Ratios
Hello,
We are talking about two things here: relative load; and varying safety factors. The diagram in the Apprentice will give you the relative load. Let's say it showed that your bobstay should be 50% stronger than your jibstay, and that the jibstay was 5/16" wire, with a breaking strength of 12,500lbs. The bobstay, however, would not see a strain 50% greater than 12,500lbs, but a strain 50% greater than the load on the jibstay. Again, hypothetically, let's say that the actual maximum load on the jibstay was 4,600lbs. We would put a safety factor of at least 2.5 on that for cruising, which would get us up in the neighborhood of the break for 5/16". And 50% over that load, 6,900lbs., would be the bobstay load, but we'd probably put at least a 2.75% SF on that, landing us somewhere between the breaks for 3/8" and 7/16". At this point we'd stop and think a bit: is that jibstay load realistic? If it is high, then maybe 3/8" would be fine for the bobstay. But if all the numbers make sense, there'd really be no appropriate choice other than 7/16".
This is the kind of thing we go through, in much greater detail, when spec'ing every rig component, for clients or for consultations. Lots of wrinkles and variables to consider, but it's part of how you get the right rig.
Fair leads,
Brion Toss
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