How to Size, Rig and Inspect Dock Lines and Fenders

• Bow and stern lines stop the boat moving forward, aft and away from the dock.
• Spring lines run from a midship or quarter cleat forward or aft along the dock. They stop the boat surging fore and aft, which is what grinds fenders and gelcoat against the dock.
• Breast lines run straight across to the dock. They are handy for short stops but can snatch hard in waves.

Discover Boating suggests three lines (bow, stern and a breast or spring) for a short side tie, and a four-line "box" in a slip: two bow lines and two stern lines, crossed where you can. Add springs on a side tie and on bigger boats or windy, tidal slips.

Nylon is the standard because it stretches and absorbs the jerk of wakes and gusts instead of passing it straight to your cleats. Three-strand nylon is tough and easy to splice; single-braid stays soft in big sizes; double-braid is strongest and nicest to handle but stretches a little less. Polypropylene floats and is cheap, but it breaks down quickly in sunlight and abrades easily, so keep it for light, occasional use, not a permanent slip.

One widely used retailer chart (West Marine) gives these starting points for nylon dock lines:

Go up one size for a heavy boat, an exposed slip or a storm-prone area, as long as the line still fits your cleats and chocks. Bigger lines also last longer because there is more fiber to lose before they get weak.

Permanent lines are cut and spliced to fit your slip; transient lines are longer, with an eye splice at one end, and travel with the boat. As a rough guide, many owners carry spring lines about as long as the boat and bow and stern lines at least two-thirds of the boat's length. In tidal water, lines must be long enough to let the boat rise and fall through the full tide without coming bar-tight at low water. Longer lines led at a shallow angle handle tide much better than short, steep ones.

• Leave lines snug but not tight. They need slack to stretch and absorb shock.
• Cross the stern lines in a slip so each one is longer and gives more stretch.
• Lead each line fairly through its chock with no sharp bends.
• BoatUS warns that a cleat is weakest when the line pulls straight up or out at a right angle to its base, and that two lines per cleat is usually the most a cleat should carry.
• Finish on the dock cleat with a proper cleat hitch (a turn around the base, a figure-eight, then a locking half-hitch). Extra wraps are fine; a pile of random knots is not.

Chafe is a leading cause of dock line failure. Put a chafe guard wherever a line passes through a chock, wraps a piling or crosses the edge of the dock, and where the eye rides on a cleat or post. Leather, rubber, fabric and polyester sleeves all work. BoatUS notes that in storm conditions, plastic hose used as chafe gear can trap heat in a nylon line that is stretching against a chock and help it fail from the inside; a sleeve that lets water reach the line is a better choice for a storm. Tie or tape the guard so it cannot slide off the wear point.

Run each line through your hands from end to end and look for:

• Fuzzy, flattened or cut spots, especially at chocks and cleats
• Stiff, hard or crunchy line and faded color, which point to UV and salt damage
• Glazed or melted-looking fibers, a sign of heat from chafe under load
• Broken strands or pulled yarns and loose, slipping eye splices
• Heavy mold or grit packed into the fibers

Replace a line that shows real wear or stiffness; do not just turn it end for end and hope. Looks can fool you: when Practical Sailor and New England Ropes pulled 10- to 12-year-old nylon dock lines to destruction, the best one kept only about half of its original breaking strength, and most kept far less, even away from chafe spots. The same tests found a splice costs roughly 10 percent of a line's strength and a bowline about 30 percent, and wet nylon is weaker still. Old lines are cheap to replace compared with the boat. West Marine suggests inspecting eye splices and attachment points at least once a season. Rinse salty or gritty lines in fresh water with a mild soap now and then and let them dry before storing.

A great line on a weak cleat still lets go. Look for loose fasteners, cracks or crazed gelcoat around the base, and green or rust stains that mean water is getting in. BoatUS stresses that cleats which take real loads should have proper backing plates under the deck. If a cleat moves, have it rebedded and backed before the next storm season.

Match the cleat to the line. The BoatUS Foundation's cleat tests quote a common maker's rule of thumb of roughly an inch of cleat length for every 1/16 inch of line diameter (about a 6-inch cleat for 3/8-inch line, 8-inch for 1/2-inch). If you move up a line size, check that the line still fits and the cleat is big enough. In the Foundation's rope tests, lines bent around a pulley broke at roughly half their rated strength, and knotted lines failed right at the knot. Treat a line's rated strength as a best case, and give it a generous size, smooth chocks and good chafe gear.

West Marine's rule of thumb is about 1 inch of diameter for a cylindrical fender (or 2 inches for a round one) for every 4 to 5 feet of boat length, with these typical cylindrical sizes:

• 15–20 ft boat: 5 to 5.5 in diameter
• 20–25 ft: 6 to 6.5 in
• 25–35 ft: 8 to 8.5 in
• 35–50 ft: 10.5 to 12 in

For quantity, the same guide suggests one fender for every 10 feet of boat length, plus one, and more at an exposed dock. When in doubt, go a size up: a fender that is too small squashes flat and lets the hull touch.

• On a floating dock, set fenders just above the waterline so they stay between the hull and the dock edge.
• At a fixed dock or with tides, recheck height as the water changes.
• Cover the bow, the widest part of the beam and the stern on the dock side.
• At pilings, fenders get pushed aside as the boat moves. Use a fender board: a plank across two or more pilings with fenders between the board and the hull.
• Tie fender lines to a solid fitting with a knot you can adjust quickly, and pull the fenders aboard once you are under way.

Most inflatable vinyl fenders should feel firm but not rock-hard. Polyform, for example, says its fenders take about 2 psi (2.1 lb): fill until the walls are fully expanded, but you should still be able to press the wall in about 1/4 inch with your thumb. Over-inflating shortens fender life and can void the warranty. Tighten plastic valve screws by hand only, never with a power driver. Wash fenders with a mild cleaner, check for splits near the rope eye, and replace any that will not hold air. Check your own fender maker's recommendation.

Ahead of a named storm or a forecast gale:

• Put on more and bigger lines, doubled where your cleats allow, each with its own chafe gear.
• Run long spring lines and lines to the far side of the slip so the boat can rise with surge without hitting the dock.
• Strip off canvas, cushions and anything that adds windage.
• Use a heavy fender board with several large fenders at pilings. BoatUS reports that boards with only two fenders tended to break or bounce out.
• Follow your marina's storm plan and see our hurricane prep guide for the rest of the boat. Never stay aboard in a storm.

Write down when each line and fender was bought and what you found at each check. Add a photo of how the lines are rigged in your slip, so you or a friend can reset them exactly the same way after a storm or a haul-out.