Jack Lines

To sailors, jacklines (one word) are safety lines running down either side of the deck. We clip to these with tether lines to prevent falling overboard. However, to the lubber--fact the broader meaning of the term--"jack" describes something of utility value, odd jobs, or regular. In old English it could refer to one of the peasantry, a common seaman, or anything common; jack stays, jack of all trades, regular jack, lazy jacks, jack s__t.... 

Jack line (two words), from Websters Dictionary:

jack line, noun.

1:  a small rope or line

2:  a rod or steel cable connecting a central pumping engine with each of two or more oil wells which it powers 

 I keep 2 of these utility (Websters definition 1) jack lines in the cockpit pockets at all times. They are about 30' x 3/8" with a wire gate climbing carabiner on the end, and serve many purposes, both routine and contingency.

 
• Jibe Preventer. Clipped to the boom-end mainsheet bail, led forward under the midships cleat, and back to a winch, it creates an adjustable preventer that can be instantly released. 
• Boom Downhaul. Because of the beam of catamaran, the mid-ships cleat is sufficiently far forward for a preventer. From this location it also pulls the boom down, making for a useful vang substitute when the traveler is all the way out and the mainsheet is eased.
• Twing line. When broad reaching it is often advantageous to haul the spinnaker or genoa sheet down.
• Clearing overrides. Place an accender (also kept in a cockpit pocket) to the sheet, clip the jackline to the ascender, and lead to a spare winch. faster than a gripping hitch and good up to about 1500 pounds.
• Recovering fouled anchor. If the anchor is fouled on drift wood or trash, often getting it clear is as simple as hanging the anchor upside down. Raise the anchor as far as possible with the windlass, clip the roll bar and secure to a bow cleat, and slack the chain. the junk should slide off. 
• Jury rigs. Not to long ago one of the sling hold the dingy parted while under spinnaker (had been lazy and had not triced up the dingy, a mistake I will not make again). To get the dingy out of the water fast I simply clipped the lifting eye on the dingy, pass the jackline through the eyes on the davits, and took the jackline to a winch. The dingy was up in less than a minute. I immediately triced it up!

Spinnaker sheets can also be used this way in many cases, having the advantage of being pre-rigged through a turning block to a winch. Of course, if the chute is up that's not much of an option. But there are applications:

• MOB (man overboard) recovery. Throw the Lifesling to the swimmer and pull him near the boat. Clip the spinnaker sheet to the Lifesling, up to a snatch block on the boom bail (we keep rescue blocks in the cockpit pockets), and winch away.
• Anchor recovery. Unlike the jack lines, these are long enough to reach the bottom and back to a winch. Lead the line over the roller, clip either the roll bar or the tripping eye and winch.