Jacklines are used along the hoisted edges of sails to make it possible to pull the sail away from the mast or the stay while it is lowered. They are especially useful in reefing mainsails that are secured to the mast with slides or slugs trapped in the spar. As the hardware comes down the mast and stacks up at the stopping point on the track or slot, the jackline permits the sail to be pulled away from the mast and down to the boom where it belongs. Headsails that are secured to a boom also benefit since the distance from the clew to the stem is longer than the distance from the clew to the stay -- when the sail is lowered, it must pull away from the stay if it is to be lowered all the way.

First let me describe the principle behind a jackline. It is a line running from the tack of the sail up the luff a distance roughly equal to the width of the sail at its base. This line is secured to the sail at both ends. At intervals of from 26 to 30 inches along this length, the line is threaded through pairs of round brass thimbles that are sewn to the edge of the sail. Hardware to attach the sail at the intervals is threaded on the line between each thimble pair. When the sail is hoisted, the line is tight and the hardware is pulled tightly against the edge of the sail between the thimble pairs. When the sail is lowered, this hardware falls away from the edge of the sail on the relaxed line. It really is a very simple concept.

Grommets are installed along the sail to secure the ends of the jackline. Two more grommets are installed at each intermediate point. Brass thimbles are sewn to these intermediate grommets as show in the figure below.

Secure one end of the jackline. A splice yields the most attractive and compact attachment. (But John Somerhausen, whose prompting led to this posting, suggests a knot here so the jackline can be tightened when the sail is reefed -- this would be especially helpful if the sail had more than one row of reef points.) Then thread the line through the first interval thimble, then the attachment hardware (the figure above shows a special "jackline snap" that is cast with a single hole -- in the case of a mainsail slug or slide a thimble would be whipped to the attachment harware's bail), then through the second interval thimble. Continue in this fashion to the last interval set. Finally, attach the free end of the line at the final grommet. The jackline should be secured so it is under some tension when the sail luff is pulled to its full length.

That's all there is to it!