I would use a FEM code (
http://en.wikipedia.org/wiki/Finite_element_method_in_structural_mechanics).
For a simple rule of thumb method, I have to know the geometry of the rig. For a beach cat, you could assume to have the weight of crew, helmsman and half boat weight on the shroud, apply a margin of safety of 3...4, then you have a rough idea of the loading. E.g. 2-up beach cat of 100kg weight: (2 x 80 + 50) x 3 = 630 kg. This requires at least a 3mm wire. Keep in mind that a margin of 3 is low, and requires a regular check. Lazy guys could go with a 4mm wire and less regular checking.
Cheers,
Klaus
That load is the vertical component of the shroud's load, which is actually higher.
Divide the load by the cosine of the angle between the shroud and the vertical before aplying the safety margin.
If the angle is close to 30 degrees the load is close to 15% higher than estimated.
The safety factor for steel wires can be smaller than 3 to 4. I guess the high figures avoid the need to calculate dynamic loads, which can be very important.
