Dock lines used when your boat and/or watercraft is away from its regular slip or dock is called transient dock lines. One end of the line has an eye that is easily passed around a cleat and the bitter end is used to make adjustments. There are dozens of combinations of diameters and lengths.
Dock lines should be made from nylon, a synthetic fiber that has a superior combination of strength and stretch. Nylon is strong (althought it shrinks and loses about 10-15% of its strength when wet), durable, and stretchy (three-strand nylon stretches up to 16% of its length loaded to 15% of its breaking strength), so it absorbs shocks. Low-stretch lines, like old worn-out polyester double braid used for sailboat running rigging, are less desirable because they transmit shocks from waves, loading up and loosening dock cleats and your boat's deck hardware. There are three main types of rope construction for dock lines; three-strand, double braid and mega braid.
Three-strand line has a knobby finish, is easy to splice and is the most affordable. Three-stand is available in several colors so you can color-coordinate your dock lines to match the color of your trim or canvas.
Double braid is somewhat stronger for a given size, has about half of the three-strands stretch, and is available in many colors so you can color-coordinate your dock lines to match the color of your trim or canvas as well.
Mega braid is a 12-strand single braid from New England Ropes, single braids are very supple and limp, so they are easy to coil and handle. Mega braid is frequently the choice for boats above 70'. It is harder to splice, so boaters may want to order custom size rope and it comes in white or black.
The cart below will show the recommended dock line diameter. Larger lines will wear longer but will stretch less.
Up to 27" - 3/8" dock line
28' - 31' - 7/16" dock line
32' - 36' - 1/2" dock line
37' - 45' - 5/8" dock line
46' - 54' - 3/4" dock line
55' - 63' - 7/8" dock line
64' 72' - 1" dock line
Over time, dock lines can look a bit green and covered with algae, or stiff and gray. If so, it may be time to consider replacing them. It's also a good idea to keep extra lines on board in case you find yourself in need of additional dock lines or spring lines. If docking inbetween two planks you may need to use double bow and stern lines to keep the boat away from both docks.
The spring lines run from the same cleats but in the opposite direction. They keep the boat from yawing, and hold it in a parallel position to the dock. While you may not use four lines every time you tie up, using a forward spring will make your boat sit more securely at the dock.
The length of your dock lines depend on the length of your boat. Bow and stern lines should be about two thirds the length of the boat. Spring lines, used to keep the boat parallel to the dock should be as long as the boat.
Lines can become damaged when the lines rub on surfaces. This is called Chafe. It is inevitable, but can be reduced by not changing the angle of the line abruptly and by using abrasion-resistant pads, such as lengths of leather or hose, called chafing gear. Even smooth, large radius survaces will abrade nylon and polyester lines over time.
Chafe guards provide a sacrificial surface that can take the damage without reducing the breaking strength of the line. Commonly, docks will have eyebolts, rings, or galvanized cleats where the dock lines are made fast. The type of hardware on the dock or piling determines the best type of dock line splice and chafe protection. for example, you may want to have an eye splice around a thimble, and a galvanized shackle when connecting to a ring or eyebolt.