So regarding which single mast rotation position is most mast protective under spi, what am I missing?
This is simple to answer; you are forgetting the leech-and-mainsheet combo that acts as a backstay. (when the mainsheet is not slack)
What you do by 90 degrees rotation, mainsail traveller near to centre of the rearbeam and with the mainsheet on, is to support the mast in its weakest plane by a tensioned leech.
The stronger plane of the mast (fore-aft) is then presented to the side of the boat. Meaning that the mast top, which is unsupported in a sideways direction, can count on the stonger plane to keep it from bending to much and collapsing.
Naturally the spi is mostly pulling foreward and here the leech tension keeps the mast top from bending forward and this is alot more important then the strength of the weaker plane of the mast. Of course if you dump the mainsheet in a situation like that ....
What happens when you derotate the mast. First you rotate the stronger plane more to the fore-aft direction of the boat where its strength is pretty useless as the leech tension is more then enough to stabilize the mast top. At the same time you are presenting more of the weaker plane to the side-to-side forces. These side forces are smaller then the fore-aft forces BUT the weaker plane of the mast is disproportionally weaker in that way so in fact you are increasing the risk of damaged.
That is the reason for the following rule of thumb when sailing under spinnaker :
-1- 90 degrees rotation on your mast
-2- pull the mainsheet on tight
-3- NEVER EVER uncleat or dump the main during a gust or a fully powered up spinnaker.
These three rules are simple enough for any bozo to remember and that saves alot of money to both the owner (bozo) and the builder (warranty)
As you have discovered yourself know. The REAL workings of the boat under spinnaker are alot less easy to understand and misconceptions can quickly end up in damages.
More experiences sailors are introduced to the more nuanced techniques of trim under spinnaker when it is clear that they understand what is going on and can be trusted to understand what can and can't be done safely.
Wouter
