What scope ratio should I use?

The conventional working scope is 5–7× water depth (measured at the hawsepipe) for chain in calm conditions, increasing to 7–10× in heavy weather and 10–12× for storm or hurricane anchorage. The greater the scope, the flatter the catenary, and the less vertical pull is applied to the anchor — increasing holding power. Wire rope (rare on commercial vessels) requires roughly twice the scope of chain because it does not contribute weight to the catenary.

How is swing circle calculated?

Swing circle is the maximum distance the stern can sweep around the anchor. It equals the chain veered + the vessel's length overall, plus a small allowance for chain catenary lift. A 250 m vessel with 8 shackles (220 m) of chain in 30 m water has a swing circle of roughly 470 m radius. Always check the swing circle clears all charted hazards and adjacent vessels at every wind direction.

What is a shackle of chain?

Standard length: 1 shackle = 27.432 metres (15 fathoms = 90 ft). UK Royal Navy uses 12.5 fathoms (~22.86 m). Each shackle is colour-coded with paint marks on the links — typically a red link, white links, or numbered tags so the bridge knows how much chain has been veered.

Does this calculator account for tidal range?

No. Compute scope using the maximum expected water depth at the anchorage (high water + tidal range + sea state surge). If you anchor at low water with a 5× scope, that scope shrinks to 3× at high water — likely insufficient holding.

Anchor scope & swing circle

Plan an anchorage by working out scope ratio (chain length ÷ water depth) and the swing circle the vessel will sweep. Conventional working scope is 5–7× in calm, 7–10× in heavy weather, 10–12× in storm anchorage. Always compute against the maximum expected water depth at the anchorage (high water + surge).

Water depth (m)

Measured at the hawsepipe at HW.

Chain veered (shackles)

1 shackle = 27.432 m (15 fathoms).

Vessel LOA (m)

Result

Chain veered: 219 m
Scope ratio: 7.3× water depth
Swing circle radius: 469 m
Swing circle diameter: 939 m

Scope is in the working range. Increase to ≥7× if heavy weather is forecast, or ≥10× for storm conditions.

Notes & caveats

· The calculator assumes chain (not wire). Wire requires roughly 2× the scope because the wire does not contribute weight to the catenary.
· Swing radius approximation: chain length + LOA. The actual chain catenary lifts above the seabed under heavy load, slightly shortening the effective horizontal distance — the calculator gives the maximum.
· The 5–7×, 7–10×, 10–12× ranges are conventional seamanship; consult the vessel-specific anchoring instructions in the bridge procedures manual where provided.
· Holding power also depends on bottom type (mud > sand > gravel > rock) and anchor type. This calculator gives only the geometric scope.

Sources: standard seamanship; IACS UR A1 (anchoring equipment); see also scope of cable, anchor watch, running moor.