Enter the anchor line length and the water depth into the calculator to determine the swing circle radius, or provide any two values to find the missing variable.
Anchor Swing Circle Formula
The calculator runs two modes. The quick estimate adds deployed chain to vessel length. The accurate mode resolves the chain into its horizontal component, then adds the distance from the hawse pipe to the stern.
Quick: R = C + LOA
Accurate: R = sqrt(C^2 - (D + H)^2) + S
Drag circle: R_drag = sqrt(C^2 - (D + H)^2) + G
- R = swing circle radius from the anchor position
- C = chain deployed (1 shackle = 27.432 m)
- LOA = vessel length overall
- D = water depth
- H = hawse pipe height above the waterline
- S = distance from hawse pipe to stern
- G = distance from hawse pipe to GPS antenna
- R_drag = drag circle radius used for GPS anchor watch
The quick mode is conservative because it treats the full chain length as horizontal reach. The accurate mode subtracts the vertical drop from the hawse pipe down to the seabed before computing the horizontal component, which gives a tighter and more realistic radius. The optional drag circle uses the GPS antenna offset instead of the stern offset, so you can compare what the bridge GPS will actually display against the chartwork.
Reference Tables
Use these values to sanity check inputs and to set anchor watch alarms.
| Shackles | Meters | Feet | Fathoms |
|---|---|---|---|
| 3 | 82.3 | 270 | 45 |
| 5 | 137.2 | 450 | 75 |
| 7 | 192.0 | 630 | 105 |
| 9 | 246.9 | 810 | 135 |
| 11 | 301.8 | 990 | 165 |
| Holding ground | Typical scope (chain : depth) | Notes |
|---|---|---|
| Mud and clay | 6:1 | Best holding for stockless anchors |
| Sand | 6:1 to 8:1 | Reliable, watch for ripples and shoals |
| Gravel and shingle | 8:1 | Risk of dragging in fresh winds |
| Rock and coral | Avoid if possible | Anchor may foul or fail to set |
| Heavy weather, any bottom | 10:1 or more | Walk back extra and prepare second anchor |
Worked Examples
Example 1, quick estimate. A 200 m bulker pays out 5 shackles of chain in open water. Chain in meters is 5 × 27.432 = 137.2 m. Swing radius = 137.2 + 200 = 337.2 m. Plot a 337 m circle from the anchor position.
Example 2, accurate mode. Same vessel, depth 40 m, hawse height 3 m, hawse to stern 185 m, hawse to GPS antenna 162 m, with 7 shackles deployed. Chain = 192.0 m. Corrected height = 40 + 3 = 43 m. Horizontal reach = √(192.0² − 43²) = √(36864 − 1849) = √35015 = 187.1 m. Swing radius = 187.1 + 185 = 372.1 m. Drag circle for GPS = 187.1 + 162 = 349.1 m. Set the anchor watch alarm just inside the drag circle.
FAQ
Why is the quick estimate larger than the accurate result? The quick formula assumes the chain lies flat along the seabed. In reality part of the chain rises from the seabed up to the hawse pipe, so the true horizontal reach is shorter than the deployed length.
Should I use swing radius or drag circle on the bridge? Use the swing radius for chartwork and clearance from hazards or other vessels. Use the drag circle, which is centered on the GPS antenna offset, for the anchor watch alarm on the GPS or ECDIS.
Does this account for catenary? No. The accurate mode treats the chain as a straight line from the anchor to the hawse pipe, which is a standard simplification. The result is slightly conservative because real chain sags, putting the bow a little closer to the anchor.
How do I convert shackles? One shackle equals 15 fathoms, 27.432 meters, or 90 feet. The calculator handles the conversion when you select the unit.