I have the '87 of basically the same boat. I recently bought a new North main.
With my old sail, I had to have more pre-bend (slack backstay) to keep the sail manageable, and even still, when the wind strength was up, I was at the limit of my ability to flatten it out. Two problems with old sail; very stretchy, and permanently over stretched. With this sail, I would have close to 2 sections of prebend to accommodate the old sail.
With my new sail, I only need about 1 section worth of prebend (a section is about 7" on our masts I think). This is pretty normal for fractional rigs. With 1 section of prebend and a slack backstay, I now get the maximum draft I would want. From there, I can add outhaul, and backstay to flatten the sail once I need to manage the sail power. The new sail has virtually no stretch (at least compared to my old sail), so it is very predictable and much easier to flatten when necessary.
There was a good discussion on this site about pre-bend and mast manufacturers typically suggest a section or so of pre-bend for fractional rigs, and sailmakers will expect this to be the case unless you tell them otherwise.
If your sail has draft stripes, then you can, or you can have your sailmaker do some analysis on your sail (see my sample attached). This will really tell you whether you mast bend is well matched to your sail. UK has a tool (
http://www.uksailmakers.com/accumeasure) that lets you do this on your own. I have used this to tune up my new mainsail. For me, the overall pre-bend is now pretty good. Since doing the first analysis (attached), I got a softer top batten, and I now get slightly more camber at the very top, and my 3 draft stripes have similar amounts of camber.
Chris
