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What is the narrowest webbing that can be to make slings using the beer knot? I've used it on 1" webbing, but wondered if it would work on 9/16" webbing.

Would expect you are more limited by your patience in sliding the one end through the other. It's a pain. Can't think of any reason you couldn't use the thinnest tubular webbing you can find.

I'm more curious why you have any need for the knot. It's more of a "trick" not than anything else. Serves no real purpose that the basic ol' water knot can't do just as well...and it's difficult to inspect.

I've used beer knots in 15mm webbing, which is almost indistinguishable from 9/16". But they are a LOT harder to tie than in 1" webbing -- I use a knitting needle or something similar to push the webbing down inside the outer tube. Don't expect to do normal 9/16" beer knots in the field.

I use beer knots for 10' webbing slings that I never plan to untie; the slings get used for emergency harnesses, "comfort straps" to lend a hand, and quick loops over boulders for removable anchors. Beer knots have more strength than water knots. Partly it's a gimmick to keep people from borrowing my webbing loops and removing the knot. Other than that kind of use, they are a PITA. If you are tying rap slings on the spot, just use water knots.

I prefer the beer knot for it's strength (20% reduction) and compactness. The double fisherman's has 20% reduction but is less compact. Compare that to the water knot's 50% strength reduction (!) and you can see why I'm concerned with strength on narrow webbing. Craig Connally (The Mountaineering Handbook) recommends the double fisherman for making slings on-the-climb with narrow webbing. I wasn't sure that the inside diameter of 15mm webbing would be enough to stuff an end in for 10 inches, but good to hear it can be done. I'd rather make my own slings at 1/3 the cost of sewn slings since I'm poor.

I believe the 80% strength value for beer knots is from testing with 1" webbing.