of freedom requires that.

But humans are herd animals; if we see a group, we follow the crowd. 

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Reeving Eye

 If you are going to pull your halyards, imagine the things that can happen if your tape, thread, or paper clip splice comes apart:

• The reeving/mousing line falls to the ground, the halyard goes to the masthead, and you have to climb and get it. Annoying or bad, depending on your skills.
• Both lines fall to the ground. You will have to climb and tread a new line down the mast, not snagging anything or placing the line where it will chafe against wiring and anything sharp. Much worse.
• The reeving line falls to the ground and the halyard collapses into a piling inside the mast, tangling and wrapping around fasteners, wiring, and through-bolts. This can take days to sort out, if you can sort it out. You'll want to sell the boat. If you contract it out, it can cost thousands.

Instead, connect the halyard to the reeving line with a reeving splice. Do it right.

• Remove about 8-10 inches of core. There is no need to taper it, just cut it square. Milk the cover back out until it extends 8-10 inches beyond the core.
  
• Make a very small eye from the cover by burying it back into itself. You don't need a fid, just a length of rigging wire bent into a tightish hairpin. Ideally, the buried cover touches the end of the truncated core, but this is looks not function. Lock stitch to keep it in place. The eyes should be 30-50% line strength, depending on the details. Very strong and no larger than the line itself.
  
• Lash the two lines together with heavy whipping twine.

 As a bonus, the eyes will be there next year AND there is no need to whip or otherwise finish the line ends.

Another possibility is to simply stitch the cover flat against itself. Just as functional.

I'll post a video on sewn eyes as soon as I find it. That's a slightly different thing, but it is often the only way to splice a used line.