From an engineering standpoint I would guess they are there to keep the stays from sliding upwards as G described. Stays only 'push out gently' on the shrouds, they generally don't/can't supply the sorts of forces you are thinking about in the directions you are thinking to change the tensioning. Those forces are rather large and I honestly can't think of a way you could change wire tension short of adding more shrouds and attached everything at the spreader. but that would reduce tension on the side you put more wires on, ie the same force would now be spread between the two wires. The question would then be why do that, since you already have shrouds at that point attached near the foot of the spreader. If the spreader was actually able to be strong enough to change the tensions between the upper and lower half of the wire those forces would have to go somewhere. Unfortunately, if the spreader was rigid enough to do that it would act like a giant torque lever and probably bend/break your mast. Spars are not designed to take anything but compressive forces. Look at the way the spreader is attatched to the mast, it looks like you could just bend it down, right, not a lot of up and down strength. But you can push it into the mast directly with a great deal of force, right?
The mast holds largely compressive loads (straight down), the shrouds are under direct tension in the direction of the wire. and the stays have a small compressive load pushing them into the mast. None of the spars can stand much 'sideways' forces but can take tremendous compressive forces, and of course ropes and wires can't stop any sideway force, ie they are pure tension. I'm not at my home computer (photoshop) so i'll have to wait to draw the diagram.
Last edited by tenuki; 05-25-2007 at 04:59 PM.