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post #28 of Old 08-19-2008
Telstar 28
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If you did a bit of research, you'd realize that the breaking strength of tethers and jacklines needs to be far higher than what you might expect. The momentary shock loads on a jackline can be very, very high. A person falling across the boat and getting stopped by the tether may generate upwards of 20 G's of force on the tether. If they weigh 180 lbs., which isn't all that heavy, that means the shock loading is 3600+ lbs. Then add in the mechanical advantage caused by pulling at a near-perpendicular angle to the jackline... and you've got some really serious forces involved.

Also, on a typical small sailboat, where the halyards are 3/8" polyester double braid, the breaking load of the line is about 4400 lbs. If tying a bowline only has at best 60% of the breaking strength of the line, you're now looking at 2600 lbs... Which isn't much more than 10:1 for a lot of the sailors I know—and that assumes the line is in perfect condition, without chafe or UV damage—not generally the case.

Also, consider that the shock loading caused by falling may be an order of magnitude more than what you actually weigh, the safety margins aren't all that large.

BTW, this is one reason I upgraded my halyards to spectra cored lines of the same get a greater safety margin even with some wear and tear on the lines.

Originally Posted by montenido View Post
Alright, great thread, but I have to weigh in also. Yes, I am fairly new to sailing, but I am not new to using webbing, ropes and carabiners in lifesaving conditions. I also bought a pair of 40' jacklines on ebay. Very well stitched with good quality hardware and top-quality webbing. People, these materials are so strong that they will never feel your weight if/when you fall and are restrained by them. The 1" webbing is rated for well over 1000 lbs breaking strength. The same holds for well-tied knots in quality rope. When you go up the mast using your good condition halyard (tied with a bowline), it is so much stronger than the load that you are applying that the safety factor is probaly more than 10:1.

Check the knots and stitching of any sewn loops in any safety gear, no matter the brand name or the amount of $ spent. That is just prudent.

Keep up the great threads. I am learning so much from you all.


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Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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