^^ As I try to think about an approximation, if we are talking about guys hiking to windward, more things come to mind:
- Are they together or spread (it isn't a point load)?
- What is the worst case (boat flat, really leaning out)? Are they leaning outside the railing, between the top and lower? I wonder if they can press the rail with more than a small fraction of body weight even so.
But bending a stanchion sure seems possible.
But back to the calculations.
Let's assume 4 guys would just about fill a 2 meter space. That makes it a uniform load, not a point load. As a very good first approximation, distributing a load reduces its effect by 50%, so we will take the load as 400 pounds. Static, I can't see how a man could rest more than 25% of his weight on the rial without it cutting him in half soon, but for calculation sake, I'm going to assume 100% as a dynamic factor. I cannot believe that 4 men could put 100% of their mass on the line at the same time, but dynamically, with an accidental jibe, maybe the dynamic value would get there.
If we assume 6" sag in 6' (6" for each 36" half, or a 6:1 ratio), the tensile force will be about 6x/2 = 3x = 1200 pounds without using trig or catenary calculations. Why divided by 2? There are 2 x 36" line halves supporting the side load. If the stanchions and pulpit/pushpit were rigid the line sag would be a bit less, but I'm betting the results is the lines sags a good 6 inches under that load, probably considerably more and something bent.
So conceptually, you are not far off, excepting the fact that a distributed load is much less "forceful" than a concentrated load. A detailed treatment would look at this as a suspension bridge calculation. I got the same answer this way (enter 1200 pounds, 6 feet, 32 ft/s2, and 133/32--the reason for the 133/32 is to convert pounds to pounds mass).
So the lifeline is at about 1200/5000 = 24% load, which is a reasonable engineering factor for something like this at peak load. I guess someone did the math. Lots of variables, though. I wouldn't want to test it on my boat--bent parts seem certain. For example, that 800-pound load is in what direction? Out, down, or hybrid (seems most likely)? If 50/50, then there is a 560-pound outward force carried on the stanchions and line tension. Line tension will carry some tiny portion of it, depending on the angle to the next stanchion (zero if straight), but mostly, they are going to want to bend or rip out.
Assuming each stanchion is carrying a 280-pound horizontal force 10 inches off the deck, and assuming the base is 3" wide with the bolts in 1/2" (many are more narrow), the upward force on the bolts is 280*12/2.5= 1344 pounds. They better be through bolted, since screws won't hold that. A single 1/4" SS bolt will fail (~ 1200#) and 2 would be strained once crevice corrosion sets in.
So I'm pretty sure there will be some damage.
What about the pulpit? It gets the full load, but the leverage is better. The mid-line load won't be too bad, but if 1200 pounds were attached to the top line...
1200 x 24/12=2400 pounds on a single 1/4" bolt. Failure is certain. Other boats may have stronger designs, but I bet something bends. Who is willing to hook a genoa sheet to the pulpit and grind HARD? Or just grind more gently on a life line? I doubt any of us want the damage.
The conclusion is these systems will survive a worst case hit with damage. That's the way I see it.