Rode & ChainAnchor, chain and scope sized to your boat — not to a guess.

Scope & Holding

Scope is rode length divided by rise — and the rise is not the charted depth.

It is depth, plus the tidal range you will see before morning, plus the height of your bow roller above the water. Leave out the last term and a small boat in shallow water can overstate its scope by a fifth.

The anchor holds because of the angle the rode makes at the seabed. That angle starts where the rode leaves the boat, not where it enters the water.

Add the tide before you set, not after

A four-foot tidal range means a 7:1 scope at low water becomes something considerably less at high. Anchors drag on a rising tide, in the dark, when the boat you carefully anchored clear of is suddenly much closer than it was.

The standard scopes

5:1 is a minimum for fair weather with someone on watch. 7:1 is the overnight standard. 10:1 is for heavy weather. All-chain rode can hold at somewhat less because the chain's own weight keeps the pull horizontal; rope needs more, because it floats and lifts the shank.

The number nobody calculates: swing radius

More scope holds better and swings wider. In a crowded anchorage the limit is usually not holding power — it is whether the circle you will swing through is clear of other boats, moorings and shoal water.

That radius is not the rode length. It is the horizontal leg of the triangle, and it is worth knowing before you settle in next to someone.

A snubber is not optional on all chain

Chain has no stretch. Every wave shock loads the windlass and the bow fitting directly, and both are designed for static loads. A nylon snubber absorbs that, and it also quiets the boat enough to sleep in.

5 articles

5:1 scope in 35 ft of water with 4 ft of tide

Rode to pay out
210ft
5:1 over 42 ft of rise — 35 ft depth + 4 ft tide + 3 ft bow height
water206 ft swing radius42 ft rise210 ft of rode35 ft depth + 4 ft tide + 3 ft bow
Rode length210ft
Total rise35 depth + 4 tide + 3 bow — measured from the bow roller, not the waterline42ft
Swing radiusthe circle you need clear of other boats and shoal206ft
Swing diameterif the wind boxes the compass412ft
Nominal scope5:1
Effective scopethe usual small-boat setup5:1
Rode typerope with chain leader
At the 7:1 overnight standardmore than you are paying out now294ft

Notes

  • Measure the rise from the bow roller, not the waterline. The anchor is held by the ANGLE of the rode at the seabed, and that angle starts where the rode leaves the boat. On a boat with 3 ft of bow height in 35 ft of water, leaving it out overstates your scope by 7%.
  • Add the tide before you set, not after. 4 ft of range means a 5:1 scope at low water becomes 5:1 at high — anchors drag on the flood, in the dark, when the boat you anchored clear of is suddenly close.
  • 5:1 is a fair-weather, someone-on-watch scope. Overnight practice is 7:1, which here would be 294 ft of rode. Short scope is the most common reason anchors drag.
  • Rope pulls upward on the anchor shank as it tensions, which is why it needs more scope than chain. A chain leader of at least a boat length keeps the pull at the anchor horizontal and takes the chafe on the bottom.
  • Set it properly once it is down. Back down at low revs until the boat stops moving with the rode taut, then take a transit on shore and watch it for a few minutes. An anchor that has not been set is just a heavy object lying on the bottom.

Scope and rode length

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