[svaletheia]

Hello,

I'm relatively new here (joined a while back, had some strange happenings in life around then, and am hopefully now back to 'normal' - whatever that is - for the next while).

Regardless, I've been running calculations for my Allied Princess ketch rigged boat per the Riggers Apprentice book and have noticed some VERY odd things about my current rigging situation.

For starters, here is what I have directly measured using a reasonably high quality digital caliper:

Twin forestays: 5/16"
Main shrouds, upper, and double lower: 1/4" according to my caliper, but rigger measured as 5/16" (and I measured across the widest part as well).
Main backstay, split around mizzen: 1/4" (same comment as above)
Mizzen shrouds, upper and double lower: 5/32"
Triatic? stay was not measured, but is present.

All are 1x19 SS of unknown composition.

Now for the tricky bits. According to the chart in Riggers Apprentice for RM30, the approximate RM30 for my boat (mid-line value) at 36' LOA and 27' LWL (at 30 degrees of heel its approximately 30' LWL I am told) is 30000 pound-feet. With a beam of 11 feet, chainplates nearly at the gunwales, and a safety factor of 1.5, that leaves a total force of 8182 pounds. Since this is a ketch, I understand that I apply 100% of these loads for mainmast calculations with an appropriate safety factor (2.25-2.75 seems usual?) and apply 100% of the same load to the mizzen, but with a reduced safety factor, closer to 1, given that the mizzen should only see the full load in extreme circumstances when its pretty much 'caught with its pants down' so to speak.

Running the numbers for the typical load percentages carried by a single-spreader design with twin lower shrouds, I get the safety-factor-adjusted load expectation of the cap shrouds at 9205 lbs.

Here is where things go a little sideways: 1/4" 1x19 SS, even 302 to be generous, has a breaking limit of 8200 lbs, according to Loos. 316 is worse, at 6900 lbs.
Now, 5/16" is 10600 lbs for 316, and 12,500 for 302, if I read the charts right.

So my questions are:

1. Is my rig likely undersized?
2. Did I calculate the RM forces properly and apply the theory to the ketch rig correctly?
3. If I'm going to replace all the standing wire rigging anyway, as a precaution, should I go with the 5/16" that I calculated or stick with what the rig has right now (its circumnavigated under the previous owner and the current rigging at least once)?

Bonus question:
4. Is the tension on the twin forestays supposed to be half as tight as the ordinary tension if there were a single forestay? My engineering theory tells me yes to maintain total tension on the mast, but my statics course was a long time ago and I'm not so sure we covered redundant member loading thoroughly. This is a really annoying part of my rig - how slack the forestays are which makes my leading edge of my genoa really sloppy. The rigger told me this was normal for twin forestay rigs and to just deal with it.

Many thanks!