10:1 scope in 12 ft of water with 10 ft of tide

Rode to pay out

260ft

10:1 over 26 ft of rise — 12 ft depth + 10 ft tide + 4 ft bow height

Rode length	260ft
Total rise12 depth + 10 tide + 4 bow — measured from the bow roller, not the waterline	26ft
Swing radiusthe circle you need clear of other boats and shoal	259ft
Swing diameterif the wind boxes the compass	517ft
Nominal scope	10:1
Effective scopethe usual small-boat setup	10:1
Rode type	rope with chain leader
At the 7:1 overnight standardyou are at or above it	182ft

Charted depth: 12 ft
Tidal range: 10 ft
Bow height: 4 ft
Scope: 10
Rode: rope with chain leader

Measure the rise, not the depth

Scope is rode length divided by the rise — and the rise is not the charted depth. It is depth plus tide plus the height of your bow roller above the water:

12 ft + 10 ft + 4 ft = 26 ft

At 10:1 with rope with chain leader, that is 260 ft of rode.

The bow height is the part people leave out. The anchor holds because of the angle the rode makes at the seabed, and that angle starts where the rode leaves the boat, not where it enters the water.

Tide, before you set

A tidal range of 10 ft means scope calculated at low water shrinks as the tide floods. 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 number nobody calculates: swing radius

259 ft. That is the circle you need clear of other boats, moorings and shoal water — 517 ft across if the wind boxes the compass overnight.

More scope holds better and swings wider. In a crowded anchorage that trade-off, not holding power, is usually what limits you.

Set it, then check it

Back down at low revs until the boat stops moving with the rode taut. Take a transit on shore and watch it for a few minutes. An anchor that has not been set is a heavy object lying on the bottom, and it will tell you so at three in the morning.

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 4 ft of bow height in 12 ft of water, leaving it out overstates your scope by 15%.
Add the tide before you set, not after. 10 ft of range means a 10:1 scope at low water becomes 10:1 at high — anchors drag on the flood, in the dark, when the boat you anchored clear of is suddenly close.
10:1 is a proper overnight scope. The limit on most nights is not holding power, it is the 259 ft swing radius — you need that much clear water in every direction, and so does everyone anchored near you.
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.

Nearby sizes

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