We are starting to manufacture jack lines for sailboats. They are constructed from 9800 pound polyester webbing with a sewn eye in each end. When we sew the eye we have to calculate how to sew it so it has the required 5000 pound strength. Knowing the thread size tells us the theoretical strength per stitch. Then it is easy to calculate how many stitches are required to produce the necessary strength. The question is does this really work? The only way to tell is to load test samples.
Most of what we test does not require a tree on one end and dump truck on the other. Even with this set up our first attempt resulted in pulling the dump truck up the driveway with 4000 pounds of load. A bit of weight in the bed and using the real brakes rather than the parking brake fixed that problem.
If you want more information on how to calculate strap strength check out the blog post over at our Westpac Marine site.