Clewmanship is not a word. Neither is clewology, but I'm learning it anyway. A clew the name for a specific corner of a sail. That corner is on one end of the foot of a sail. The foot, as you may imagine, is the bottom edge of the sail, which is closest to the deck of the boat. The clew is nearest to the back, or stern of a vessel. It is also the corner that comes under the most strain.
The clew ring, pictured above, is fastened to the sail in several ways. First, we wrap strips of nylon webbing around the hardware and tack those down with the sewing machine in between layers of sail fabric. Then we hand-stitch the ring into place. There are many layers of fabric at that reinforced corner. So, before I hand-stitch, I use a Dremel to pre-drill holes around the ring. After I seize the ring into place with sail thread, I cover the area with a leather patch, which you can see in the photo below. A few hundred more like these and I just might master the tao of clew.