Join Date: Oct 2009
Location: Swarthmore, PA
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Re: A couple wisely decides to head back in...
Structurally looser life lines (and jack lines) will be 'stronger' vs. impact and 'side forces' (load applied at ~90° to the long axis of the wire.
Mathematically, with a 'tight' cable or line if you apply a load perpendicularly, the resultant loads 'along' the cable or line begin to approach 'infinity' (dividing by the trigonmetric SINE of the angle .... dividing by 'near zero' winds up being a very LARGE number, or a VERY large applied stress); however the innate elasticity/stretch of the cable or line at max. stretch will relieve those angles (making those angles larger than 'zero' at the two attachment points of the cable or line).
Suffice it to say that your lifelines need to be well over ten times stronger than the max. impact load expected and applied at 90° to the long axis of the wire ... and then you always add a 'safety factor' on top of that.
Obviously you dont want 'sloppy loose' lifelines as that promotes a 'functional' failure.
Rx: dont make your life lines or jack lines 'bar tight'; a little 'slop' in the wire is 'useful' from a 'strength' viewpoint.
Maybe a picture would help. I drew this up a couple years ago. One of the biggest mistakes people can make is trying to relate the breaking tension of lifelines to their own weight. As you point out, it needs to be many many times greater that your own weight:
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1998 Catalina 250WK Take Five (at Anchorage Marina, Essington, on the Delaware River)
1994 Mason 44 Firefly on loan from my BFF (West River, Galesville, MD)
1991 15' Trophy (Lake Wallenpaupack)
1985 14' Phantom (Lake Wallenpaupack)