There are quite a few choices out there when it comes to selecting docklines. It's important to consider the type of docking and rafting you do. It helps to have two sets of docklines — a temporary set to use while you are away from your permanent slip and a set for your permanent slip, unless you're concerned about someone taking those lines while you're away.
Nylon is a strong, durable synthetic and is exceptionally well suited for docklines. You will commonly see it in either Laid or Braid Construction.
Laid line is composed of three or more twisted strands, a design that has withstood the test of time. Although it can kink, making it awkward to coil, and stretch more than braid, three-strand twisted line is very strong, resistant to abrasion and snagging, and offers good value. But some stranded nylon is quite soft in isn't very suitable for use aboard. This is frequently sold in hardware stores, is relatively very inexpensive, and is usually not marked for marine use.
Braided line is composed of many small strands, making it extremely strong and flexible. It runs smoothly, will not kink or rotate, and is easier on the hands. These small strands wear evenly, making braided line more resistant to chafe from pilings and cleats-the greatest threat to healthy docklines. However braided lines are more likely than laid line to catch on piling splinters or other snags and thereby become damaged to some degree. They're often available in a selection of colors
Factory-spliced eyes make sense. The splice, formed by interweaving parts of the rope, creates a permanent eye while sacrificing little of the line's inherent strength. It cannot "undo" as can knots used to form the eye. You could splice the line yourself. For laid line, splicing is simple, but braid requires a special fid and some practice.
Boats under 20' generally use 3/8" line; 20-30' boats, 1/2" line; 30-40' boats, 5/8"; and boats over 40', 3/4". If your boat is heavier than average, or has greater windage, you may want to choose the next larger size. Too much is far better than too little.
As a general rule docklines should equal two-thirds of your boat's overall length. Spring lines should be considerably longer. You will need to be able to use these to bring your boat alongside while docking. You may need to pay the spring line out to ease your boat to a stop in the right position and as the helmsperson brings the boat alongside. This is an art in itself, but very important. You should have lines of various lengths available to use depending on the circumstances as you come alongside a dock and the pilings and cleats on the dock. The shortest should be approximately the same length as your boat.
How to Use Them?
Unless your boat is unusually heavy or will be subjected to severe conditions, a bow line, stern line, and at least two spring lines are recommended. Spring lines keep the boat snugly near the dock, preventing it from moving fore or aft, while allowing for the rise and fall of the tide. Of course, if your permanent slip has outboard pilings as well, you'll need an additional bow and stern line. You should also have enough lines to double up when expecting very bad weather.
If you're away from the boat much of the time, lines should be longer to allow for high water from storms and tides. Short lines are a prime cause of damage. Also, put the eye end of your dock lines around the cleat, bitt, etc. aboard, if you'll be adjusting lines from the dock. Conversely, if you spend most of your time aboard, put the eye around the fitting or piling ashore and adjust from the boat. TideMinders ™ are balls with holes in them through which you can string the piling end of your line so that the line will ride up and down on the piling with tides or storm surges. You may find them very helpful.
How to Care for Them?
Chafe is the nemesis of any line (good or bad). Although inevitable, it's not uncontrollable. In a permanent docking arrangement, the chafe problem is most chronic. First, chafe-proof your boat as you would childproof your home. Examine chocks, cleats, bitts, posts, and other hardware for rough edges. Next, consider how you lead the lines, being careful to avoid acute angles, which present opportunities for abrasion. Last, get defensive. Chafe protection can range from heavy cloth such as old blue jeans wrapped around the line at the point of contact to ready made chafe guards attached to the line where it passes through the chocks. Research has shown that chafing material that doesn't let moisture and air into the line can result in excessive heat buildup as the line moves back and forth within the material. Therefore, it would be important to avoid using materials that can do this, such as a plastic or rubber hose. TideMinders help avoid chafe where the line goes around the pilings. Whether you're in your permanent slip, or just passing through, regularly check the condition of your lines for signs of wear. The investment in your boat could depend on it.
See also Choosing the Right Rope