Calculation for crossarm stays on cat

[mikereed]

I have a 46' cat with typical forward crossarm bracing consisting of a "seagull striker" or martingale with a wire stay passing over the top and connected at the ends of the spar. The boat has been undergoing a rebuild during which the stay has disappeared. I need to replace it but do not recall the diameter of wire used.

My question; is there a calculation to determine the forces and hence wire diameter on these stays or does one just use a "rule of thumb" such as using the same diameter wire as the caps and forestay?

Thanks,
Mike