Some tips for choosing the right rope for mooring a boat include using the correct rope material, selecting its diameter based on the boat's length and considering the rope's construction and length based on the type of mooring and the size of the boat. Other factors to consider include the rope's abrasion resistance rating and its deterioration potential.
Marine-grade ropes are usually made from nylon, polyester or polypropylene. Polypropylene ropes are typically bright yellow and found on many boats, and it is the type of rope used for pulling water skiers and other types of recreational floating equipment. Polypropylene has a very low breaking point, deteriorates in sunlight and does not splice well, so it is not ideal for mooring. Polyester is strong, but it does not stretch well, which makes it less effective in allowing the boat to move naturally with the waves and water level while it is tied up. Most mariners prefer nylon because it is inexpensive, extremely strong, stretches nicely and splices well.
Generally, 3/8-inch-thick ropes are suitable for boats up to 25 feet in length. Mariners should add an additional 1/8 inch in diameter for each 10-foot increase in boat length, and they should pick mooring ropes that are about the same length as the boat with a few feet added if it is tied to a fixed pier where high and low tides are a factor.