February 04, 2023
The size and length of a boat's mooring lines are important factors to consider when outfitting a boat.
A properly outfitted boat should have at least six dock lines on hand. Two (each) bow and stern lines and two spring lines. The length of the lines depends upon how and where you moor your boat.
Docking lines, which are used to tie the boat to a dock, need to be long enough to reach from the boat to the dock and still have enough slack to allow for movement caused by tide, wind, and waves. Bow and stern lines should typically be at least 2/3 times the distance between the boat and the dock. Spring lines should be equal to your boat's length.
When it comes to the size of a boat's dock lines, it is important to choose a strong line to withstand the forces it will be subject to. The strength of a line is measured in breaking strength, which is the amount of force required to break the line.
Use only 3-strand twisted or braided nylon (no polypropylene or manila). Chafe gear – water hose, clear vinyl tubing, sewn-on leather (tape is ineffective).
Bowlines: Typically made of nylon or polyester, and sized according to the length of your boat. See table below.
Stern lines: Typically made of nylon or polyester,
Spring lines: Typically made of nylon or polyester,
Fender line: Typically made of nylon or polypropylene,
In severe weather conditions, some people choose to use dock lines of a larger diameter, or even “double up” some of the lines. But notice that while making them stronger, this may decrease their cushioning effect, just when your dock and deck hardware need it the most. A better option may be to add additional dock lines secured to previously unused hardware and tie-off points. As always, good seamanship requires good judgment and analysis of every situation. Increased attention to dock line chafing gear is always prudent in severe weather.
Yacht Size Primary Line Size Spring Line Size
(overall length) (diameter) (diameter)
Up to 27 ft 3/8” 3/8”
28 ft to 31 ft 1/2” 1/2”
32 ft to 45 ft 5/8” 1/2”
46 ft to 54 ft 3/4” 3/4”
54 ft to 63 ft 7/8” 7/8”
64 ft to 72 ft 1” 1”
It is important to note that these are just general guidelines and the specific size and type of line may vary depending on the size and type of boat, as well as the intended use of the line. It is always best to consult with a professional or the manufacturer for the appropriate size and type of line for your specific boat.
In the University of Virginia School of Engineering laboratory, a series of tests were performed on nylon and polyester line (also known by its DuPont trade name, Dacron®) - BoatUS: www.boatus.org/findings