The science of anchoring

Anchoring: a practical guide

The International Convention for the Safety of Life at Sea, 1974, (SOLAS) requires that anchorhandling winches and associated equipment should be installed in accordance with the requirements of a properly recognised classification society. Unless designed for a special purpose, the International Association of Classification Societies (IACS) states that:

‘anchoring equipment is intended for temporary mooring of a ship within a harbour or sheltered area…The equipment is therefore not designed to hold a ship off fully exposed coasts in rough weather or to stop a ship when it is moving or drifting.’

Hidden dangers and holding grounds

The SOLAS requirement for passage planning is from berth to berth, or to an anchorage. It is vital to consult up-to-date charts, sailing directions and navigational warnings when planning each stage of a journey along with local advice, if available. When anchoring, it is essential to check the chart, on the largest scale possible, for hidden dangers such as undersea cables or submerged wrecks that could compromise the plan.

If there are any obstructions on the seabed then a prudent mariner must allow a suitable safety margin between the planned anchorage position and any marked obstructions. Bear in mind that global navigation satellite system positions can be subject to error. The undersea cable, for example, may not actually be in the exact position shown on the chart. The nature of the seabed may also indicate whether there is good holding ground. For example, the holding power of rock and mud is less than shingle and sand.

Be wary of choosing an anchorage in deep water. The IACS requirements for most vessels are that the anchor windlass must be capable of lifting an anchor in water depths of 82.5 metres, ie the weight of the anchor and three shackles of chain. In deeper waters, it may be difficult to raise the anchor. This could potentially damage the windlass and the vessel may have to steam into more shallow waters when the time comes to raise the anchor. If you have to anchor in deep water, you should first confirm whether the vessel has a Deep Water Anchorage (DWA) notation.

Plans and calculations

The holding power of an anchor, whatever the type, depends on the position of the anchor shank and the flukes digging into the seabed. The plan should ensure that enough chain is paid out that the catenary of the chain leaves the anchor stock lying on the seabed. The amount of chain will depend on the depth of water and the strength of any anticipated wind or current.

One formula to use when deciding on the amount of anchor cable to veer is:

This is the minimum recommended amount of cable to use. More anchor chain will ensure there is reduced likelihood that the anchor will drag. However, in a restricted anchorage, it may only be possible to pay out a limited amount of cable because of the vessel’s swing circle. In this case, you should be alert to the increased possibility that the vessel’s anchor may drag and plan accordingly, for example, by keeping the engines on immediate notice.

When planning to anchor, the navigator must also consult weather forecasts, predicted sea and swell conditions and understand the likely maximum currents that may be expected at an anchorage. The IACS has made calculations for anchor sizes depending on windspeed, current and ship size, the details of which are readily available online.

Anchors are designed for use in harbours and sheltered areas. If the vessel is likely to yaw or pitch, then the mariner must be alert to the possibility of dragging. A 40-degree yaw can increase the loading on an anchor by a factor of three.

The calculated under-keel clearance (UKC) must also be considered. The force of the current acting on the vessel will increase as the UKC reduces. Consequently, if there is a tidal range, there may be an increased possibility of the vessel dragging at low water, due to an increase in these forces as the UKC reduces.

Deciding not to anchor in a certain location is also part of the planning process.

Anchors away!

Assuming that, after thorough risk assessment, the Master decides to anchor, the first action is to ensure that everyone involved is properly briefed, (eg which anchor, number of shackles, correct PPE, etc.) As the vessel approaches the anchorage, the anchors are cleared of their sea fastenings.

While it is possible to let go an anchor on the brake from its stowed position (‘from the pipe’), the prudent navigator will walk out the anchor (lower it under power) to just above the water. This ensures that the anchor hasn’t stuck in the hawse pipe and prevents the (admittedly small) chance of damage to the hull. It is good practice to only walk out the anchor when the ship’s speed is very low, in case the brake fails and the anchor is let go (released from the brake) inadvertently. An unplanned dropping of the anchor could cause damage to the windlass, lead to the vessel sheering dangerously or damage subsea installations, such as pipelines or cables.

On larger vessels the anchors can weigh many tens of tonnes, and the prudent navigator will want to minimise the risk of the anchor cable paying out too quickly. This would make applying the windlass brake to control the cable speed very difficult, if not impossible. Larger vessels, typically over about 30,000 tonnes deadweight, should either walk out the anchor to the seabed and then let go or, alternatively, completely walk it out. If the anchor is walked out to the seabed, the chain should be stretched to ensure the anchor flukes penetrate the seabed properly.

Whether an anchor is let go or walked out, the speed of the vessel over the ground must be slow. The vessel should ideally be moving astern when the anchor touches the seabed. Too fast and the anchor will drag and, if walking out the cable, may cause damage to the windlass. However, the vessel’s speed over the ground must not fall to zero. If the speed is zero, the anchor cable will pile up on the seabed and the cable will not be properly stretched. Furthermore, the chain could become tangled, causing problems later on when the anchor is retrieved.

Once anchored the vessel’s swinging circle should be plotted and a safe anchor watch maintained. Finally, don’t forget to exhibit a black ball and the appropriate anchor lights!