Anchor holding power

An anchor's ability to prevent a ship from dragging depends on its design (holding power coefficient), the seabed type, the length of cable paid out (scope), and the chain catenary. IACS Unified Requirement W18 governs anchor equipment selection for classed ships. The officer of the watch must be able to recognise dragging before the vessel reaches a danger position.

Holding power coefficient (HHP)

• · HHP = (maximum horizontal holding load in kN) / (anchor mass in tonnes × g); expressed as a dimensionless multiple.
• · Conventional stockless anchors: HHP 8–10 in mud, 3–5 in hard ground.
• · IACS W18 High Holding Power anchors (e.g. Spek, AC-14, Flipper Delta): minimum HHP 14 certified by class test in soft clay.
• · Super High Holding Power (SHHP) anchors: HHP ≥ 70 (e.g. Stevpris for offshore mooring); not typically used for ship anchoring.
• · IACS UR W18 defines the design anchor mass (equipment number calculation) and requires HHP certification for new-build vessels; replacement anchors must match the certified type.

Seabed types and holding

• Soft mud / silt: flukes penetrate deeply; very high holding once set. Risk: anchor may bury so deep it is difficult to break out.
• Sand: good holding, flukes penetrate well; holding power increases with scope. Most common good anchorage type.
• Sticky clay: excellent holding when flukes penetrate; risk of fluke clogging and poor resetting after dragging.
• Gravel / shingle: moderate holding; anchor may slide. Supplement with extra scope.
• Rock / coral: poor holding; anchor catches on features unpredictably; may cause irretrievable fouling. Avoid if alternatives exist.

Scope ratios and catenary

• · Scope 3:1: calm or sheltered anchorage; minimum acceptable for normal conditions.
• · Scope 5:1: standard open anchorage, Beaufort 4–5.
• · Scope 7:1: fresh to strong wind (Beaufort 6–8); most holding power for the chain mass.
• · Scope 10:1: storm or hurricane conditions; near-maximum benefit from catenary.
• · Chain catenary: the weight and curve of the chain in the water keeps the pull on the anchor nearly horizontal even in surge; this is far more effective than wire rope of the same breaking strength, which has minimal catenary.
• · In very deep water (beyond about 80 m) the chain lies almost straight and catenary benefit is lost; extra scope must be paid out or a sentinel (kellet) weight added to restore the dip.

Dragging-anchor warning signs

• · GPS position drift downwind/current consistently over successive 15-minute position fixes.
• · ECDIS anchor watch alarm triggered (set radius typically 0.1–0.3 nm around the anchor position).
• · Chain snubbing sharply — the anchor is tripping and resetting repeatedly rather than holding steadily.
• · Vibration or grinding felt through the hull or chain: anchor dragging over hard bottom.
• · Bearings to fixed objects ashore (lights, towers, transit marks) changing steadily on the same side.
• · Rapid or continuous depth change on the echo sounder as the ship moves to shallower or deeper water.

Actions on dragging

• · Alert the Master immediately; call standby on the engines.
• · Veer more cable if safe to do so (increases catenary and may re-set the anchor).
• · Use engines ahead and slightly into the wind to reduce tension on the cable and assist re-setting.
• · If available, let go the second anchor; position the second anchor 1–2 shackles ahead of the lead anchor and use both in a tandem arrangement.
• · If dragging cannot be stopped and a danger position is threatened, heave up and proceed to a safe anchorage or sea.

Sources: IACS Unified Requirement W18 — Anchoring equipment (anchor mass and chain cable); Det Norske Veritas (DNV) Rules for Ships Pt.3 Ch.3 — Anchor and chain equipment; US Navy Anchor Handling Manual (NAVSEA S0400-AA-PRO-010, 2019 ed.). See also equipment number, scope, passage planning.