Thanks, Wouter, for your input, I always appreciate your taking the time to share your perspective!

Yes, I was already very well aware of the need to use the mainsheet leech tension as a "backstay" - I didn't mention it in my post but rather assumed it would be the case (ie, it would be a constant). As you point out, even with adequate leech tension and the traveller centered, there is a risk of mast breakage if the the mast is not sufficiently rotated.

You said that the leech tension is sufficient to support the mast in the fore-aft direction of the boat (regardless of mast rotation) opposing the mostly forward pull of the spi, and when unrotated, the reduced support of the minor chord of the mast will allow it to bend and possibly break abeam. I understand what you are saying and the logic you are using but I wonder if there's more to it than that - an if there aren't some lessons that might be learned from those nuances? I'd like to try to take it a bit further...

The issue comes down to better understanding the sideways forces on the mast that will cuase it break when unrotated (assuming that the leech tension and spi pull counteract each other in the fore-aft direction of the boat).

I would think that the spi, like other foils, has a center of effort perpendicular to the foil, so the pull would have a foreward, leeward, and upward component. Looking at it from above I would guess it might be pulling approximately 20 or 30 degrees to leeward off the bow (fore-aft direction). If the traveller is centered, the leech tension acting on the top of the mast would be almost directly aft. If the traveller is slightly off center, as if often the case, the force would be to leeward of aft, maybe 10 or 15 degrees.

So if you map all these forces on a vector diagram, if my crude analysis is correct, an approx 90 degree mast rotation is needed to provide the counterbalancing (static resistive) force to the (dynamic) leeward forces generated by the spi and slightly off-center traveller. In that scenario, significantly less mast rotation does look like the mast would be insufficiently supported against those forces, especially in a gust, (although slightly less than 90 degrees looks like it would be fine as well). Interestingly, with a slightly off-center traveller, the leech tension is actually part of the problem - instead of contributing to mast support a portioin of it's pull is actually pulling in the same direction as part of the spi's pull (to leeward) instead of opposing it. This suggests that the more you ease the traveller, the more critical it is that the mast remains fully rotated and vice versa.

If the traveller is centered fully, according to the vector diagram, the optimal mast rotation to protect the mast looks to be less slightly than 90 degree rotation (by 10 or 15 degrees). But if you wanted to have the option to periodically partially dump the traveller in gusts, it would seem that 90 degrees rotation would be a good and safe overall compromise.

I'd appreciate your insights on these thoughts as I hope that a better understanding will be useful to me when learning to sail with the spi.

Jerry