NWslackline guide to longlining (Part 1): Anchor SlingsThis will be the easiest topic for us to cover, because there really isn’t that much to talk about. We need to discuss the different types of anchor slings, and the different ways you can rig them. In a broad sense, the two types of slings are “homemade” and “manufactured”. Homemade slings are anything you have bought as raw material and knotted yourself. I have explained in the past how to make the two basic types (a big loop, and an eye-and-eye sling). If you’re going to make a sling, especially that might be used on a longline, make it a big loop. The benefit of a loop sling is that it’s twice as strong as a single ‘strand’ of webbing. Of course the knot reduces the strength to some extent (figure 40%, to be safe), but the doubling of strength by making a loop compensates for this somewhat. If you fold it in half and use it as a double loop, that’s yet another way to add strength. The knot issue with strength reduction is the main reason I prefer to longline on manufactured slings. The benefit of manufactured slings (the type I will talk about below, anyway) is that they have a safety factor: the working load (“WLL”) is usually 1/6 or 1/7 of the breaking load (“MBS” or “UBS”), so a 5300lb WLL sling is very, very strong. And for what it costs to make a large nylon loop sling and then double it over, you can basically buy a spanset (“polyester roundsling”).
As a general rule, I try to keep the gear that is further out on my rigging as strong as possible. Another way to think about this is to say that the webbing I walk on is the weakest link, and as you head outward in either direction each piece of equipment should get stronger and stronger. The idea is that if anything breaks it will be near the middle. If something broke at the end, you’d have more metal gear flying, potentially causing more harm, than if something broke in the middle. So the webbing might be rated for 4000lbs. The pulleys for 6000. The rope in the pulleys for 7500. The rigging plate for 12,000. The shackle on the rigging plate for 54,000. The sling on which the shackle sits: 60,000. I am skipping some equipment, but hopefully you see the general idea here. Your slings should be STRONG. If you use a homemade sling, use a couple wraps around the anchor (tree, etc) to accomplish this. In general though, for longlining, just invest in some manufactured slings.
The main type of manufactured slings used by highliners and longliners is endless-loop polyester roundslings. In slackline, they are often called “spansets”. Within the rigging industry the color/strength is standardized. I usually use green, although many other people use purple (a little less strong) and a rare few use yellow, blue, gray, tan, or some other color. The length of the sling is the length of the loop, not the material used for the loop. To restate this, a 6 foot spanset would use 12 feet of material. I own several different sizes, but the size I use 90% of the time is the 8 foot spanset. I would recommend buying two 8 foot spansets, and possibly a 4 foot, plus an extra 1/2″ or 3/8″ shackle (or steel carabiner). The two 8 foot spansets will wrap almost any tree you’d encounter, and on the off chance you need to wrap a HUGE tree at one end, you can attach the 4 foot spanset to one of the 8 foot ones using the shackle, giving you a little over 12 feet of total wrap. In my longline bag I carry two 8-footers, a 4-footer, and a spare shackle — this has always been adequate.
|Tree width (ft)||Spanset to use|
Here is a quick chart of the tree width that a given spanset will safely wrap. You can always use larger spansets on smaller trees (an 8′ spanset will obviously wrap a 1′ tree just fine), especially if you just wrap it twice around. As you can see from the chart, most urban and suburban trees (those under 150-years-old) will be covered by an 8ft spanset. With a 4ft extension you could wrap a very large tree, indeed.
There are just a couple rigging methods to cover. If you buy a spanset, it should come with a stitched-on rubber tag that explicitly shows this. There are basically three ways to use a loop to rig, and each affects the strength somehow. The nice thing is that generally we will use the “basket” technique for wrapping a tree or metal pole, which doubles the rated strength of the loop. Bonus! Basically just avoid the “choker” method, and you’re good to go.
The last hazard to mention is that the more of a “tri-load” you place on the anchor sling, the greater the forces. Although your line may have 2000lbs of tension, this tension translates into the sling at an angle, and the more oblique the angle, the greater the force multiplier. The force on each side of the sling is a function of the slackline tension (“F”) divided by the-cosine of half of angle theta (theta being the angle between the two ends of the sling). Confused? See the chart at right. Basically, as a general rule, try to keep to 60 degrees between the two ends of the sling (and avoid going over 90) and you should be plenty safe. This is one reason we like to use spansets — they are so burly that you don’t have to worry much about doing any math while rigging; as long as you make reasonable choices the sling should be 10 or 20 times stronger than any loads you put on it.
Last but not least, please please please pad ANYTHING you attach your slings to. The only anchor I might not pad would be a thick metal pole with a very smooth surface and no corners what-so-ever (I once damaged a sling by wrapping an octagonal pole that had the most rounded corners you can imagine). If you are super broke, you can use thrift store bath towels cut into strips for padding (although if you are super broke, longlining might not be for you haha). If you are anything less than super broke, you might want to consider a Towel Tube (shameless plug for NWslackline’s $5 padding solution).