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Securing Your Boat To The Dock
May 29, 2011 7:36 PM | Tagged as Dock lines

Securing Your Boat To A Pier 

Expensive hull damage to your boat is most likely to happen when it’s improperly tied up to the dock. When strolling along the docks of marinas, I’m often amazed by the type and size of lines used (even clothesline!). It's also wondrous to see the ingenious, unrecognizable knots some skippers use to secure their very expensive boats in a slip. If the wind comes up or somebody leaves a large wake, dock lines and fenders are the only things keeping a boat from smashing into the pier or seawall. Here are some basic rules of thumb about dock lines.

Any well-equipped boat should have at least six (6) dock lines: two (2) bow lines, two (2) stern lines and two (2) spring lines. The dock lines should be at least 2/3 of the length of your boat and the spring lines should be at least the full length of your boat.

The size (diameter) of your line depends on the size and weight of your boat. The following is an approximate guide.

Line Diameter
Boats under 20 feet = 3/8"
Boats 20 to 30 feet = 1/2"
Boats 30 to 40 feet = 5/8"
Boats 40 to 60 feet = 3/4"
Boats over 60 feet = 1"

If your boat is heavy for its size, consider going up one size in diameter. A boat cannot be damaged by having its dock lines a bit oversized, but I've seen lots of damage done because lines were too small!

One word about knots in your dock lines: A knot will reduce a line's breaking strength by as much as 50%. Avoid them as much as possible! The only knots a dock line should encounter are those that you tie around cleats or pilings. The art of tying nautical knots is an essential skill all boaters should learn.

Good dock lines are expensive, so wherever your dock line goes through a chock or other hardware, use a chafing guard around it. Chafing guards can be purchased in a ship's store, or you can fashion your own, less expensive models, out of an old piece of garden hose.

Nylon is the best choice for dock lines. In my opinion, it is the only choice! Nylon line comes in two types. First, there is 3 strand twisted, also called "laid" line. This type of line is fine for most purposes, as it is strong and stretches to help absorb shock. It does, however, have the disadvantages of kinking easily and being rough on the hands. Coiling laid line clockwise helps to avoid kinking.

The second type of nylon line is braided line. This type rarely kinks and is much easier on the hands. Its disadvantages are that it tends to chafe more easily than twisted line and it's more expensive.

Polypropylene line (called "poly") is a poor choice for dock lines. Poly does not stretch, it has a very low breaking strength and degrades quickly in direct sunlight.

To illustrate why nylon lines are preferable to poly, here are some of their respective breaking strengths.

Diameter  Nylon Poly
  (Lbs. breaking strength) (Lbs. breaking strength)
3/8" 4,000 2,100
1/2" 7,000 3,900
5/8" 11,000 6,000
3/4" 15,000 8,000

 
[Note: For finding safe working loads for the above lines, calculate 1/5 the breaking strength. For example, the working load of 3/8" nylon line = 800 lbs

The following diagram shows some examples of properly secured boats: 

0

Remember, good dock lines and good fenders are your first line of defense against hull damage. Use nylon line that's correctly sized for your boat's length and weight, and learn how to tie a proper cleat hitch, at the very least.


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