Freestanding Rig Effect

That's interesting about the D1s. This little boat does not have lower diagonals. I've read elsewhere on this forum that the lowers do 60% of the work to hold up the rig and that the uppers tend to control the bend. I'm starting to see that this is absolutely true.

Thanks for triggering the notion.

 

Not sure what the OP was trying to accomplish by winding his turnbuckles up at the dock. Without knowing the max load the shrouds take when sailing how do you know what to do at the dock.
Also the sketch doesn't show lower shrouds. The sketch sort of indicates that the spreaders are swept and that the chainplates are aft of mast centerline ,if so, and assuming the lower shroud chainplates are also aft of mast centerline the lower shrouds help prevent the mast from bending. Even giving all that unless the spreaders are really swept back the angles are pretty small, you won't see much headstay tension change with shroud tension. Changing shroud tension will change mast bend characteristics, headstay tension is more effected by backstay and mainsheet tension.

 

Without lower shrouds, shortening the spreaders will reduce the mast bend and more proportion of shroud tension will be passed on to the headstay.
Sweep angle of spreaders also play a role in tuning this sort of rig. Some highly developed dinghies with this sort of rig even have adjusters on them. Adjusters are not used when underway, but tuned to expected conditions of the day.