Hi Phil,
1 You don't want to tie off a jib halyard that is attached to the top of the jib to anything that is not between the top of the lower furling drum and the bottom eye of the forestay.
2 It's difficult to work out from your Chat entry what's left from your old wire jib halyard - I assume you have a flexy wire halyard with eyes at each end, and you still have the twisted shackle and metal block that are attached to the forestay shackle at the bottom of the upper furler swivel.
http://www.sprint15.com/manuals/parts_list/section9.jpg shows the relevant parts and configurations. If you don't have even a double-eyed wire halyard, I'd make up an "interim" replacement out of something like 2mm dyneema (low stretch high breaking strain) rope - it's got to be just thin enough to go through the metal block...... Get enough to allow for tying off at each end. If you're in doubt about this, I watched multi-National Champion George Carter do just that at a Nationals to replace a wire halyard that was clearly not going to last out, and it didn't slow him down in subsequent races..... You'll still need about the same length of almost any old string to tie onto the free end (not the one that gets shackled to the top eye of the jib luff) for use only when lowering the jib, tying off when not in use, raising the jib when needed, and untying completely when the jib is hoisted and in use (I use the spare length available from my furling line).
3 When you hoist the jib, use a short length of thin rope to tie the eye of your wire halyard end to the lower forestay eye or to a shackle you put through it (eg to replace the missing snap-link shackle). If you've replaced the wire halyard with dyneema or the ilk, tie it directly.
TIP from BRIAN PHIPPS is to have the jib quite low on the forestay, to open the gap between the top of the jib and the mast/mainsail - otherwise the airflow through the slot can be adversely affected. At this stage the bottom of the jib is still loose.
4 Then use the tip from Rob Bowen, but ultimately from Brian Phipps, of getting the cat head to wind and applying as much mainsheet tension to the (already hoisted) mainsail as you think you'll use during day's sailing - err slightly heavy on tension if you think the wind is going to get stronger on the water. Use another short length of thin rope to tie the eye at the foot of the jib luff to the top of the lower furling drum, or the shackle there, or whatever, tensioning it before you tie it off so that the jib luff is tight, but not tighter than the forestay. You'll end up with quite a lot of string in this area.
5 If you always "feel" the rig tension need to stretch the forestay and jib luff to the point at which it was set, you'll eventually get a feel for the tension needed, and probably be able to dispense with the main-sheet tensioning part.
6 If you've used string to tie just about everything off, don't be tempted to mark it until it's been used a few times so slack anywhere in the "system" has been worked out. But you could then mark the string for different wind settings.....
7 The above is based on "what will get you going" on the few occasions when you use the jib. It may not be "fully class legal".
8 For what it's worth, my tip is "always sail with no more rig tension (rig NOT jib luff tension) than allows you to push the mast through 90 degrees with just single finger pressure". If you do this, then when you sail "downwind" the jib will belly out nicely given the relatively low apparent wind, enabling you to sail at a slightly better downwind angle, but still at speed.
In another Chat thread there have been discussions about shroud length, which inevitably went into mast rake and rig tension. The advice from Mark Aldridge applies equally to sailing with the jib :-
"Just set it about right and then practice.
When you've practiced once, practice again and again and again.
...and keep doing it.
This will reap more benefit that worrying about the rig!
Mark Aldridge"
I'd just add : when you do get good at sailing your Sprint 15, you'll be able to experiment with small variations in rig tension and mast rake to see if you can fine-tune the aggregate of your acquired boat handling skills, body weight and the boat set-up to further advantage...
