You may or may not know this already, but the strongest knot is actually no knot at all! As soon as you add a knot to a rope you reduce that rope’s strength. However, there are knots that are stronger than others, so a better question might be, which knot weakens the rope strength the least?
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Which Knot Is Strongest?
There are thousands of different types of knots, so I’ll try to sum up different applications
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The Figure 8 Knot On a Bight is an extremely strong knot, averaging a break at around 9.81 kN (2205 lbs force) with 7mm Edelweiss Cord in one series of tests, compared with the Bowline, the Reever Loop, the Alpine Butterfly Loop, the Zepplin Loop, and the Zepplin Loop Whip. You can see the video results for this particular set of data from hownot2.com here. It’s no wonder that climbers everywhere use the figure 8 knot when tying their harness into a climbing line.
The Double Fisherman’s Knot was the strongest bend in this test case, which never broke in these tests, rather the rope broke at around 19.15 kN (4305 lbs force), which was the strongest when compared to the Flemish, Reever, Sheet, Alpine Butterfly, and Zeppelin Bends.
Why is there such a difference in breaking strength between knots and bends? Well it all depends on where the force of the rope is applied. Some knots bring the tension to the knot itself, while some bring tension to the rope--it all depends on how the rope is used.
Much of this data comes from hownot2.com--they have done thousands of break tests with hundreds of different types of gear, rope, knots, etc.
The Bimini twist is an impressive fishing knot that uses wrapping around the line so as to spread out tension. The Bimini twist is touted to have 100% line strength and is confirmed in these tests here:
As mentioned before a knot can’t increase the strength of a line. One way to avoid making a knot is to use a seizing which is essentially sewing thread to make a loop. In these tests, here seizings were made with 68% efficiency (3577.6 lbs force out of 5200 lbs line test).
How Strong Of Rope Do I Need?
So because we know that knots can decrease the strength of your rope, you can make sure that you have strong enough rope by doing a couple calculations:
Assume the starting rope strength (new) is about 80% of its manufacturer-rated breaking strength. For example, 2000 lbs manufacturer’s minimum break strength is 1600 lbs. Next, assume the least efficient knots by taking only 40% of the rope strength after that. Bringing a 2000 lb rope strength down to 640 lbs, which is about 30% of the original 2000 lb rating.
If your life depends on a rope, then you want to make sure the weight you put on it is max 30% of the line’s manufacturer’s minimum breaking strength.
What about shock load?
Shock load refers to the temporary stress a rope goes through when you fall or swing where the stress rises for an instant and then goes down. Many ropes actually have a maximum number of falls they are rated to since these shock loads put a lot of stress on the rope.
For Putting Up a Tire Swing
Shoot for a rope minimum break strength of 1500 lbs--this means you will have 450 lbs of rope strength including knots. This should accommodate most people and the shock load associated with swinging.
Or, you can tie multiple ropes to increase the strength.
For each rope you add to the tire swing you can add that number to the max strength of the swing. For example if you have a rope with a minimum breaking strength of 1000, then 30% of 1000 is 300 lbs. But, if you have 4 ropes tied to the tire swing where the weight is distributed evenly among the ropes, you really have 1200 lbs of weight you can put on the swing.
For Putting Up Windchimes
You don’t need anything super strong for windchimes but you don’t want your windchimes to clatter to the ground.
If you want to go overkill you can use 3/16 line from Home Depot here. This is 90 lb line, but if you have some knots in the line and if the wind really kicks up, your rope will handle 30 lbs easily.
If your wind chimes are smaller and in the 3-10 lb region, you really would be totally fine with this 1/8 inch line from Home Depot.
Make sure you get a line that is weather resistant. Cotton ropes will eventually lose their strength in the sun and getting wet and drying again hundreds of times. Polypropylene can lose its strength in the sun but it is resistant to water.