Join Date: Nov 2007
Location: SF Bay area
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The last couple of posts are reinforcing my thinking about tension.
Regarding breaking strength of soft shackles, while you are using a knot, the knot is made up of two lines loaded in one direction, and then hooked onto a loop, essentially made up of two lines loaded in the other direction. So on the side of the shackle that has the knot/loop hookup, you've got double the strength. So I would say that that side of the shackle has a breaking strength approximately equal to the strength of the underlying rope (doubled because you've got two lines equally sharing the load, half because of the knot).
The other side of the shackle is stronger, because no knot, but who cares, weakest link and all. So the breaking strength on either side of the shackle is the strength of the underlying rope. But now the shackle, when closed, is again folded in half, like a line going over a sheave. The load, hanging from the bottom of the shackle, is supported equally by both sides of the shackle, so we again get to double.
So it seems, more conservatively (taking the 50% hit from the knot into consideration), the breaking strength of a soft shackle is twice the breaking strength of the underlying rope.
s/v Laelia - 1978 Pearson 365 ketch