Shiphandling, Manoeuvring and Anchoring

Prerequisites

You should have completed the earlier lessons in this course on bridge watchkeeping, passage planning and the COLREGs, and you should be comfortable with basic stability terms (draught, trim, block coefficient). From yacht or small-craft handling you already know propeller walk, the effect of wind on a high-sided hull and how to anchor with a scope of chain. This lesson scales those ideas up to a ship of 100 to 400 metres that answers her helm slowly, carries her way for miles and has an anchor weighing several tonnes.

At OOW level you will rarely berth the ship yourself, but STCW Table A-II/1 requires you to know the effects of deadweight, draught, trim, speed and under-keel clearance on turning circles and stopping distances, the effects of wind and current, squat and shallow water, and the proper procedures for anchoring and mooring. The MCA oral examiner will test all of them.

Learning Objectives

Explain the pivot point and how it moves with headway and sternway.
Define advance, transfer, tactical diameter and drift angle, and read them from the ship's manoeuvring data.
Describe stopping distances, the crash stop and why a large ship cannot stop quickly.
Explain propeller walk on single and twin screw ships.
Explain squat, bank effect and ship-to-ship interaction, and how to reduce them.
Describe the effect of wind and current on a large ship at low speed.
Plan and carry out an anchoring, choosing between letting go and walking back, and keep an anchor watch.
Prepare the ship for pilot boarding safely.

Why Big Ships Handle Differently

A loaded VLCC of 300,000 tonnes displacement at 15 knots carries an enormous amount of kinetic energy. Her rudder is large but small compared with her mass, so her rate of turn builds slowly and her momentum keeps her moving long after the engine is stopped. Three things follow:

Everything takes time. You have to think ahead in minutes and miles, not seconds and metres.
Low speed means poor steering. The rudder works because of the water flowing past it. Slow down too far without engine power and you lose control, especially in wind.
Shallow and narrow water changes everything. The ship's own pressure field reacts with the seabed, the banks and other ships.

Your references are the wheelhouse poster, pilot card and manoeuvring booklet, which give turning circles, stopping distances and engine data at different draughts. Read them before you need them.

The Pivot Point

When a ship turns, she rotates about a point on her centreline called the pivot point. It is the point about which the bow and stern appear to swing as the ship turns. It is not a fixed point like the centre of gravity: it moves as the ship gains or loses way.

Stopped in the water, with no headway or sternway, the pivot point lies close to the centre of gravity, roughly amidships.
Making headway, the pivot point moves forward, typically to about a quarter of the ship's length from the bow.
Making sternway, it moves aft, to about a quarter of the length from the stern.

The diagram below shows why this matters.

Pivot point position with headway and sternway, and the lever arms for rudder and bow thruster

The turning effect of any lateral force is the force times its distance from the pivot point. Going ahead, the rudder at the stern acts on a long lever arm (about three quarters of the ship's length), so the stern swings out strongly. A bow thruster or a tug forward, close to the pivot point, has very little lever and so little turning effect. Going astern the opposite is true: the rudder is now near the pivot point and almost useless, while a bow thruster or forward tug has a long lever and is very effective.

Practical consequences for the OOW:

When turning ahead, the stern swings out to the side opposite the turn. In a narrow channel, near a buoy or close to another ship alongside, watch the stern, not just the bow.
Bow thrusters lose effect quickly as speed rises; most are of little use above about 2 to 3 knots. Check your ship's thruster data.
When the ship is moving astern, steering with the rudder is unreliable. Expect the stern to seek the wind and the propeller to walk it.

The Turning Circle

The turning circle is the path followed by the ship's centre of gravity when the rudder is put hard over and held there. It is measured in trials, usually in deep water at full sea speed, both to port and starboard, and the results go on the wheelhouse poster.

The diagram above shows the terms you must know:

Term	Definition
Advance	Distance travelled along the original course from the point where the wheel is put over to the point where the heading has changed by 90°
Transfer	Distance measured at right angles to the original course from that same starting point to the 90° point
Tactical diameter	Distance at right angles to the original course from the starting point to the point where the heading has changed by 180°
Final (steady) diameter	Diameter of the circle once the ship settles into a steady turn; a little less than the tactical diameter
Drift angle	Angle between the ship's fore-and-aft line and the tangent to her track; the bow points inside the circle and the stern sweeps outside it

IMO's Standards for Ship Manoeuvrability (Resolution MSC.137(76)) set limits for ships of 100 m and over: the advance should not exceed 4.5 ship lengths and the tactical diameter should not exceed 5 ship lengths. A typical full-form ship turns tighter than that, with an advance of about 3 to 4 lengths.

What changes the turning circle

Speed. At steady speed the turning circle in deep water is roughly the same size at any speed; what changes is how quickly you go round it. But if you increase engine power while the rudder is hard over (a "kick ahead" from slow speed), the extra flow over the rudder tightens the turn without much increase in headway. That is the basic trick of low-speed handling.
Draught and trim. A loaded ship has a larger turning circle than one in ballast. Trim by the stern usually increases the diameter; trim by the head makes the ship turn more readily but steer badly.
Shallow water. In shallow water (depth less than about twice the draught) the turning circle can double in size. This is the single most important fact for confined waters: the ship turns far wider than the poster suggests.
Wind and current. A turn in a current is carried bodily with the water; a turn across the wind is affected by the windage of the hull and accommodation.

Using it in practice

Advance and transfer place the wheel-over point in pilotage waters, so the ship arrives on the new track rather than overshooting it, and they tell you how much sea room a large collision-avoidance alteration needs.

Worked example: a 90° turn

Your container ship is 280 m long. The pilot card gives an advance of 840 m and a transfer of 420 m at full sea speed, deep water, loaded. You plan to alter 90° to starboard at a channel junction.

Advance is 3 ship lengths, transfer 1.5 lengths.
The wheel-over point must be about 840 m (0.45 nm) before the intersection of the two tracks, measured along the current track, if you use full helm. With a more gentle rudder angle, such as 15° in a planned turn, both distances will be greater, so allow more.
If the depth there is only 1.3 times your draught, expect the advance to grow considerably. Reduce speed earlier, use more rudder, and consider a controlled rate-of-turn alteration rather than a "hard over" turn.

Stopping the Ship

Stopping distance

A large ship's stopping distance is measured in ship lengths, and in nautical miles at sea speed. IMO's manoeuvrability standards set the track reach in a full astern crash stop at not more than 15 ship lengths, though the Administration may accept up to 20 lengths for very large ships. A loaded VLCC at full sea speed may take 2 to 3 nautical miles and 15 minutes or more to come to rest.

There are three ways to slow down, and they differ greatly:

Method	What happens	When to use it
Reduce speed in stages	Ship slows over many miles; full steering throughout	Normal arrival, approaching a pilot station or anchorage
Stop engine and coast	Ship slows slowly by friction alone; steering fades as speed falls	Gentle approaches, when distance is ample
Crash stop (full astern)	Shortest distance, but steering is lost and the ship sheers unpredictably	Emergency only, to reduce the force of an unavoidable impact

The crash stop

When a crash stop is ordered, the main engine goes from full ahead to full astern as quickly as the machinery allows. On a slow-speed diesel the engine must first stop, then be restarted in reverse on compressed air, so there is a delay before any astern power is felt. Once the propeller turns astern the flow over the rudder is disturbed, steering is lost, and the propeller's transverse thrust (see below) swings the bow, often to starboard on a right-handed single screw ship. The ship may end up well off her original track and swinging.

Know the limits of your own engine: how many air starts are available, the time from full ahead to full astern, and any barred speed range. Many collisions have been made worse because the OOW relied on a crash stop when a large, early alteration of course would have avoided the danger entirely. In open water, a turn usually avoids the danger in less distance than a stop: compare your advance (perhaps 3 to 4 lengths) with your crash-stop reach (perhaps 10 to 15 lengths).

Propeller Walk (Transverse Thrust)

A propeller does not only push the ship forward or backward; it also pushes the stern sideways. This is transverse thrust, or propeller walk, and you met it on small craft. The cause is mainly the difference in water pressure and flow between the upper and lower blades.

For a single, right-handed fixed-pitch propeller (turning clockwise when going ahead, seen from astern), the most common arrangement:

Going ahead, the stern is pushed slightly to starboard, so the bow tends to swing to port. The rudder easily counters this once the ship has way on.
Going astern, the effect is much stronger: the stern walks to port and the bow swings to starboard. With little or no headway the rudder cannot counter it.

That is why a ship with a right-handed propeller prefers to berth port side to: the astern kick to stop her also pulls the stern towards the berth.

Controllable pitch propellers (CPP) turn in one direction all the time and the blades change pitch to go astern. Most are left-handed (turning anticlockwise when viewed from astern), so going astern the stern walks to starboard. Always check the pilot card: it states the direction of rotation and the effect.

Twin screw ships usually have outward-turning propellers. Their transverse thrust cancels out when both go the same way, and using one ahead and one astern gives a strong turning couple with little headway.

Squat and Shallow Water

What squat is

When a ship moves through water, the water passing under and around her hull speeds up. Faster-moving water has lower pressure (Bernoulli's principle), so the ship sinks bodily and changes trim. This reduction in under-keel clearance is squat. It is small in deep water but becomes dangerous in shallow water and narrow channels, where the water has less room to flow past.

Squat in shallow water and bank effect in a channel

As shown in the upper part of the diagram above:

Full-form ships (block coefficient above about 0.7, such as tankers and bulk carriers) squat more at the bow: they trim by the head.
Fine-form ships (block coefficient below about 0.7, such as container ships) tend to squat more at the stern.
Squat is roughly proportional to the square of the speed. Halve your speed and you cut squat to about a quarter.
Squat is greater in a confined channel than in open shallow water, roughly double.

Estimating squat

Barrass's widely used approximation, taught for the MCA orals, is:

Maximum squat (metres) = Cb × V² ÷ 100 in open shallow water
Maximum squat (metres) = Cb × V² ÷ 50 in a confined channel

where Cb is the block coefficient and V is the speed through the water in knots.

Worked example. A bulk carrier with Cb 0.82 is in a dredged channel at 10 knots. Squat ≈ 0.82 × 100 ÷ 50 = 1.64 m. If her static under-keel clearance was planned at 1.8 m, she is left with about 0.16 m, which is far too little. Slowing to 7 knots gives 0.82 × 49 ÷ 50 = 0.80 m of squat, leaving about 1.0 m. That is why passage plans set speed limits as well as minimum under-keel clearance, and why company UKC policies include squat.

Signs of shallow water: the ship becomes sluggish on the helm, speed drops for the same revolutions, vibration increases and the wake may turn muddy.

Bank Effect

In a narrow channel, a ship running close to one bank experiences an uneven pressure field. Water is squeezed between the bow and the near bank, building a high-pressure "cushion" that pushes the bow away from the bank. Further aft, water accelerates between the quarter and the bank, creating low pressure that sucks the stern towards the bank. The lower part of the diagram above shows this.

The result is that the ship sheers away from the nearer bank, sometimes violently, towards the middle of the channel or across it into oncoming traffic. To counter it you keep the ship near the centre of the channel where possible, slow down (the forces again rise with the square of speed), and hold some helm towards the near bank to balance the sheer. Experienced pilots sometimes use bank effect deliberately to help a ship round a bend, but the OOW should treat it as a hazard to anticipate.

Interaction Between Ships

Every moving ship carries a pressure field with high pressure at the bow and stern and low pressure along the sides amidships. When two ships pass close, especially in shallow or confined water, these fields act on each other. This is interaction.

Overtaking in a channel

Overtaking is the most dangerous case, because the ships stay close together for a long time:

As the overtaking ship's bow approaches the other ship's stern, the two high-pressure areas push the overtaken ship's stern away and the overtaking ship's bow outwards.
When the ships are alongside, the low-pressure areas amidships draw them towards each other bodily.
As the overtaking ship's bow passes the other ship's bow, the overtaken ship's bow may be drawn towards the overtaking ship's low-pressure side and then pushed off, so she can sheer across the overtaking ship's stern or bow.

Head-on passing in a channel

The bow fields first push the bows apart; as the sterns approach, the sterns may be drawn together and the bows swing towards each other, while each ship is also feeling her own bank.

How to reduce interaction

Reduce speed before the ships come close, not while alongside. Interaction forces rise with the square of the speed, but you need enough speed to keep steering.
Increase the lateral distance as much as the channel allows.
Keep the encounter short. When overtaking, agree by VHF (through the pilots or VTS where applicable) and have the overtaken ship slow down so the overtaking ship passes with a good speed difference and spends less time alongside.
Be ready on the helm and engine, with a helmsman in hand steering and the master on the bridge.
Remember small craft: a tug or launch close under the bow of a large ship can be drawn in and run down or capsized.

Wind and Current

Current

A current moves the whole body of water, and the ship moves with it. In open water this is simply set and drift for the navigator. At low speed in confined waters it becomes a handling problem:

Stemming the current (heading into it) gives you good steering at low speed over the ground, because the water flows past the rudder even when you make little progress. That is why ships try to berth and anchor heading into the stream.
A following current needs more speed through the water to steer, so stopping distances grow; a cross-current sets the ship sideways, more so as speed falls.

Wind

The wind acts on everything above the waterline. Container ships, car carriers, cruise ships and ships in ballast have very large windage. Some general effects:

At speed, the hull grips the water well and the main effect is leeway and some weather or lee helm, depending on where the windage lies relative to the pivot point. A ship with high accommodation aft tends to have her stern pushed downwind, so the bow comes up into the wind. The OOW applies helm and a leeway correction to the course.
When stopped, most ships lie with the wind roughly abeam or slightly abaft the beam, drifting to leeward.
Going astern, the stern tends to seek the wind (back into it), because the pivot point moves aft and the bow, with less grip, blows off.
At low speed, a strong beam wind can overpower the rudder entirely. Know the wind speed at which your ship can no longer hold her heading at manoeuvring speed; the pilot card or master's standing orders may state it.

Watch the ECDIS or radar for the difference between heading and course over ground and correct for it.

Anchoring a Large Ship

Anchoring a ship is a planned operation, not a quick stop. The master usually conns while the OOW (or chief officer) leads the anchor party forward, or the OOW assists on the bridge with position fixing, engine orders and communications. You must understand the whole sequence.

Choosing the position

The master or passage plan will choose the anchorage, considering:

Depth suited to the ship and her windlass, and good holding ground (mud or sand is good; rock is poor).
Sufficient swinging room: the radius of the swinging circle is roughly the length of cable paid out plus the ship's length, and it must clear other ships, shallows and charted cables or pipelines.
Shelter from the expected wind and sea, and the effect of the tidal stream.
Any local regulations: designated anchorages, VTS permission, prohibited anchoring areas.

On the chart or ECDIS, plan an approach heading into the stronger of wind or stream, the let-go position, distance-to-go marks and the swinging circle.

Scope

The diagram above shows the essentials. The anchor holds only if the pull on it is close to horizontal; that needs enough chain for part of it to lie on the seabed, and for the weight of the chain (the catenary) to absorb shock loads. Cable is measured in shackles: one shackle is 15 fathoms, or 27.5 metres. Each shackle is marked so the anchor party can report the length paid out.

Common rules of thumb for the number of shackles:

1.5 × √depth (in metres) for normal conditions. In 25 m of water, 1.5 × 5 = 7.5, so about 7 to 8 shackles.
More chain in strong winds, strong streams, poor holding or for a large ship; less only if swinging room forces it.

Letting go versus walking back

Method	How it is done	When it is used
Letting go	Anchor and cable run free under gravity, controlled by the windlass brake	Moderate depths, small to medium ships, usually up to about 20 to 25 m of water
Walking back	Windlass motor engaged, anchor lowered under power ("in gear") all the way, or to just above the bottom, then let go or walked out	Deep water, large ships (VLCCs, large bulk carriers), rocky bottom; anchors and cable are very heavy and a free fall could run away with the cable or damage the windlass

The danger of letting go in deep water is that a heavy anchor gains so much speed that the brake cannot stop it. The cable may run out to the bitter end, the brake linings can burn out, and people forward can be injured. Company procedures normally require walking back above a certain depth; follow them. Also check the windlass's rated lifting capacity, which limits the depth from which you can recover the anchor.

The anchoring sequence

Prepare the anchor in good time: windlass power on, anchor cleared away (lashings and securing devices removed), anchor walked out of the hawse pipe until it is clear, brake on and windlass out of gear (if letting go). Anchor party in hard hats, goggles and safety shoes, with a radio checked.
Approach on the planned track at low speed, heading into the wind or stream. Fix the position frequently and report distance to go.
Take off headway so the ship is almost stopped over the ground, with a slight sternway, at the let-go position.
Let go (or walk back) on the master's order. The anchor party reports the cable "on the bottom" and then the number of shackles as it pays out ("two shackles in the water").
Pay out cable steadily as the ship drops back, so it lies along the seabed rather than piling on top of the anchor. Avoid snubbing it.
When the planned scope is out, apply the brake. The anchor party watches for the cable to rise and tighten, then slacken as the ship comes back. When the cable leads steadily and the ship is riding to it, the anchor is "brought up".
Exhibit the anchor ball (or anchor lights by night) and switch off the steaming lights. Record the time, position, depth and scope in the log. Inform VTS or port control if required.

The anchor watch

At anchor the ship is still at sea and the OOW's responsibilities continue. MGN 315 and the ICS Bridge Procedures Guide expect the OOW to:

Fix the position frequently by more than one method (radar ranges and bearings of fixed objects, GNSS), and check that the ship stays within her swinging circle. Set an anchor watch or guard zone on ECDIS and radar.
Watch the weather, tide and sea state, and the readiness of the engine. Warn the master early of a rising wind or a change in forecast.
Keep a proper lookout and show the correct lights, shapes and sound signals (COLREG Rules 30 and 35).
Check the cable lead and tension; a heavy surging cable or vibration through the deck suggests the anchor may be dragging.

Signs of dragging: the position moving steadily out of the swinging circle, bearings changing steadily in one direction, the ship lying beam-on to the wind rather than heading into it, and the cable leading out tight and then jerking.

If the ship drags: call the master, call the engine room for immediate readiness, pay out more cable if room allows, warn ships nearby, and be ready to heave up and re-anchor or put to sea. Many large ships in a severe gale are safer heaving up and steaming offshore than relying on the anchor; anchors on large ships are not designed to hold in very strong wind and sea.

Pilot Boarding

Pilot transfer is a high-risk operation that has killed pilots. SOLAS Chapter V, Regulation 23, sets out the requirements for pilot transfer arrangements, supported by IMO Resolution A.1045(27) and the IMPA poster on required boarding arrangements.

Key points for the OOW

Rig the ladder on the side the pilot asks for, normally the lee side, at the height requested, and well clear of any discharges.
A pilot ladder alone may be used for a climb of not less than 1.5 m and not more than 9 m. Above 9 m, a combination arrangement (accommodation ladder with pilot ladder) is required. Check the current SOLAS requirement and your ship's arrangement.
The ladder must be certified, in good condition, secured to strong points (not to the rails) and with spreaders correctly spaced. Each step must rest firmly against the ship's side.
Provide a lifebuoy with self-igniting light, a heaving line, adequate lighting at night (lighting the ladder and the boat, not dazzling the pilot), and stanchions and handholds at the point of access.
A responsible officer in radio contact with the bridge must supervise the transfer and escort the pilot to the bridge.
On the bridge: agree the speed and heading for the pilot boat (commonly about 6 to 8 knots with a lee made by altering course), keep a good lookout for the boat, and avoid sudden helm or engine movements while it is alongside.
Inspect the ladder before every use and log the inspection.

Once on the bridge, the master and pilot exchange information (pilot card, pilotage plan, defects); the OOW remains responsible for the watch.

Worked Example: Arrival, Anchoring and a Rising Wind

You are OOW on a loaded Panamax bulk carrier (225 m, draught 13.5 m, Cb 0.84, single right-handed propeller) arriving at an outer anchorage. The master has the con; you are fixing the position and handling engine orders and communications.

Approach. The planned anchor position is in 22 m of water, mud. The approach track heads 260°. The wind is from the west at force 5 and the stream is setting 080° at 1 knot, so the approach is into both. The master reduces from 12 knots at 5 nm out, in stages, to dead slow at 1 nm. You report distance to go every cable from 5 cables and the ship's speed over the ground from GNSS.

Letting go. The depth is 22 m, within the company's limit for letting go. With the ship stopped over the ground and a slight sternway from a short astern movement (the bow swinging to starboard with transverse thrust, as expected), the master orders "let go starboard anchor". The anchor party reports cable on the bottom at two shackles. Using the rule of thumb, 1.5 × √22 ≈ 7 shackles; the master chooses 8 because the forecast shows the wind increasing. The ship brings up with 8 shackles in the water.

Anchor watch. You plot the anchor position and draw a swinging circle with a radius of 8 × 27.5 m + 225 m ≈ 445 m (about 2.4 cables), set an ECDIS anchor watch alarm and a radar guard zone, and take radar ranges of two headlands every 15 minutes.

Rising wind. At 0300 the wind backs south-westerly and rises to force 8. Your fixes show the ship has moved 1.5 cables outside the swinging circle in the last 20 minutes and is lying beam-on. You call the master, ask the engine room for immediate readiness, and warn the ship anchored 6 cables to leeward on VHF. The master decides to heave up and steam to sea until the gale passes. During heaving, you use engine ahead slow to ease the weight on the cable, as the master orders, so the windlass is not overloaded.

Common Mistakes

Trusting deep-water manoeuvring data in shallow water. Turning circles and stopping distances can double. Slow down early and allow more room.
Relying on a crash stop instead of a turn. In open water, a bold early alteration usually avoids danger in far less distance than stopping.
Ignoring the stern. When turning ahead, the stern swings out. Near buoys, banks and other ships, the stern is often what hits.
Going too fast in a channel. Squat, bank effect and interaction all rise with the square of the speed. Halve the speed, quarter the effect.
Letting go in too deep water. A runaway cable can destroy the windlass and injure the anchor party. Follow the company limit and walk back.
Passive anchor watch. Setting an alarm and not checking fixes, or not acting on a dragging trend, has led to many groundings.
Poor pilot ladder rigging. Ladders tied to rails, missing spreaders and no officer in attendance are among the most common port state control findings, and pilots have died as a result.

Summary

The pivot point lies about a quarter of the length from the bow with headway, and a quarter from the stern with sternway; it decides which controls are effective.
The turning circle is described by advance, transfer and tactical diameter; IMO limits are an advance of 4.5 L and tactical diameter of 5 L. Shallow water greatly enlarges it.
Stopping takes many ship lengths; the crash stop loses steering. A turn is usually the faster way to avoid danger.
A right-handed single screw ship's stern walks to port when going astern; check the pilot card for CPP and twin screw arrangements.
Squat ≈ Cb × V² ÷ 100 in open shallow water and about double in a confined channel; it rises with the square of speed.
Bank effect sheers the ship away from the near bank; interaction draws ships together when alongside and can make them sheer. Slow down beforehand and keep apart.
Anchor into wind and stream, with scope of about 1.5 × √depth shackles or more, letting go in moderate depths and walking back in deep water. Keep an active anchor watch.
Rig pilot ladders exactly as SOLAS V/23 requires, with a responsible officer in attendance.

Check Your Understanding

Where is the pivot point of a ship making headway, and why does that make a bow thruster ineffective at speed?

Answer: About a quarter of the ship's length from the bow. The bow thruster is close to the pivot point, so its lever arm, and therefore its turning effect, is small. Thrusters also lose effect as water flows past the hull at speed.

Define advance, transfer and tactical diameter.

Answer: Advance is the distance along the original course from where the wheel is put over to where the heading has changed by 90°. Transfer is the distance at right angles to the original course to that 90° point. Tactical diameter is the distance at right angles to the original course to the point where the heading has changed by 180°.

A bulk carrier with a block coefficient of 0.8 is in a confined channel at 9 knots. Estimate her maximum squat, and say what happens if she slows to 6 knots.

Answer: Confined channel: 0.8 × 81 ÷ 50 ≈ 1.3 m. At 6 knots: 0.8 × 36 ÷ 50 ≈ 0.58 m. Squat varies with the square of speed, so a modest reduction in speed gives a large gain in under-keel clearance.

Your ship is running close to the starboard bank of a narrow channel. What will happen and how do you counter it?

Answer: Bank effect: high pressure between the bow and the bank pushes the bow to port, away from the bank, while low pressure aft draws the stern to starboard towards the bank, so the ship sheers to port. Reduce speed, move towards the centre of the channel, and hold some starboard helm (towards the bank) to balance the sheer.

Why is a crash stop often a poor choice for avoiding a collision in open water?

Answer: A loaded large ship may take 10 to 15 ship lengths or more to stop, steering is lost once the engine goes astern, and the ship sheers unpredictably. A bold early alteration of course needs only about the advance (3 to 4 lengths) to clear the danger and keeps the ship under control.

Which way does the bow swing when a ship with a single right-handed fixed-pitch propeller goes astern from rest?

Answer: The stern walks to port, so the bow swings to starboard.

When should you walk the anchor back rather than letting it go?

Answer: In deep water (beyond the company's limit, often around 20 to 25 m), for very large ships with heavy anchors and cable, or on a rocky bottom, because a freely running anchor may gather so much speed that the brake cannot hold it.

List four signs that the ship may be dragging her anchor.

Answer: Any four of: position moving steadily out of the swinging circle; bearings or ranges of fixed objects changing steadily; the ship lying beam-on instead of head to wind; the cable leading out taut and jerking or vibration felt through the deck; the ECDIS or radar anchor watch alarm activating.