Here is an interesting new way to rig and handle the trimming of jib sheets - the lines that pull back on and position the after corner of the jib (the clew). Disclaimer: I learned about this technique from a sailing friend who is happy with this setup on his boat, but I have not yet tested this on my own boat. Nor have I seen this method reported elsewhere in rigging or sailboat manuals. I would be interested to hear from other sailors who have tried this.
The basic idea is to attach a secondary jib sheet (S) to the sail and lead it down to a secondary block that is forward of the primary (P) jib sheet. While the primary sheet pulls the jib clew more aft (tightening the foot of the sail), the secondary sheet pulls the clew more downward (tightening the leech of the sail). Together they allow positioning the clew exactly where you want it for best sail shape without having to move the sheet block forward or aft.
How It Works
The standard setup for jibsheets uses a single sheet on each side that, led to a block at the appropriate place on deck, pulls the jib clew both down and back. The tension of both the foot and the leech of the sail results from a single sheet pulling in essentially two directions: down and back. The sheet block has to be in just the right position to balance the pull in both directions for the best sail shape. Then if you change your point of sail and ease or tighten the sheet to trim the sail, you may also have to adjust the position of the block for the perfect balance of these directional vectors. The same is true if, with a furling jib, you furl the jib in or out, changing the size and position of the sail's foot and leach.
In this system of double jib sheets, however, the primary sheet pulls mostly back and the secondary sheet pulls mostly down, so that you can make any needed adjustments to trim the jib without having to move the sheet block, as in the standard setup.
How to Rig
Positioning the blocks for the primary and secondary sheets depends on the boat and sail and may require some experimentation. The block for the secondary sheet should be far enough aft that this sheet would never pull the clew forward when the sail is fully unfurled. (In the sketch above, the secondary block would be too far forward for a jib that as usual sweeps further back. This sketch is simplified just to illustrate the general idea.)
The primary sheet block would likely stay where it already is on the boat if fixed, or moved to its most aft position if mounted on a track.
In use, you would generally trim the jib first using the primary sheet to get the desired tension on the foot of the sail for best sail shape. Then the secondary sheet can be tightened as needed to flatten the leech.
On most boats, secondary sheet winches will also be needed, an added expense that may prohibit experimenting with this setup.
Advantages of Double Jibsheets
The primary rationale for double jib sheets is that the jib can be shaped just as perfectly as desired - for any point of sail, any degree of jib furling - without having to move the position of the sheet block car as in the traditional setup.
This has two advantages. First, you don't have to leave the cockpit (as is needed on many boats) to move the jib block car on a track along the deck. That is easier for solo sailors and perhaps safer in some conditions.
Second, you avoid the difficulty of moving the jib block car when there is significant sheet tension on the block. Often because of enormous pressure on the sheet it is impossible to move the car at all with the jib pulling. You either have to let the sheet out and the sail flog while you adjust the car, or guess the best position when on the other tack and experiment back and forth. With a secondary sheet, however, you can change the position of the clew and thus the sail shape at any time without these difficulties.
Obviously the double sheet setup requires more hardware and for most boats a second set of winches. The sailor who designed this system thought of it when installing a new pair of larger jib sheet winches on his boat: why not just keep the smaller original winches too instead of replacing them? So he mounted the new primary winches farther aft and used the originals for the secondary sheets.
The other downside is the need for care when tacking or gybing. The secondary sheet is loosened and released first, and the tack is performed as usual with the primary jib sheets. Then the secondary is tightened as needed. There is always some risk of the lines tangling during the tack, but this is minimal when the tack is well executed.