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Old 23-06-2009, 15:57   #1
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Jacklines

I'm looking to put jacklines on for an upcoming ocean leg (just a Chesapeake bay sailor with this boat so far). I saw some debate elsewhere about the merits/demerits of webbing vs steel cable, primarily centered on stretch.

Looking online I see the stretch characteristics for webbing are 15 - 22% at 80% load. This seems like an awful lot to me, so now the question:

Why not use low stretch rope, e.g. halyard line? Granted its not flat, but low stretch, easier to work with than cable, etc.

Thoughts?
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Old 23-06-2009, 16:21   #2
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Use dacron webbing not nylon.
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Old 23-06-2009, 16:22   #3
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Line is round.
If you step on it your foot will take off and you fall. Hopefully not over the side or on a rib cracker. If you step on webbing your foot doesn't take off.
Run the the webbing close to the center of the boat and tighten it up as tight as you can. Then it will not stretch as much.
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Old 23-06-2009, 16:40   #4
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"Granted its not flat," And that's the be-all and the end-all of the story. Step on a tubular (flat) webbing and nothing much happens. Step on any kind of wet round line or cable, and you're in motion, and you just can't afford to let that happen.

Good tubular webbing is plenty strong enough. If you presoak it in water for ten minutes before you install it, you'll get most of the stretch out of it when you install it. It stretches when wet, tightens up a bit as it dries.
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Old 23-06-2009, 16:53   #5
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I use dacron lines run up both sides of the boat. Since I only rig them when heading offshore and use one everytime I leave the cockpit there's not much chance of it causing me to go over. I always use redundant terminations on the lines. I cleat the fore ends on two cleats, on the aft ends I double bolen to the davits then cleat the tails.
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Old 23-06-2009, 17:14   #6
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A jackline saved my life so I gotta pipe up about this. The job (in my opinion) of a jack line is to keep you on the boat, not just attached to it, Your concern about stretch is a good one but rope is not the way to go. I think the best set up is to have a double tether (at your harness) which allows you to click onto aother line while still attached to the boat. This gives you more options for jackline layout and shorter pieces of webbing which will stretch less. I like two on each side of the cabin top, then a short piece for the narrow foredeck.
I don't know if I am describing this right, but my big point is- when clicked into your jackline you should be able to take a flying leap off the boat and only get yanked back at the gunnels. Many people die or get injured when the jackline is too loose and they are just pulled along in the water.
I practice this idea, and a few years ago at 3 am in the Atlantic, I was flung backwards by a bad wave and watched that tether go tight just as my butt hit the lifeline, whew!


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Old 23-06-2009, 18:12   #7
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All of the ideas listed above are valid ideas and shoud be carefully considered. As a single handler my biggest fear was to fall overboard and watch my boat sail over the horizan.

The goal is to stay on the boat.

Webbing is best, anything else will roll under your foot.

When going forward at night/in a heavy seas, consider doubling the tether around the jackline and crawling up the pitching deck. ALWAYS, go forward on the high side of the boat and use two tethers to cross over to the problem if on the low side.

I once had a jib sheet wrap around my neck and jerk me to the end of the tether, while forward of the mast, but that's another story.

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Old 23-06-2009, 18:45   #8
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A cable jackline, with no stretch, must be carefully engineered.

Beware the tight-rope effect. I once broke a 5000# cable assembly with my hands, demonstrating the stress involved. It was not that hard; one hard jerk. The sailor was mad... and then relieved.

You need to have some give in the system somewhere to absorb impact and to reduce peak loads.

Also consider the length of the tethers. They should keep you on the boat, not catch you when you fall off. Additionally, being dragged in the prop zone would really suck.

I use 1/2" line, but I have reasons that may not matter for you:
* It is a cat and I run the jacklines above the deck, from the top of the hard-top to the front beam. I actually use them as railings/handlines
* Line stands sun better than any webbing. I single-hand a lot and leave them rigged.
* I have enough beam that some stretch is fine.
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Old 24-06-2009, 01:50   #9
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Originally Posted by thinwater View Post
Beware the tight-rope effect. I once broke a 5000# cable assembly with my hands, demonstrating the stress involved. It was not that hard; one hard jerk...
Could you explain please?
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Old 24-06-2009, 07:26   #10
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Simple engineerig statics.

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Could you explain please?
It is like a lever.

Straight line - load is 100%.

45 degree bridle, or 2 anchors set open hawse - each leg is STILL 100% because of the re-direction.

In a tight rope situation, if the ratio of the length of the line to the deflection at breaking is 20:1, then the stress in the line will about 10x the load, and it is not hard to create 500 pound jerk force.

In actuality, the steel line and fittings will bend a little and stretch permanently, making this failure less likely. However, that leaves the line slack, and this owner had re-tensioned it several times. After all, it didn't seem to have taken any hard falls. Thus, it was primed for failure.

Any jackline system has to allow for several feet of side stretch in order to avoid massive stress multipliers. It isn't the force of the fall that rips out jackline attachments - it is the stress multiplier.

That said, the shorter the fall the better and the lower the forces, so it is a matter of compromise. As an engineer, it is only steel cable that makes me very nervous, unless carefuly thought-out. Larger backing plates and 10,000 pound design, perhaps. Then it would be very good and quite perminant.
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Old 24-06-2009, 09:52   #11
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It is like a lever.

Straight line - load is 100%.

45 degree bridle, or 2 anchors set open hawse - each leg is STILL 100% because of the re-direction...

... It isn't the force of the fall that rips out jackline attachments - it is the stress multiplier...

... As an engineer, it is only steel cable that makes me very nervous, unless carefuly thought-out. Larger backing plates and 10,000 pound design, perhaps. Then it would be very good and quite permanent.
Thanks Thinwater.

It’s like a lever or a hoisting sling; where a 45 degree sling angle (bridle of equal legs) loads each leg to about 0.707 x the load, and the total load (force) is 1.414 (2 x 0.707) times the weight.
A tighter jackline, that only allowed a 30 degree sling angle, doubles the total loading (100% on each equal leg).
A slacker jackline, that allows a 60 degree load angle multiplies the force by only 1.154 x the load.

Of course, these multipliers only apply to static loads, not dynamic moving forces , where the slacker jackline allows more acceleration, resulting in a greater force on the jackline (when falling crew brought up short).

Someone more knowlegable than I will have to compute the ideal jackline tension (& therefore angle).

It would require a 3.5" wide single-ply, or 2" wide two-ply Nylon sling (Eye & Eye) to achieve roughly 10,000 lifting capacity. A typical 1" sling (1-ply) as commonly used for jacklines, has a basket capacity of about 3,000 Lbs.

Nylon Web Slings lose fifty to sixty percent of their strength, after 36 months of continuous exposure to sunlight.

Goto ➥ The Crosby Group 2009 General Catalog
Then select “Rigging Information”, then select “Sling Angle Information”
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Old 24-06-2009, 17:52   #12
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I like the load diagrams in the GordMays post above.
But as Thinwater pointed out a few posts before, it’s the Shock Load that kills.
It is for this reason I prefer webbing.

Rock climbing years ago I fell over 30 feet before the line snagged me and then stretched. I was doing about 30 mph when the line started stretching. The “impact” wasn’t bad.
If it had been wire I would have broken something or been extruded through the harness.

If I been stopped in 6” by a wire falling at 30 mph the force on the line would have been 2000 lbs.
Can anyone guess (or calculate) how much I weigh. Or the G force.

The point is falling on a boat the maximum stress you will likely put on your tether line is no more than 2 times your body weight. This is a vertical fall of 6 feet.

If we use a nylon jackline that over 25 feet deflects or “bends” 1 foot under a light load and deflects or “bends” 3 feet when you fall at 2 times your body weight the force on the jackline is 20 times your body weight.
If you weigh 200 lbs. the maximum load is 4000 lbs. Something to think about.
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Old 24-06-2009, 19:24   #13
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Thanks for a good discussion and getting my thinking straight on this.

I agree that some stretch is needed, but was considering stem to stern lines on each side (in addition to the much shorter permanent jacklines in the cockpit) and so was concerned about so much stretch, as I agree that the point is to not go over rather than be dragged along behind. I will go with webbing, but the point made about short runs hit home, so they will not be in one long run, but in multiple shorter runs (with double lanyards to always be clipped in).
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Old 25-06-2009, 03:16   #14
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... I will go with webbing, but the point made about short runs hit home, so they will not be in one long run, but in multiple shorter runs (with double lanyards to always be clipped in).
I suggest that you consider having full length jacklines fabricated, and shackle them to the toe rail to create the multiple shorter runs.
This would allow you to unshackle them, if it turns out you find you don’t like the shorter runs, in use.
This will also be cheaper, since a major portion of the cost is sewing the "eyes" (more expensive than a shackle, which is a "spare" until you decide to use it).

Since nylon loses fifty to sixty percent of it's strength after 36 months of continuous exposure to sunlight, jacklines cannot be fitted and forgotten.
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Old 25-06-2009, 05:50   #15
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The two lines I run, run over the dodger, near the mast, and down to bow. My harnesses are on a short tether that just let me reach the shrouds. Done in 5/8 polyester doublebraid they are very stout and don't lie on deck. I have 2 rolls of ex heavy polyester webbing 5000lb test onboard that I use in my canvas business. Ill stick with the 5/8. Can't believe people are using nylon. I gave up using it on bimini strap because it was degrading in a matter of a few months to where it wouldn't even hold a bimini up. I wouldn't recommend anyone trusting their life to nylon webbing after any amount of exposure.
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