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Old 22-04-2010, 07:46   #1
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Jacklines - Impact Force Calculations

I've read a lot of posts where sailors argue back and forth the strength requirments for jacklines. I've spent a lot of time falling on ropes as a rock climber, am familiar with OSHA requirments for rigging fall protection systems, and have engineered all sorts of refinery equipment. I thought I might float some figuring out there for the group to consider.

The calculations are thumbnail and don't go beyond high school physics. I'm sure there are flaws and I know there are many assumptions; it's not as though the motions of a boat in a storm are well defined! However, I am confident the concept is correct and that the results are characteristically correct in their predictions.

The central message is to examine jackline systems in terms of energy absorption and not strength. Climbing equipment manufacturers have know this for many years and more and more, OSHA sees this too. There have been too many serious injuries caused by a drop, a strong anchor, and a neck-snapping, gear-breaking stop at the end.

Enjoy.

Sail Delmarva: Sample Calculations for Jackline Stress and Energy Absorption
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Old 22-04-2010, 08:40   #2
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I've spent a lot of time falling on ropes as a rock climber,
Pretty crook rock climber, eh?



So you took up sailing to wreck a few boats?

3 points on the article..I really can't understand the US measurements as I use metric. Pounds really gets me. I think the article was saying 1 inch (25mm) tubular webbing jacklines are OK is one crewman is washed overboards with full wave preasure.

Thats fine.

however, I have never seen anyone with tubular webbing jacklines its all still flat ribbon. (I wanted to make a safety harness from tubular, btw. Much more comfortable)

UV was not underestimated, but what is reality is that most people won't change their jacklines for several years and they are worth didly-squat!

I removed ours form Sea Life and now use them as sail ties. We use shrouds, 1 inch SS rails and the SS lifelines as a last resort. in weather where waves wash the deck no one goes forward at all

if someone wants to give me 30 meters 25mm tubular I will gladly accept!


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Old 22-04-2010, 08:51   #3
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I have heavy Wichard webbing strap jacklines, I believe they are rated far in excess of 6000 lbs. I have 6000 lb test webbing on the cabin top that was custom made by a sail shop. It cost me maybe $25 including the custom fitment and is fully sewn on both ends for a distance of about 8" back. They did a killer job. I plan to replace it every 2 years whether it needs it or not. The Wichards I keep belowdecks when not in use and when I go sailing I rig them. They clip aft and cleat forwards.

About the only place I still need to rig is the cockpit itself.

What do the calculations suggest for both energy absorption and peak force on the padeyes?
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Old 22-04-2010, 09:19   #4
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Originally Posted by MarkJ View Post
Pretty crook rock climber, eh?



So you took up sailing to wreck a few boats?

3 points on the article..I really can't understand the US measurements as I use metric. Pounds really gets me. I think the article was saying 1 inch (25mm) tubular webbing jacklines are OK is one crewman is washed overboards with full wave preasure.

Thats fine.

however, I have never seen anyone with tubular webbing jacklines its all still flat ribbon. (I wanted to make a safety harness from tubular, btw. Much more comfortable)

UV was not underestimated, but what is reality is that most people won't change their jacklines for several years and they are worth didly-squat!

I removed ours form Sea Life and now use them as sail ties. We use shrouds, 1 inch SS rails and the SS lifelines as a last resort. in weather where waves wash the deck no one goes forward at all

if someone wants to give me 30 meters 25mm tubular I will gladly accept!


Mark
The UIAA site is all SI units, so you can find comfort there. US engineers have to be unit bilingal, so we forget.

Yes, it apears that 1-inch (25mm) tubular webbing is OK for one crew member, if the jackline is no longer than 30 feet. The safety margin is limited but reasonable. The standard says 6,000 pounds for sound reasons.

1-inch tubular webbing is only ~ $0.17/foot, so you are talking about only $16.00! Cheap.
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Old 22-04-2010, 09:29   #5
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I have heavy Wichard webbing strap jacklines, I believe they are rated far in excess of 6000 lbs. I have 6000 lb test webbing on the cabin top that was custom made by a sail shop. It cost me maybe $25 including the custom fitment and is fully sewn on both ends for a distance of about 8" back. They did a killer job. I plan to replace it every 2 years whether it needs it or not. The Wichards I keep belowdecks when not in use and when I go sailing I rig them. They clip aft and cleat forwards.

About the only place I still need to rig is the cockpit itself.

What do the calculations suggest for both energy absorption and peak force on the padeyes?
Regarding padeyes, the calculations sugest that you need some distance to slow down. The example showed that ~ 2.5 feet was enough and I believe that 6-foot OSHA tethers provide just about that amount of extenion. Not a coinsidense - that is what they are intended for. If the pad eye is in the cockpit (at the helm) less is needed, because the crewman will not reach 20 ft/sec.

If you watch the rope testing video, I believe they show a shock absorber being pull-tested.

I don't know of any marine version that has this sort of sewn-in absorption capacity. I imagine it is coming. Compared to climbing and construction work where vertical falls are common, the need on sailboats is far less. However, the NEED would be for a crewman that clipped himself to the cabin top pad eye near the mast and was then thrown off; the impact at the end of the tether could be as deadly as the fall. This has been well proven in climbing and construction expereince.
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Old 22-04-2010, 09:36   #6
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I suppose I was talking about the padeyes that hold the jackline, not to directly clip into.

Although it should be noted I use a highly elastic tether designed specifically for this purpose that CAN if necessary be clipped directly to a solid object and in the 2 M it lets you fly, meet the maximum force requirements. Supposedly.
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Old 22-04-2010, 09:54   #7
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I suppose I was talking about the padeyes that hold the jackline, not to directly clip into.

Although it should be noted I use a highly elastic tether designed specifically for this purpose that CAN if necessary be clipped directly to a solid object and in the 2 M it lets you fly, meet the maximum force requirements. Supposedly.
The padeyes at the ends of the jacklines need to be rated for the same force as the jacklines; 6000 pounds. In general, a normal pad eye is not designed for this sort of force in shear; the Whichard catalog shows a good design. It would not be normal to add shock absorption there; that is what the jackline is for.

The OSHA-style tethers are NOT elastic. That is a widely held misconception. They have a section that is bar tacked in a folded patern that begins to extend when the trigger force (550-900 pounds) is reached. Watch the UIAA rope test video. This method had been proven in the field over 20 years and it does work dependably, though not as smoothly as a slightly elastic jackline.
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Old 22-04-2010, 10:02   #8
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Right-o. I actually have the Wichard elastic jackline tethers, btw. Most of my jackline gear and padeyes are Wichard. Good to get confirmation they make a solid design.
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Old 22-04-2010, 10:25   #9
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Right-o. I actually have the Wichard elastic jackline tethers, btw. Most of my jackline gear and padeyes are Wichard. Good to get confirmation they make a solid design.
Do remmember that the elastic tethers do not absorb energy; they simply keep things neat.

BTW, the Whichard jacklines are 6,000-pound rated. It is odd that the jacklines and tethers get the same rating, when the stress on the jacklines is many times greater. But climbing harnesses have always been rated for FAR greater load than a rope could ever deliver. There are some durability and robustness issues too.
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Old 22-04-2010, 10:31   #10
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Good point about the neatness versus energy absorption. Although I remember at one point reading that they were - again supposedly - able to be clipped to a solid point. I wonder if that's just because of their length then? Interesting thoughts about it. I certainly need to clip to solid points periodically - it makes much more sense to clip to a ring on the mast with the 1m short tether when working the mainsail than it does to stay tethered to the deck a half meter below and a quarter meter behind me with a 2 meter tether. The length of the tether most likely prevents me from reaching that 20ft/sec you were talking about, in < 1m of travel distance. I suppose I mightn't want to clip to a solid point with the 2m tether.
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Old 22-04-2010, 10:33   #11
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I would try to avoid using tubular webbing opting for a more dynamic rope instead, something in the 10-10.5mm range would be fine. Imagine tying a 170lb sack of bricks and dropping it 3 feet above it's anchor point for a total fall of 6ft. Tubular webbing has no stretch and can seriously injure you. Webbing is only good for holding things not catching falls. As far as force absorption climbers use these things called screamers that are simply a bar tacked nylong sling that expands once 500lbs of force is put on them. Yates Ice Screamers - Mountain Tools They are used to save their anchor points from ripping out during a fall not the rope itself which is not really needed on a sailboat.

Any time your dealing with falling human beings and ropes you want to think dynamic. Our bodies are too fragile and can be seriously injured by even 3ft falls onto static lines. Go dynamic

Here's a video of Dean Potter taking a huge whipper onto a dynamic rope. Dude's got cajones the size of watermelons. He'd be dead if that fall was on a static rope or tubular webbing.
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Old 22-04-2010, 10:40   #12
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The reason you use tubular webbing is another safety issue - it doesn't roll or slip when you step on it on a wet deck.

I'll take that advantage which I use every time I go forwards. Not to mention the load on a jackline is sideways to its tension direction (which, yes, translates into elongation stress, but in a way that maximizes energy absorption in the material as well as providing a much greater distance for deceleration). Its not pure "falling shock".

Flat tubular or flat woven webbing for me, anytime, over round anything.
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Old 22-04-2010, 10:48   #13
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slipping on ropes sucks. I agree that webbing has it's purpose on a boat if the falls are short and not a free falling shock. I happend to use rope because I have old climbing ropes laying around. Just thought I share my knowledge next time anybodys fooling around with other situations we're they're using ropes or webbing and there is the potential for a fall.
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Old 22-04-2010, 10:54   #14
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I would try to avoid using tubular webbing opting for a more dynamic rope instead, something in the 10-10.5mm range would be fine. Imagine tying a 170lb sack of bricks and dropping it 3 feet above it's anchor point for a total fall of 6ft. Tubular webbing has no stretch and can seriously injure you. Webbing is only good for holding things not catching falls. As far as force absorption climbers use these things called screamers that are simply a bar tacked nylong sling that expands once 500lbs of force is put on them. Yates Ice Screamers - Mountain Tools They are used to save their anchor points from ripping out during a fall not the rope itself which is not really needed on a sailboat.

Any time your dealing with falling human beings and ropes you want to think dynamic. Our bodies are too fragile and can be seriously injured by even 3ft falls onto static lines. Go dynamic

Here's a video of Dean Potter taking a huge whipper onto a dynamic rope. Dude's got cajones the size of watermelons. He'd be dead if that fall was on a static rope or tubular webbing.
You will also note that Dean uses tubular webbing for slacklining, because rope is too elastic. A dynamic rope jackline would give a softer catch, but you would land in the water as it stretched 4-6 feet to one side. It also rolls underfoot.

I think the sailing gear folks got the webbing part right. Like the 3 little bears: cable is too stiff, nylon rope is too soft, webbing is just right.
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Old 22-04-2010, 11:24   #15
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My understanding is that nylon tubing and webbing loses strength when wet. Anybody know by how much? I thought it was on the order of 20%. I don't think I saw that the calculations.

The biggest problem I see for high latitude sailors is that any synthetic jackline runs the risk of getting melted if pulled against the stack for the cabin heater, a real risk on my boat. I'm seriously thinking of converting to SS cable for this reason. (For my deck jackline only. I run two jacklines; one to the bow stem, and one around the mast for reefing the main. The jackline to the mast will not contact the stack, I think. The deck jackline will hit the stack for sure if I fall the wrong way. Just recently discovered this potential problem.)

I've learned the hard way that turning off the heater on really cold days (or more likely nights) turns the deck into an ice skating rink. So sometimes it's necessary from a safety perspective to keep the heater going while sailing. This in turn compounds problems for jacklines not normally contemplated by more temperate weather sailors.

In the past, I have avoided the problem of SS cable rolling under foot by sheathing it in nylon tubing (this was on a previous boat which I sailed out of Alaska).
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