In the 90s, the Boat US Foundation performed a study of deck cleat strength. Testing was performed using 6-inch cleats of a number of materials and designs, which were pulled from several directions. The standard vendor recommendation is 1/16-inch of line size for each inch of cleat, so these cleats are recommended for use with 3/8-inch line (breaking strength 4,200 pounds, working load limit 525 pounds). All but the nylon cleat had working load limits (assume 4:1 safety factor for metals) greater than nylon rope. Most were nearly as strong as the rope, but only two were stronger than the rope in all directions. We can expect strength to go up roughly as the square of size, roughly matching rope strength as we go. Only a few broken cleats were noted among the boats damaged by Hurricane Irma. More commonly, the cleats pulled out of the deck.
Obviously, cleats need a firm backing. A strong, well-engineered deck helps, but backing plates are essential (see How Big Does a Backing Plate Need to Be? PS August 2016).
Watch the angle formed when the line goes through the chock. You may not be able reduce the downward angle, but any pressure against sharp edges will weaken the line by 20 to 50 percent. Avoid sharp turns, which forces fibers on the outside of the turn to carry most of the load. Try to keep the runs straight, and pad the turns with heavy chafe gear.
Bottom line. Cleats and chocks must be sized for the required line size, which for storm-mooring a 35-foot yacht requires 7/8-inch line and 14-inch cleats. However, it is a rare 35-foot boat that has cleats over 10 inches. This is a challenge.
For the full report on cleat strength from the BoatUS Foundation, see https://www.boatus.org/findings/16/.
