Rationale: (Al's note: This was written in 1981 and still has validity but on SHADES, we now have a further modification of this system that will be explained under "Forestay Tackle" below.)
    The forestay serves no structural function once your jib is up and properly tensioned. At that point it becomes slack and does nothing but interfere with your jib tickers (wind indicators). What you could use, however, is a straight-line reference from bow plate to the hounds (where forestay is attached) in order to check how far the luff of your jib is sagging off to leeward while you are close-hauled. To accomplish this, we replace the bottom 18" of stay by a shock cord loop rigged in such a way as to take up slack as the jib luff is tensioned. This keeps the forestay wire under some tension (and fairly straight) and out of your jib at all times. As a back-up to the shock cord we use a short length of thin line (3 mil.) which we tie between the end of the wire and the shackle (or cotter pin) at deck level.
Extraneous note: The photo below shows one other item that sort of has to do with the forestay - installation of a spinnaker sheet catcher to help prevent the sheet from going under the boat. Ours is simply thin stainless steel wire the ends of which are wrapped around the two front screws under our forestay plate. You'll note that it gets a lot of abuse:
.....

Installation: The shortening of the wire part of your stay must be done with good wire cutters and a swaging tool. Before doing this, you take three feet of ¼" shock cord, and make a loop out of it by overlapping the two ends about 1" and putting a seizing around the overlap with whipping line. Then hoist your jib to maximum tension (for a blow), attach the loop via shackle or cotter pin at the bow while detaching the forestay there. Keeping the seized part of loop away from bends, stretch the loop parallel to the jib luff until the cord begins to stretch. At the same time pull the stay tight parallel to jib luff. Mark (with tape) the point on the stay that is reached by the top of the loop. Cut the wire there and swage in a loop with thimble. Use a small shackle to connect wire loop to shock cord. Tie one end of the back-up line (with a bowline) around cotter pin or through shackle at deck and the other with a round turn and several half hitches through wire loop.
Note: I discovered that forestay wire is very stiff and hard to swage and have replaced my forestay with wire cannibalized from an old main halyard which is much easier to work with!

Rationale:  The jib halyard/magic box combination is easily used to put up to 400+ lbs. of tension on the rig.  Such pressure translates into mast compression which in turn becomes pre-bend when you push forward on the mast about 1 to 2 metres above deck level.
    The problem with using the jib halyard to get the required tension is that pre-bend is only needed in light airs when such tension removes all jib luff sag for which all jibs must be designed.  This in turn makes for an overly flat luff entry which makes the steering groove unacceptably narrow and steering consistently within the groove impossible.
    What we therefore do is crank up the tension with the halyard and then take up any slack in the forestay with the aforementioned tackle. Once the stay tackle is well tightened and cleated, it keeps the rig tension required for pre-bend while the halyard can be slackened off to give us any amount of luff sag that feels comfortable.

On SHADES, we have cut enough off the bottom of our forestay to accommodate a 30-40 cm. tackle which consists of

• about 1.5 metres of 4 mm. pre-stretch line
• this is tied to the becket of an RWO #R4959 (V-jamming block with becket intended for windsurfing use) attached to the bow plate where the forestay used to be attached
• from the becket, the line goes up about 30 cm. to an RWO micro-block (#R1530) which has been swaged into the new loop at the lower end of the forestay
• the line then leads down around the sheave of the R4959 before exiting through the V-jammer which we have facing aft

The amount of pre-bend you get depends on

• how far aft your spreaders are angled
• the amount of rig tension applied
• the inherent flexibility of your mast section

NOTE: Abbott masts are harder to pre-bend but word from the Fanshawe Fleet is that using a minimum diameter (¼”) mast pivot pin facilitates such bend.