Anchoring a Ship: Scope, Cable Length, Swinging Circle -

Introduction

Anchoring holds the ship in a chosen position using the anchor and its cable. Good anchor work needs the right position, enough cable, and a careful check that the anchor is holding.

Choosing the Anchor Position

Choose a location dedicated for an anchorage.
Check the available depth in relation to the draught. Areas with too much depth cannot be used for anchoring, and areas with too little depth for the draught cannot be used either.
The area should be clear of cables, pipelines and any other underwater obstructions.
Type of seabed: preferably soft and cohesive, for example mud, clay or firm sand.
The direction and speed of the current.
The wind direction, force and state of the sea.

Figure 1. Neither too little nor too much water under the ship is suitable for anchoring.

Cable Considerations

The anchor is most efficient when it is subjected to a horizontal pull by the cable on the seabed. When the angle of inclination of the cable at the anchor is less, more holding power is exerted by the anchor. Thus when the anchor is dragging, drop more cable to increase the holding power.

Figure 3. The smaller the angle of the cable at the anchor, the more holding power the anchor gives.

Why a Longer Cable Holds Better

Let h be the vertical distance from the hawse pipe to the seabed, and L the length of cable paid out. If the cable is taken as a straight line (ignoring the weight of the chain), the angle θ it makes with the seabed is given by:

sin θ = h / L = 1 / scope

The more scope, the smaller the angle:

Scope 3: θ = arcsin (1/3) ≈ 19.5°
Scope 5: θ = arcsin (1/5) ≈ 11.5°
Scope 7: θ = arcsin (1/7) ≈ 8.2°

A smaller angle means a more horizontal pull on the anchor. In practice the weight of the chain also lies along the seabed and flattens the angle further.

Amount of Cable to Use

Factors to consider:

Holding characteristics of the anchor
Length of stay
Strength of wind, tidal stream or current, and sea or swell
Swinging room
Type of cable
Depth to draught ratio

The length of cable, in shackles, can be found with these formulas:

Standard condition: [(Depth of water in metres × 2) + 90] / 27.5
Strong wind or current: [(Depth of water in metres × 3) + 140] / 27.5

One shackle of cable is 27.5 m long, so dividing by 27.5 gives the answer in shackles. For example, in 20 m of water:

Standard: (20 × 2 + 90) / 27.5 = 130 / 27.5 ≈ 4.7 shackles

Strong wind or current: (20 × 3 + 140) / 27.5 = 200 / 27.5 ≈ 7.3 shackles

In practice you would round up to the next whole shackle, so 5 shackles in the standard case and 8 in the strong wind case.

Scope

Scope is the ratio of the length of the anchor cable to the vertical distance from the hawse pipe to the seabed. The more the scope, the better the anchor will hold the ship.

Scope = L / h, where h = depth of water + height of the hawse pipe above the water

Figure 2. The scope is the ratio of the length of cable paid out to the vertical distance from the hawse pipe to the seabed.

For example, with 20 m depth and a hawse pipe 10 m above the water, h = 30 m. With 5 shackles (137.5 m) out, the scope is 137.5 / 30 ≈ 4.6.

Dragging of the Anchor

An anchor is said to be dragging when, instead of holding, the ship drags it along the bottom. This is due to strong current, strong wind or insufficient cable paid out.

How to Find Out if the Anchor Is Dragging

By watching the bearing of two fixed lights or objects in line.
By observing the anchor cable. If it comes to a long stay, then a short stay, then a long stay repeatedly, the vessel is dragging.
By plotting the fix of the ship and confirming that her position is within the swinging circle.
By checking the speed over ground on GPS.
By checking the distances of objects ahead and astern of the vessel.

Figure 5. Two fixed lights or objects that were in line show movement when they open out.

Swinging Circle

Radius of swinging circle = Length of cable in metres + LOA

Sometimes, instead of LOA, the distance is measured up to the bridge, because the equipment measures everything from the CCRP, which is typically on the bridge:

Radius of swinging circle = Length of cable in metres + Distance from the bow to the bridge (CCRP)

The reasoning is simple. The anchor is the centre. The hawse pipe can be as far as the length of cable from the anchor, and the stern is a further ship length behind the hawse pipe. For example, with 5 shackles (137.5 m) out and an LOA of 200 m, the radius is 137.5 + 200 = 337.5 m. In practice the true horizontal distance to the hawse pipe is a little less than the cable length, so this is a safe figure.

Figure 4. As wind and current change, the ship can swing anywhere inside the circle.

More You Should Know

Ground Tackle

The anchor and its fittings are called ground tackle. This includes the anchor, the cable, the windlass that heaves it, the hawse pipe through which the cable runs, the chain stopper that holds it, and the chain locker where it is stowed. Anchor cable is measured in shackles of 27.5 m, and the cable is marked so the deck party can report how much is out.

Anchoring Procedure

Plan: choose the position from the chart, check depth, seabed, swinging room and obstructions, and decide how much cable to use.
Prepare: have the engine room ready, make sure communications with the forecastle work, and make the anchor ready for letting go, with the brake on.
Approach: come in slowly, heading into the wind or current, whichever has more effect on the ship.
Let go: when the ship is stopped over the ground (or just moving astern) at the chosen position, let go the anchor and pay out the cable while the ship eases astern.
Check holding: watch the cable come taut, check bearings and GPS, record the position, time and length of cable, and set an anchor watch.

Anchor Lights and Shapes

Under the Collision Regulations (Rule 30), a ship at anchor shows a black ball in the fore part by day. At night a ship of 50 m or more shows two all-round white lights, one in the fore part and one near the stern and lower than the forward one. A ship of 100 m or more must also use her working lights to illuminate her decks. A ship under 50 m may show one all-round white light where it can best be seen.

Figure 6. Ships of 50 m or more show two white lights at night. Smaller ships show one.

Anchor Watch

Use the GPS anchor alarm and radar range rings, and check visual bearings at regular intervals.
Watch the wind, the current and the weather forecast, since a change in any of them can make the anchor drag.
Keep the engines ready at short notice when conditions are bad.

If the Anchor Drags

Inform the master and have the engines made ready.
Pay out more cable to increase the holding power, and use the engines to take the strain off the anchor.
If that does not stop the dragging, heave up and re-anchor, or put down a second anchor as the master decides.
Warn nearby ships and the port control or VTS if there is any risk to them.

Weighing Anchor

Heave in slowly with the engines ready. As the cable comes up and down (vertical), the ship is nearly above the anchor. When the anchor breaks out of the bottom, it is reported as anchor aweigh, and from then on the ship is no longer held and must be controlled by the engines and helm.

Conclusion

A ship anchors safely when the position is well chosen, enough cable is out for the conditions, and the holding is checked and kept under watch. Knowing the scope and the swinging circle tells you how much room you need, and the dragging checks tell you at once when something changes.

Anchoring a Ship: Scope, Cable Length, Swinging Circle