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Keep in mind that the forces on the line are not the same as the forces on the shiv. For instance.. lets say you hang a 100 lb weight on the halyard (where you would normally hang a sail, but for now, it's just a normal old weight).
Then, you pull on the other side of the halyard (the side you'd normally tie off to a cleat when you have the sail raised). So, at this point, you are pulling on one end of the halyard, and the 100 lb weight is attached to the other side.
Lets say you hoist the weight off the deck about 2 feet, and hold it steady at that height. With you doing that, you'll be pulling down with 100lbs of force. The weight, of course, would be pulling with the same.
At that moment, the rope will be supporting a total load of 100 lbs, but the shiv/pulley will be supporting 200 lbs.
In conclusion, whenever a rope turns 180 degrees around a turning block, the load on the block is double the load on the rope.
Calculating the load transfered from the sail to the halyard is less straightforward, and depends on a lot of factors. Those factors include: Sail weight, bolt rope vs. sail slugs, cunningham tension, halyard tension, and even, to a certain extent, wind speed. However, wind speed has less of an effect than you might think.. most of that force is spread out along the spars.
Depending on the level of accuracy you are looking for, you might consider buying/renting a gauge that measures linear loads (a fishing scale, if you want). Attach it to your halyard and go sailing. Many other methods/calculations will get you a ballpark range, but with so many variables, only measuring it gets you there.
It really is a lot lower than you might think. Consider that the rope you are using has never broken, or likely even come close (at least a result of load force). So determine the working load of that rope and figure that's the upper end of your loads, perhaps. Just another thought.
... or I'm wrong.
Living aboard, currently in the Chesapeake
O'Day 37, still new to us