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So now I am going to date myself.What is the logic used to determine the spreader heights? It might be a bit of an illusion in the photo but the lower spreader looks relatively high? I guess the d1s mean it can be set higher?
Back in the 80's we had mast that bent around 5-6%, so by that, my 10.7m 89er mast, back in the 80's we would design to bend and operate with 1/2m bend most of the time going up to 640-650mm of bend in a gust and if it went 700mm it would fall down.
This is what my father would have called a active batten rig.
We got very anal about the luff curve, not only the amount but also the position/Type of the curve and we would marry that curve multiplied by a factor in the roach, but modified so we would get the upper quartile the feather off first.
And we developed this spreadsheet that would give us all of these number and it would also spit out spearder poke and toe.
Poke it the distance past a straight line sideway, toe is for-n-aft.
Top spreader is a no brainer, you dial in the type of wire your are using, and from that you work out the effective angle that you need (sideways) and then pitch of the spreader all had to do with expected AWA, and that varied from 22-27°.
But your 100% correct, if you have an effective set of D1's, (which I have) you start your measurment at the top of the D1's.
Today with squarehead, mast bend is 1.5 - 2%, it may get to 3%, you are no were near the critical numbers, so you have to be a idiot for the mast to fall down (or somethng breaks, which proves you were a idiot, because you did not check), plus today you have the Andrew Lectie's of this world who do all of it.
Not sure people will get all of this, but it is critical.
The original 18teen rigs and later the 49er and 29er rigs, and by rigs I mean mast and sails (talking upwind now) were design by me, I dictated luff and leach profiles, areas and I did the mast. Ian (Maka) McDiarmid then worked with me, sure made suggestions, 99% of the time they were incorporated, and we made some truley outstanding rigs, but they were driven by one brain, supported by another, working as a team.
Now you have mast makers who make a mast and then sail-makers who fit a sail to a mast, 99% of the time.
The recent 3Di-ing of 49er/FX has been both painful and iluminating. The mast and sail were developed at different times, by diffrent people on different continents.
It has been painfull, but we have had very supportive sailors, and by garnishing their thoughts and feed back, hopefully, and all indicators suggest this, we have hit the nail on the head!
Longivity of the 3Di jib is presently 3 fold! The mens and womens WC 1st place where acheived with Jibs that had seen serious use. Use that would have rendered previous incarnations of the jibs in the trash can. Mains could well live more than a year.
Mains are interesting, squarehead, stress the sail far less because the mast bends far less. And the Mast is stressed far less because of the Squarehead, it can bends less. WIN - WIN!
But back to it, not quite sure how I engaged Andrew, but speaking to another Taswegen today who commented that Andy was soft, quite, understated but quite extrodinary with what he did.
This was my baby, firstly I was blown away with the amount of data feed back I got, and the freedom with which I got it. A lot of you would have no idea, but I checked and double checked everything I got from Andy against my own data base, and by the time Andy and Tim (Willets - C-Terch) started talking ie's WRT the mast, I had a lot of confidence in what Tim/Andy where proposing.
Back to spreaders.
My stay base is 25° and Andy has set my spreader angle (the same) at 25°.
I probably would have had them further fwd, 22-23° maybe, but it would be more toe than angle, but given that the mast is 112mm for-n-aft and the bend is so much less, all that is going to happen is the D1 tension will be higher.
I may have been more anal about the height, but again, everything is so much simpler with the Squarehead!
Long way to answer a simple queastion!