Guide

Dock lines and fenders, explained

What each line does, how thick and how long, which fenders, and where every number on this site comes from.

The four lines

Alongside a dock, a boat usually lies to four lines; ACBS says "you need at least four lines". The bow line runs from the bow to the dock ahead; the stern line from the stern to the dock behind. The two spring lines run along the boat: the forward spring leaves a forward cleat and runs aft to the dock, which stops the boat moving ahead; the aft spring leaves an aft cleat and runs forward, which stops it moving astern. In a slip, Tie-Up Sheet doubles the bow and stern lines (one each side, to the dock and to the outer pilings) and puts the springs along the finger pier: a common layout, but your marina may set its own.

Diameter

Two published rules are used and the larger answer wins:

Rule A: "one-eighth inch of line diameter for every 9 feet of boat length" (Getmyboat Journal). A 27 ft boat: 3/8 in.
Rule B: the ACBS table: under 30 ft 1/2 in; 30 to 40 ft 5/8 in; 40 to 70 ft 3/4 in; 70 to 90 ft 7/8 in; 90 to 110 ft 1 in. If the boat is heavy, or lies in strong wind or rough water, ACBS says "you should choose the next larger size."
Cleat check: Boating Magazine gives "an 1/8-inch in diameter for every 2 inches of cleat length". A line too thick for the cleat will not make a proper cleat hitch.

This site rounds Rule A up to the next 1/8 in, and where two ACBS rows meet (40, 70, 90 ft) it uses the larger line. Over 72 ft, Rule A gives more than 1 in, where the ACBS table has no row: the plan then shows Rule A's size and says to ask a rigger or the builder. Thicker line is tougher but, as Boating Magazine notes, gives less cushioning when pulled taut.

Length

Bow and stern lines "about two-thirds of a boat's length"; spring lines "should equal the boat's length" (Getmyboat Journal; ACBS gives the same). Boating Magazine is shorter on bow and stern lines: "half the length of the boat". Tie-Up Sheet uses the longer figure, then rounds up to a length commonly sold (10, 15, 20, 25, 30, 35, 40, 50, 60, 75, 100, 125, 150 ft): that list is this site's own.

Material and chafe

Boating Magazine recommends nylon, which "resists sun, fuel and chemical damage" and "offers enough stretch to provide some degree of shock absorption". Polypropylene "degrades rapidly from UV exposure and wear"; Dacron can "quickly grow stiff". Wherever a line runs over an edge, a fairlead or a piling, protect it with chafe gear (this site's advice; foam floats on bungee-style lines are there "primarily to protect the hull from line chafing", Boating Magazine).

Fenders

Boating Magazine (Jeff Hemmel): "An inch of diameter for every 4'–5' of boat length is a good rule of thumb for the classic cylindrical fender"; for ball fenders the article doubles it, to 2 in of diameter for the same 4 to 5 ft. How many: "Use one fender for every 10' of your boat's waterline", and "Consider three fenders the bare minimum, with one placed at the boat's maximum beam." Hang them neither too low (they dip into the water and pick up marine growth) nor too high (wave and wake push them out of place). Tie-Up Sheet rounds the size up to a common diameter (4, 5, 6, 6.5, 8, 8.5, 10, 12, 15 in).

Fixed docks and moving water

On a floating dock the dock rises and falls with the boat. On a fixed dock or a piling it does not, so a line that fits at one level can be short at another and hold the boat against the water. On Lake Erie, wind can pile water up at one end: the National Weather Service in Cleveland writes that "in an extreme case, water levels between Toledo and Buffalo can differ by more than 10 feet." The Water Level Check works out how much length a line needs across the range you expect.

Sources

All read on 11 October 2026. Rules of thumb are starting points: your boat's builder, your marina's rules and the conditions on the day come first. Tie-Up Sheet is independent and is not Cedar Point, Six Flags or Cedar Point Marina.