Boat Dock Lines: Sizes, Types & Lengths
Boat dock lines are bought by specification, not by brand loyalty — diameter, length, construction and fibre decide whether your boat rides a storm surge calmly or saws through a chock and chews a piling. Get the spec right and almost any reputable maker's line does the job; get it wrong and the priciest cordage on the shelf fails the same way. This guide works through the numbers the manufacturers themselves publish, so you can rig any boat among boats for sale with lines sized to it, not to habit.
Why This Is a Sizing Guide, Not a Ranking
There is no honest podium for dock lines. Cordage is a commodity bought by spec: two lines of the same diameter, construction and fibre from established makers differ by percentages, not by kind, and the roundup ecosystem for this category does not converge on tested winners — it recycles catalogue copy. So this post follows the evidence instead: the sizing charts, tensile tables and fibre data the rope makers publish themselves — Samson, New England Ropes — checked against BoatUS and Fisheries Supply guidance. Where those sources disagree, the disagreement is printed, not smoothed over. Makers appear as examples carrying their own published data, never as ranked winners. For the other half of the dockside pair, see best boat fenders; the full kit is in our boat accessories essentials hub.
Dock Line Diameter by Boat Length
The working chart most chandleries publish — Fisheries Supply's version runs 3/8 inch for boats to 20 feet, 1/2 inch for 20 to 32 feet, 5/8 inch for 33 to 48 feet and 3/4 inch for 49 to 65 feet — tracks the common rule of thumb of an eighth of an inch of diameter for every 9 feet of boat. Treat it as a floor: Samson's own published chart is more permissive, holding 3/8 inch on boats to 35 feet and 1/2 inch to 50 feet, while BoatUS's 2019 guidance lands between them, at 3/8 inch to 25 feet, 1/2 inch to 35 and 5/8 inch to 45. One 35-foot boat, three published answers — 3/8, 1/2 and 5/8 inch. The tiebreakers are displacement and exposure, which no chart sees: a heavy or high-windage boat, or a slip open to fetch and wakes, takes the larger size, and Samson's footnote for storm conditions is to upgrade one size. Diameter is cheap insurance bought once; gelcoat repair is not.
How Long Should Boat Dock Lines Be?
Length follows position, not convenience. BoatUS's equipment guidance states the standard split: bow and stern lines at least two-thirds of the boat's length, spring lines equal to the boat's length. Springs earn their length — a spring only controls surge at a shallow angle well fore or aft of its cleat, and a short spring is just a second breast line. Two published alternatives deserve naming. BoatUS's own Don Casey prefers a travelling set all equal to the boat's length, so no line is ever the short one in a strange slip. Samson's brochure sizes by beam instead — bow lines at one and a half times the beam, springs at three-quarters of overall length. Any of the three systems rigs a boat safely; mixing them accidentally — short springs bought by the two-thirds rule — is the failure to avoid.
Double Braid vs 3-Strand vs Mega Braid
Construction decides hand feel, strength and behaviour on the cleat. Double braid is a braided core inside a braided cover, each carrying an equal share of the load in Samson's description, the cover adding abrasion and UV protection. New England Ropes states the trade plainly: double braid lines stretch slightly less but are stronger than three-strand lines, and Fisheries Supply puts the strength edge at 25 to 35 percent at the same diameter. Three-strand answers with its own virtues, in BoatUS's summary: it doesn't snag, it is easy to splice, it is considerably less costly than braided rope — and it is stretchier, which at a dock is often a feature. Braided line's counter-flaw, also BoatUS: better abrasion resistance, but a tendency to snag on rough pilings. Mega braid — a 12-strand single braid, New England Ropes' Mega Braid the type example — is the large-boat answer, supple at diameters where double braid becomes a hose; Fisheries Supply positions it for boats of 40 feet and larger.
Nylon vs Polyester vs Polypropylene
Fibre decides how the whole system absorbs energy. Samson notes nylon is still used in dock and anchor lines precisely because its excellent elasticity absorbs shock loads. The makers' data quantifies it: New England Ropes' figures show nylon 3-strand elongating about 12 percent at 20 percent of breaking load, against 2.8 percent for its polyester double braid — nylon works like a snubber built into every line; polyester transmits the jerk to cleats and chocks. Polyester's virtues — UV resistance, minimal stretch — belong to halyards and sheets, not docks. Polypropylene is the trap fibre: it floats (specific gravity 0.91, against nylon's 1.14), which feels boaty, but BoatUS is blunt about the rest — relatively low and erratic breaking strength, poor splice-holding, and heavy strength loss in sunlight within as little as a year. Their verdict stands: leave polypropylene to the skiers.
Breaking Strength: Reading Manufacturer Charts Honestly
A breaking-strength figure is one maker's test of one product, not a property of the diameter. New England Ropes publishes its Double Braid Nylon at 4,800 lb for 3/8 inch, 8,000 lb for 1/2 inch, 13,100 lb for 5/8 inch and 20,500 lb for 3/4 inch. Compare like with like and the spread shows: in 3-strand nylon at 1/2 inch, Samson publishes 6,300 lb for its Pro-Set-3 while New England Ropes publishes 7,500 lb for its Premium 3-Strand — same diameter, construction and fibre, 1,200 lb apart. Neither is lying; test methods, fibre grades and coatings differ. Two rules follow. Quote one maker's full chart when comparing sizes, never a column stitched from several makers. And derate for real life: Samson sets the standard working load at 20 percent of rated break strength, and BoatUS notes a bowline costs about 40 percent of a line's strength where a splice retains about 95 percent — why pre-spliced dock lines, New England Ropes' factory lines with a professionally spliced 12-inch eye among them, are the sensible default.
| Diameter | Boat length (Fisheries Supply chart) | New England Ropes double braid nylon — published breaking load |
|---|---|---|
| 3/8 in | To 20 ft | 4,800 lb |
| 1/2 in | 20–32 ft | 8,000 lb |
| 5/8 in | 33–48 ft | 13,100 lb |
| 3/4 in | 49–65 ft | 20,500 lb |
The Full Set: Bow, Stern, Springs and Spares
The minimum working set is one bow line, one stern line and two spring lines, sized and lengthed by the sections above. A home slip adds a second pair of springs and, in tidal or surge-prone water, lines doubled at the weather side; the travelling set can be lighter but should be complete, because a guest dock hands you whatever cleats it has. What the set serves is the slip itself — our how much does a boat slip cost guide (publishing with this batch) covers what you are tying up to. Carry one spare line a size up: it is the storm upgrade and the raft-up line in one coil. Label lines by position if your crew rigs the boat — a spring led as a breast line because it was the one that reached is a sizing failure, not a teamwork success.
Chafe, Spliced Eyes and When to Replace
More dock lines die of chafe than of load. The wear points are fixed and knowable: every chock and fairlead, every piling edge a breast line crosses, every cleat horn under a sawing surge. Chafe gear — hose, leather or a purpose-made wrap at each contact point — costs almost nothing and is the difference between a five-year line and a one-season one; Samson's storm advice pairs the size upgrade with doubled chafe protection for the same reason. Inspect on a schedule: a fuzzed or flattened cover, a stiff or glazed section, 3-strand whose strands have begun to open, or a splice starting to slip are all retirement notices, because the failure they promise arrives at 3 a.m. in weather, not on a calm Sunday. Retire by line, not by set — the springs and weather-side lines live harder lives than the lee stern line.
Boat Dock Lines FAQs
How many dock lines does a boat need?
Four working lines at minimum — bow, stern and two springs. A permanent slip typically runs six or more with doubled weather-side lines, and one spare a size up covers storms and raft-ups.
What size dock lines for a 30-foot boat?
The working chart gives 1/2 inch; the rule of thumb (1/8 inch per 9 feet) agrees. A heavy or high-windage 30-footer in an exposed slip takes 5/8 inch — Samson's storm guidance is to upgrade one size.
Are double braid or 3-strand dock lines better?
Neither categorically. Double braid is stronger at the same diameter (New England Ropes publishes 8,000 lb for its 1/2 inch double braid against 7,500 lb for its own 3-strand) and handles more softly, but can snag on rough pilings. Three-strand is cheaper, stretchier and easier to splice.
How long should spring lines be?
As long as the boat. BoatUS's guidance puts spring lines at the full length of the boat, against two-thirds for bow and stern lines, because a spring only controls surge when it runs at a shallow angle. Short springs are the most common rigging mistake in the marina.
Bottom line: boat dock lines are a specification exercise — diameter from the chart with a size-up bias, springs at full length, nylon for its stretch, one maker's chart quoted whole, and chafe gear at every contact point. Rig by those numbers and the lines disappear into the background, which is the job. Then go back to choosing the boat they will hold, from the boats for sale at BoatDealsDirect.com.