Soft shackles are a very handy tool on a sailboat; they can securely attach something and hold incredible loads while weighing next to nothing and posing no risk of damaging the boat. Best of all, they require no tools to install or remove!
A soft shackle can be used anywhere a regular shackle would be used. The difference is a regular shackle is made of metal and can scratch your topsides or pound into your deck. A soft shackle is made of dyneema and is very soft and flexible, making it safe to use anywhere that you wouldn't want damaged by a regular shackle.
One of the most important parts of a soft shackle is the bulky knot at the end. This knot will be the weak point in the soft shackle, as stress will accumulate near it and the sharp bends of the knot will further weaken the dyneema. Several different knots have been suggested for this purpose, with the goal of minimizing stresses.
The ideal soft shackle knot is bulky and does not twist under load. To avoid twisting, the tails of the soft shackle need to both share the load of the knot. A symmetrical knot would be ideal in this situation as it will then share the loads evenly between the two tails of the soft shackle. Minimizing the sharpness of the bends of the knot would also help prevent stress points in the knot which would lead to premature failure of the soft shackle.
The Lovers Knot is a good candidate for this purpose, as it is a bulky, symmetrical knot with no very sharp bends.
Once the soft shackle is ready to tie, stretch out the two tails parallel to each other.
Take the top tail and pass it under the lower tail, around the front of the soft shackle, and then under the top tail. This will form a half hitch with the top tail.
Then take the lower tail and pass it over the upper tail, behind the soft shackle, through the loop formed by the first tail, and over the lower leg, forming another half hitch.
You want to end up with two interlocking half hitches with the tails exiting through the same section of the knot.
As you tighten the knot down, be sure to pull evenly on both tails, this will ensure that the knot is balanced and even. Be sure to position the knot very closely to the mobious splice, which will ensure that the tails are not going to move or be at slightly different lengths which could lead to more stress on the overloaded tail.
Once the knot is fully tightened, it will work well to secure the soft shackle and allow the unit hold itself together under incredible loads.