Dock Line, Fender & Anchor Scope
Line diameter and length for your boat, how many fenders and what size, and the anchor scope measured from where it is actually measured.
Dock lines
Fenders
Anchor scope
Scope starts at the bow roller
Scope is the ratio of rode paid out to the vertical distance from the bow roller to the bottom — not to the water. On a boat with four feet of freeboard anchoring in ten feet of water, the roller is nearly a third of that vertical, so seven to one measured from the surface is really about five to one in practice.
Add the tide still to come, too. Anchoring at low water in a six-foot range and calculating for the depth under you at the time is how a perfectly good scope becomes marginal by midnight, which is also when the wind gets up.
Nylon three-strand by boat length
| Boat length | Diameter | Breaking strength | Working load |
|---|---|---|---|
| up to 20 ft | 3/8″ | 3,700 lb | 463 lb |
| 20–30 ft | 1/2″ | 6,400 lb | 800 lb |
| 30–40 ft | 5/8″ | 10,400 lb | 1,300 lb |
| 40–50 ft | 3/4″ | 14,200 lb | 1,775 lb |
| 50–60 ft | 7/8″ | 20,000 lb | 2,500 lb |
| over 60 ft | 1″ | 25,000 lb | 3,125 lb |
Breaking strengths are for new line. Working load is an eighth of that — the margin covers shock loading, knots, chafe and age, all of which are working against you at once.
Things worth knowing
- Nylon is the right rope for dock lines and rode precisely because it stretches. Fifteen to twenty per cent at working load is what absorbs the shock of a wake instead of transferring it to a cleat.
- Never use polypropylene for either job. It floats, which sounds useful, but it has almost no stretch, it degrades in sunlight quickly, and it fails without much warning.
- Spring lines are what stop the boat moving fore and aft along the dock. Bow and stern lines alone let it surge with every passing wake, and surging is what saws through a line at the chock.
- Chafe kills more lines than load does. A line that parts overnight almost always parted where it crossed something, and a length of split hose over that spot costs nothing.
- Scope is measured from the bow roller, not the waterline. On a boat with high topsides anchoring in ten feet, the four feet from water to roller is nearly a third of the vertical — leave it out and your seven to one is really five to one.
How to use
- Enter your boat length and how high the bow roller sits above the water.
- Read the line diameter, the bow, stern and spring lengths, and the fender plan.
- Add depth, tide and conditions for the rode to pay out.
Frequently asked questions
What size dock lines do I need?
Roughly an eighth of an inch of diameter for every nine feet of boat: three eighths up to 20 feet, a half to 30, five eighths to 40, three quarters to 50. Bow and stern lines run about two thirds of the boat’s length, springs a little over the full length.
Why do I need spring lines?
Bow and stern lines stop the boat leaving the dock; springs stop it surging back and forth along it. Without them every passing wake works the boat fore and aft, and that motion is what saws a line through at the chock.
How many fenders?
One every ten feet, with a floor of three. Three matters more than it sounds: with only two, the hull can pivot between them and touch the dock exactly where nothing is protecting it.
What anchor scope should I use?
Seven to one overnight, five in settled conditions with someone aboard, ten or more when it is blowing. More scope keeps the pull on the anchor horizontal, and a horizontal pull is what makes an anchor dig in rather than lift out.
Do I measure scope from the water?
No — from the bow roller. On a boat with four feet of freeboard anchoring in ten feet of water, the roller is nearly a third of the vertical distance, so seven to one measured from the surface is really about five to one.
Should I allow for the tide?
Yes, and people routinely do not. Anchoring at low water in a six-foot range and calculating on the depth under you at the time is how a good scope becomes marginal by midnight, which is when the wind usually gets up.
Why nylon and not something stronger?
Because nylon stretches fifteen to twenty per cent at working load, and that stretch is what absorbs the shock of a wake instead of transferring it to a cleat. Polypropylene floats but barely stretches, degrades in sunlight and fails with little warning.
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