Interesting thread. I don't have a superwing mast or a spi yet but want to understand these issues so I did some elementary diagramming of wind direction, boat heading, and mast rotation and looked at the forces on the mast by the spi and the masts stiffness/flexibility in these scenarios (admittedly a very crude analysis). At issue, as discussed above (and I've read before) is the need to fully rotate the mast under spi to protect the mast from breaking. The problem is when I diagram out the geometry (superwing mast profile on a cat on different downwind headings with different amounts of rotation) it doesn't seem to confirm this. For example, what rotation will stiffen the mast vs allow it to bend off looks like it depends just as much on the particular heading downwind as it does on the amount of rotation. For example:

1) When heading deep downwind, less rotation puts the major chord of the mast more in-line with the wind direction so rotating completely on this course would seem to make it more bendy and breakage prone

2) When heading downwind as high as possilbe (close to a beam reach as you can under spi) max rotation puts the major chord of the mast close in-line with the wind so rotation completeley on this course looks like it would indeed prevent breakage

3) When heading on a typical broad reach (ie, 135 deg off the wind), it looks to me like you can never get the major chord of the mast in line with the wind and the best you can do is to get it diaganolly across it (still better than across the minor chord); BUT it looks like you get that scenario both totally unrotated and fully rotated - only when paratially rotated is the minor chord parallel with the wind. Since both fully rotated and non-rotated seem to be mast protective and fully rotated presents a better luff entry angle, it seems obvious why the latter is preferred.

But the above anaysis doesn't take into account the clocking of apparent wind as the boat accellerates. If at speed the wind always clocks forward to say 30 degrees off the bow then only a partially rotated mast setting would put the major axis parallel with the apparent wind (and at apparent wind speeds the boat's heading in relation to true wind diminishes in importance).

Also, in reality, when sailing downwind under spi, it is my understanding you are really never sailing a straight line. To get going initially you head up, then fall off as you gain speed and the apparent wind builds. You also sail "S" curves to maintain optimal speed and mananage gusts/waves.

It seems like it's much more complex than "rotate fully to be safe downwind" and "derotate a little in lighter air or when singlehanded to go faster but risk breaking the mast". Your course is constantly changing, you're accellerating and decelerating and as a result the apparent wind is clocking forward as you bear off initially and ocsillating foreward and aft to a lesser extent as you go through "S" curves and gusts. Plus the true wind and apparent wind are shifting with gusts. So if you just set one mast rotation for all downwind sailing it seems like at times the the fully rotated mast position is not the most protective position and in fact no one position could be. The issue as to what is the fastest mast rotation position for these different conditions is an even more complex anaysis but not part of this question...

So regarding which single mast rotation position is most mast protective under spi, what am I missing?

Thanks in advance for educating me on these interesting and complex isues!

Jerry