OP, since it sounds like you’re learning as you go here, I’ll give you another pointer.
http://l-36.com/high_strength_soft_shackle.php
This is the soft shackle I prefer to tie. It’s not the easiest to tie, but I think it’s the best for a couple reasons.
One, the knot at the end with the tails tucked back into the legs of the shackle WILL NOT pull out. UHMPE rope is extremely slippery and the simpler knots with tails hanging out can be pulled out if not seated well and monitored.
Two, the tails being tucked in doubles the OD of the shackle below the knot, that the eye cinches around. The eye is the weak point of the shackle, and the larger the bend radius you give it (the OD below the knot), the stronger the eye will be.
(EDIT: Excuse me, tucking the tails doubles the *cross sectional area* below the knot, increasing the *diameter* by 40%, not doubling it. Running low on sleep this week.)
The author over at L-36 mentions that just using bigger line is a better way of managing shackle strength than trying to optimize the design of the knots, but for 3/8”, 7/16”, 1/2” rope, going up a rope size can add significantly to the cost of the shackle (more expensive rope and you need more length), and makes the shackle fatter and harder to feed through small holes.
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