Anchor dragging in an open roadstead: veer or weigh?
MV Sagarika lies at anchor in an open roadstead in 16 m of charted depth with 4 shackles (110 m) of cable out, a scope of about 6:1, in a light northerly. The master fixes position every 15 minutes using two shore transits and a radar range to a charted shore tank. At 02:00 the wind veers to the east-southeast at 30 knots. Over the next 40 minutes the transits show a steady, unidirectional change and the radar range to the tank increases by 0.4 nautical mile on a bearing of 250°(T); the anchor symbol on the electronic chart has not moved. A container vessel lies at anchor 0.6 nautical mile to the west-southwest. Main engine standby is 30 minutes, the bow thruster is unserviceable, and the tidal stream sets 0.8 knots to the southwest. The chief officer proposes veering two more shackles and holding the anchor down rather than weighing. Port VTS requires immediate notification of any vessel dragging, and the master's standing orders say the anchor is to be weighed where the holding ground is doubtful.
1. From the fix data, decide whether Sagarika is dragging or merely swinging, and state the cable scope you would work to if you re-anchored in the same depth.
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Drift check. 0.4 nm in 40 minutes = 0.4 ÷ (40/60) = 0.6 kn along 250°(T). A vessel swinging about a fixed anchor keeps a roughly constant bearing to her own anchor and reverses direction at the ends of the arc. Here the bearing change is monotonic, the radar range to a fixed charted object increases without reversal, and the anchor position is unchanged: that is translation, not yaw. The drift track also matches the resultant of the ESE wind (pushing the ship towards about 290°) and the SW-setting stream (225°), which is the signature of a dragging anchor.
Scope if re-anchoring. Scope is measured from the hawse pipe, so use depth of water plus hawse height: 16 m + about 3 m = 19 m. In 30 knots of wind work to at least 7:1: 7 × 19 = 133 m, i.e. 4.8 shackles, so 5 shackles (137.5 m) as a minimum and 6 shackles if the holding ground is soft. Four shackles (110 m) gives only about 5.8:1 and is inadequate in this weather.
2. Assess the chief officer's proposal to veer two more shackles, taking account of the vessel at anchor 0.6 nm to the west-southwest and the tidal stream. What do you decide?
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Geometry first. The other vessel bears 247.5° at 0.6 nm; Sagarika is drifting on 250° at 0.6 kn. The drift is almost directly towards her, and at 0.6 kn the 0.6 nm gap closes in about one hour. With no thruster and 30 minutes' notice for the main engine, that gap governs the decision, not the cable.
Does veering help? Veering restores holding power only while the anchor is still embedded. Once it has broken out, the anchor is skidding or has picked up debris; 55 m of extra cable will not re-set it quickly, it lengthens the recovery job while the ship drifts, and it raises the risk of crossing the other vessel's cable. The turn of tide is not something to plan around.
Decision. Reject the proposal. Ring standby immediately, weigh anchor, and either re-anchor well clear on 5–6 shackles or stand out to seaward if the wind is still building. Log the decision and the reasons.
3. List the immediate actions and notifications you would take in the first 15 minutes, and the records you would keep.
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- Call the master; ring main engine standby at once rather than waiting for the 30-minute notice.
- Inform port VTS on the designated VHF channel that Sagarika is dragging, giving position, drift track and intentions; broadcast a warning to the anchored vessel and keep a listening watch on her reply.
- Post a lookout forward, exhibit the anchor ball/light as required, and use sound or light warning signals if risk of collision develops.
- Start weighing: heave short, use engines to take the strain off the cable and stem the drift, and be ready to slip if the anchor is fouled (buoy the cable end and record the position).
- Keep the timeline: first fix showing drift, wind and stream observations, orders given, times of engine movements and messages to VTS. These form the basis of the report to the company, flag state and port authority, and are the evidence if the other vessel later alleges a close-quarters incident.