Jett wrote:I notice in one of Roger's previous posts in this thread he chooses to clip the jib onto the forestay when racing, because of better sailshape. Though Greenlake's analysis makes sense about achieving maximum tension with the halyard alone on a jib with an internal wire, I still would think that the best luff shape would be maintained with the "two wire" approach (i.e., jib hanked to forestay and tight jib wire tension) in a stiff breeze.
What's preventing the sag is the tension, resiting sideways pressure. The maximum amount of overall tension has some upper limit, no matter how many wires, external or internal one uses. The reason for that is that the tension needs to be balanced by (swept back) shrouds. Once you increase forestay (+luff) tension too far, the mast will come forward.
On a jib with an internal luff wire, what you end up doing is tightening the halyard to the point where some, or even most, of the total tension is taken up by the luff wire and the forestay becomes slack. In some ways, it simply turns into insurance against sail/halyard failure. (Otherwise the mast can come down on such failure).
If you entirely replace the forestay, the entire tension will be on the luff wire, which will then resist sag just as well as the forestay.
Anyway, that would be my reasoning. There are sails that don't have a luff wire, and I would think that those cannot be used in a way that makes the halyard tension primary. With such a sail, you use the halyard to set the luff tension (that's tension on the cloth itself).
With an internal wire, there's usually a small line from a cringle on the sail cloth to set the luff tension. On my present jib, this line is pre-tied, so that if I tension the wire the proper amount, then the resulting luff tension should be OK. But it can be adjusted, if needed and could be made continuously adjustable if I cared.