Yeah ...I thought you'd say that
Well ...I believe that using 25% of the total load for the backstays came from the example in Rigger's Apprentice on the top of page 138 (since it was difficult to find any other rule of thumb in other design texts).
I'm assuming that you mean that 1.5 to 2 is insufficient for a factor of safety for shock loads in backstays but, again, this came directly from the "Mizzenmast Scantlings" inset on page 284 of Rigger's Apprentice. In Chapter 5 of Rigger's Apprentice the value of 2.5 is quoted a few times relative to main backstays but I'm still lacking any real good data on mizzen scantlings.
I do not actually have a mizzen staysail because the main has fixed backstays attached to the masthead that seem like they'd interfere even though my sail plan shows a staysail and Franz Schattauer might have actually made one for this boat years ago. The only stay that holds the top of the mizzen forward is the one that goes between mastheads (still don't remember what the correct term for that is). The mizzen has single uppers and double lowers for shrouds so, aside from the aft-led lowers the stick has nothing else keeping it from springing forward (and it's a real wet noodle to climb above the spreaders without those backstays).
The only situation that I can imagine where the running backs might take a decent load would be if I got pooped by a very large wave when running. It would have to be really really large though because this is a pretty dry boat with a lot of freeboard.
I believe that your description of a "tiny fraction of the total load" lines up with my experience with the rig Brion. It seems like I've seen at least one old picture of a Spencer with a ketch rig where it looked like they were using line that might have been as small as 3/8" and terminating it at either a cam cleat or regular cleat near the side of the cockpit. I do wonder if those Colligo system "blocks" actually run free well since they are decribed as "static". I don't really want to fuss around untangling things on the after deck on every tack.
John
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