1. Overview — Slow Speed Manoeuvres
Source: NAYAK, Capt. Santosha K. Theory and Practices of Marine Pilotage. Chapter 14 — Slow Speed Manoeuvres.
Syllabus: Anexo 2-B, Área II (Arte Naval), item 8 (NAYAK), Cap. 14 — manobras em baixa velocidade. Cross-list Área III, item 6.
Handling a ship that has ample speed and full use of engines and helm is one thing; handling her at slow and very slow speed is quite another, and far more critical. These manoeuvres are demanded inside harbour limits, at very close range from other vessels and shallow patches, and their whole object is a safe berthing or un-berthing with good control of the ship's movement.
Higher speed gives better control, yet it cannot be held inside the harbour. For the purpose of this chapter slow speed means a speed of less than 5 kts, with the engine running at dead slow ahead, at a lower RPM, or on astern revolutions. Every ship handler and pilot must be conversant and comfortable with slow-speed work, because the disadvantages of carrying too much speed inside the port are severe.
A further difficulty is that the ship's published manoeuvring characteristics are given for full engine power only. There is no bridge poster and no recorded data for how she behaves at slow speed with minimum power — that knowledge rests on the experience of the pilot and ship handler. The navigator is left asking how the ship behaves at very slow speed, how to control her when engine power is not available, and how to control her when the engine is available but cannot be used freely because longitudinal space is too tight.
2. When and Why Slow Speed Manoeuvring Is Used and Critical
2.1 When a vessel resorts to slow speed manoeuvring
The speed control plan fixes points where the vessel must reduce speed to a defined level. Rule No. 6 (safe speed) sets no numeric limit, so it is the pilot who, on taking over navigational duties, specifies the speed quantitatively at each reference point and leg of the passage. This plan is discussed at the master–pilot information exchange, where the pilot explains the optimum speeds, the reasons for any unusual speed, and thereby the likely frequency of engine orders. Broadly the optimum speed is proportional to the distance to the berth, modified wherever a specific external force acts on the ship.
For a fully loaded Panamax berthing starboard side on Berth No. 1 with two tugs on the port bow and quarter, the segments falling under slow speed run as follows.
| Distance from berth | Optimum speed | Reference point |
|---|---|---|
| 1 mile | 4 kts | Turning circle |
| 5 cables | 3 kts | Passing Berth No. 10 |
| 2 cables | 2 kts | Passing Berth No. 5 |
| 1 cable | 1 kt | Passing Berth No. 3 |
| 100 m | 0.8 kt | Passing Berth No. 2 |
| 50 m | 0.5 kt | 50 m from bow position |
| 25 m | 0.3 kt | 25 m from bow position |
| 10 m | 0.1 kt | 10 m from bow position |
2.2 Why slow speed manoeuvring is critical
Slow speed brings a set of difficulties that make good control hard to keep. The chapter lists ten of them.
| # | Difficulty at slow speed | What it means for control |
|---|---|---|
| 1 | General disadvantages | Difficult to turn, rudder less effective, wind catches the ship, and she takes a larger set. |
| 2 | Insufficient number of kicks | Air bottles allow a limited number of engine starts — for some ships only 12–18, for others unlimited. Frequent start/stop at slow speed consumes the starting air; the handler must ration kick starts so enough remain for the critical phase. |
| 3 | Minimum RPM not available | Only some ships offer an RPM below dead slow ahead. Without it, running dead slow continuously will not let the speed fall below its corresponding value, forcing stop-and-kick to regain helm effect. |
| 4 | Cannot rely on astern engines | Astern power is usually weaker than ahead — as low as 25%; other methods must be used to shed speed. |
| 5 | Helm useless when running astern | While running astern the helm does not work, so the ship is not under control for that period and needs other techniques. |
| 6 | Peculiar astern characteristics | Running astern brings transverse thrust, heavy vibration, loss of control and wedge effect. |
| 7 | Dangerous transverse thrust astern | On a right-handed propeller the helical flow throws the stern to port and the bow to starboard; unmanageable if half or full astern is run without correction. |
| 8 | Engine-type limits | Diesels build power quickly; turbine ships come online slowly, but are better than diesels at holding heading while reducing speed. Most ships are diesel with a right-handed propeller (LH propeller reverses the bias — bow to port, stern to starboard). |
| 9 | Handling CPP | The shaft always turns one way; astern is obtained by changing pitch. At zero pitch the propeller still turns and makes wash that hampers passing mooring lines — better to disengage and stop from the engine room, but then the engine is unavailable. |
| 10 | Twin-screw ships | Good if outward-turning (one ahead, one astern turns her on the spot even at zero speed); poor if inward-turning, since one screw's astern wash spoils the other's flow. |
3. Thumb Rules of Slow Speed Manoeuvring
A short set of working rules guides the handler at slow speed.
| # | Thumb rule |
|---|---|
| 1 | Keep the speed on the slower side and hold an engine power reserve as a contingency. Higher RPM makes the helm more effective, so a slow ship can run the engine hard for a moment to gain helm and turn away from danger. |
| 2 | When tugs become ineffective, the engine and helm are the pilot's only weapon. A slow ship can be given any higher RPM for a while to get helm effect; a fast ship cannot, because the extra RPM only creates a new danger. |
| 3 | Inside the harbour and close to the berth, every increase in speed must be killed within the next few minutes. Any speed gain may demand equal or greater astern running to compensate, so a decision to increase speed must carry an immediate backup plan to reduce it again. |
| 4 | Running the engine at higher RPM with the wheel hard over makes the speed increase only marginal, not as quick as normal. |
| 5 | Astern engines are normally weaker — 30–75% of ahead power (rarely 100%). |
| 6 | Note the number of consecutive starts and keep engine movements within limits, or the starting air is exhausted and the engine becomes unusable. |
| 7 | With the engine stopped but appreciable way on, many ships steer satisfactorily for a time depending on their directional ability — but the helm needed is larger than normal. |
| 8 | At very slow speed with the engine stopped, wheel hard over can still be used to get lateral movement of the stern. |
| 9 | Optimum speeds must be clearly defined in the berthing plan and discussed with the master, though it is not always possible to keep them at every stage. |
4. Control Options Using Engines and Helm Only
Controlling the ship is hardest at slow speed, yet speed above a certain limit is simply not acceptable inside harbour limits. Increasing speed to gain control is suicidal and invites emergencies. When an optimum speed cannot be held, the pilot turns to a graded set of options — plan a, b, c, d — the first of which use only the engine and helm.
4.1 Short bursts of ahead engine
With the engine stopped, water resistance and inertia bleed the speed off on their own, and the steering response grows sluggish in proportion to the falling speed. If the speed is falling steadily and a certain figure must be held without further increase, short bursts of the engine maintain it. A drawback of the engine-stopped state is that the ship, no longer correcting through her steering, is more prone to weather and develops a bodily drift that grows as the speed falls.
4.2 Hybrid turning
Hybrid turning combines a course alteration with a reduction of speed — needed, for example, when a loaded bulk carrier leaves a channel and must both head for a berth set at an angle and shed speed within a few hundred metres. The sequence is:
| Step | Action |
|---|---|
| 1 | With engines running, give hard over to the side of the intended alteration. |
| 2 | Once sufficient swing is generated, stop the engines while the swing continues. |
| 3 | Put the wheel amidships. |
| 4 | Give astern up to full astern as required; the transverse thrust keeps her turning the same way while the speed drops continuously. |
4.3 Wheel hard over with engines stopped
With the engine stopped, the propeller slipstream through the rudder gives little helm effect, so a larger helm is needed to steer. Wheel hard over then produces a lateral movement of the stern, whose speed depends on the ship's existing ahead speed. This is often effective to counter the wind pushing the stern away, to bring the stern towards the berth without a tug push, to reduce lateral drift of the stern, or to nullify residual transverse drift after the ship has stopped running astern.
4.4 Wheel hard over with short engine kicks
While the ship runs on with engines stopped she holds her course for a time, then begins to wander and will not hold even at hard over. Short kicks of the engine restore helm effect and stop the wander; once steady, the engine is stopped again, and the cycle is repeated to hold the course while holding the speed. The drawback is that each kick consumes starting air and reduces the kicks in hand.
4.5 Sequential wheel hard over
Where there is sufficient sea-room, alternating hard-over helm sheds speed. Each hard over breaks the ship's directional inertia and cuts the speed marginally; before she settles on the new heading, opposite hard over brings her back to the original course, breaking the inertia again. The sequence is repeated as head-room and speed reduction allow. Its limitation: once momentum is created it is very hard to reverse with opposite helm, and with engines running the ship simply picks up her lost speed as she settles back.
4.6 Astern engine at the right time
Astern engines (again 30–75% of ahead power) reduce speed only below about 5–6 kts; used at higher speed they barely slow her while stressing the engine heavily. Because steering is also lost astern and the ship takes a constant swing, there must be sea-room for the swing — particularly on the quarter — or standby tugs. The handler may therefore choose to use astern earlier, while conditions suit, to preserve a reserve of ahead power for control in a later critical area.
4.7 Transverse thrust with astern engine
Running astern, the propeller wash strikes one side of the transom continuously and throws the stern to port or starboard by propeller type; with the pivot point forward, the ship swings accordingly. This can be turned to advantage: to produce a wanted swing, to push the stern clear of an obstruction while slowing, or — on the suitable side while berthing — to shed speed while bringing the stern towards the berth.
5. Control Options With Assistance From Tugs
5.1 Engines ahead with tug back-pulling
When the engine can neither be stopped nor run astern but the ship must slow with control retained, a tug is set to back-pull while the engine runs on, keeping the helm live for course-keeping. Sometimes a tug is made fast right astern through the centre lead purely to reduce speed. Because tugs do not pull exactly astern but in an arbitrary direction, the stern drifts slightly towards the pulling side; if the pilot cannot check that drift with engine and helm, the tug is told to stop pulling momentarily and resume once the ship is in control.
5.2 Use of tugs while engines stopped or running astern
When ahead engine cannot be given at all and no other option is open, the pilot depends heavily on the tugs to hold the ship until an alternative arises. If a dangerous swing from transverse thrust or a lack of slipstream helm effect is expected, the tugs must be warned in advance and deployed at the right position before the effect develops — proactive use that saves much time.
5.3 Hooking with one tug
Hooking controls the lateral movement of the ship with a single tug while she is underway but with the engine stopped (or astern). It is done in two ways — bow hooking and stern hooking.
Bow hooking — engine stopped
A ship passing berthed vessels close on her starboard side is drifting continuously to starboard. To stop the drift, or bring her back outboard, hook with one tug and no engine:
| Step | Action and effect |
|---|---|
| a | Wheel hard over to the side of the drift (starboard). This stops the stern drifting to starboard, or moves it away from the berthed vessels; as the stern moves out the bow tends to close in, and once the swing catches to starboard it will not stop easily. |
| b | Use the forward tug to pull the bow out (at pull-minimum). This holds the bow from closing in; while the bow holds, the stern keeps coming out. |
The two steps bring her bodily out if already drifted, or stop the bodily drift to starboard.
Bow hooking — running astern
The same close-range drift, but now she must also reduce speed by running astern:
| Step | Action and effect |
|---|---|
| a | Run engines astern (wheel amidships). The transverse thrust develops slowly, the stern begins to move and the heading changes with increasing ROT. |
| b | Use the forward tug to pull the bow out (at pull-minimum), holding the bow from closing in while the stern comes out. |
Stern hooking — to hold course while reducing speed
Running astern throws the stern to port and swings the bow to starboard. To stop the stern drifting to the port margin of the channel, stern hooking with one tug checks the swing:
| Step | Action and effect |
|---|---|
| a | Run engines astern to start reducing speed; the transverse thrust builds and the heading changes with increasing ROT. |
| b | Bring the stern tug to standby to push from the port quarter; as the ROT grows, increase the push to nullify the transverse thrust. |
Stern hooking — to stop drifting while reducing speed
Here the ship is also drifting towards the shallow bank. Running astern would throw the stern further to port, doubling the drift, so the tug is used proactively:
| Step | Action and effect |
|---|---|
| 1 | Before running astern, the tug pushes enough to set up a residual drift to starboard. |
| 2 | Once she starts drifting to starboard, run engines astern; the astern movement compensates the starboard drift and holds her on course. |
| 3 | Meanwhile the tug increases power to keep a little starboard drift, or the double drift to port would soon carry her dangerously onto the shallow bank. |
5.4 Use of tugs only, without engine or steering
When neither engine nor helm is available, tugs alone control the ship — one forward and one aft, working the lateral movement of bow and stern to keep her in the safety margins. Note that the speed falls at a greater rate when the tugs work alongside: as they come to almost a right angle to the heading while pushing or pulling, they add drag and weight, increasing the lateral speed while marginally changing the longitudinal speed by their position.
6. Thrusters and Other Resources
6.1 Bow thruster only
The bow thruster is an added tool at slow speed. With enough way on and a need to slow while keeping control, the handler can use the bow thruster alone to control the heading without running the ahead engine. Besides building lateral speed towards a berth, thrusters greatly help course-keeping through a manoeuvre.
6.2 Thrusters and helm together, engine stopped
The bow thruster can perform bow hooking in place of a tug when tugs are unavailable. Combining thruster and helm with engines stopped or running, the handler can generate the lateral bodily drift needed to bring the ship alongside without a tug.
6.3 Using current in favour
Current is not measurable in real time, but tidal streams follow diurnal and seasonal patterns, so the handler can anticipate the strength and direction of the current at a given place and time. That anticipation — sharpened by experience — lets the pilot use the current to advantage; without it he may be caught unaware.
6.4 Using wind in favour — Hughes' formula
Unlike current, wind force and direction are observed in real time — an experienced eye reads them almost at once from the sea surface. The pilot uses the wind to advantage for a safe manoeuvre, and its force can be quantified with Hughes' formula for the wind force in tons.
where $\theta$ is the wind direction from the bow (relative wind direction, in degrees); $V_a$ the headwind speed in m/s; $A$ the projected area from the bow above the waterline (m²); $\rho$ the air density ($0.125\ \text{kg}\cdot\text{s}^2/\text{m}^4$); $B$ the projected area from the side above the waterline (m²); $a$ the length from bow to the wind pressure centre (point of action); $R_a$ the resultant wind pressure force in kgf (÷ 1000 to give tons); $@$ the wind pressure force angle (angle of action, in degrees); and $C_{Ra}$ the wind pressure force coefficient.
Worked example — a container vessel of 255 m LOA, 11 m draft, $A = 880$, $B = 6200$, wind of 20 kts ($10\ \text{m/s}$, recalling $1\ \text{m/s} = 1.944\ \text{kt}$) from 4 points on the starboard bow ($\theta = 45°$):
Because $C_{Ra}$ depends on $\cos2\theta$, the wind force varies strongly with direction: with the wind right ahead or astern $C_{Ra}$ is a minimum ($1.325 - 0.575$, about 43% less); at 4 points on the bow $\cos2\theta = 0$ and $C_{Ra}$ takes its absolute value 1.325; and right abeam ($\cos2\theta = -1$) it reaches its maximum ($1.325 + 0.575$, about 43% more).
The windage areas $A$ and $B$ are the projected areas above the waterline from the bow and from the side; a light-condition ship, exposing far more side area, feels a proportionally larger lateral wind force for the same wind.
6.5 Mooring lines and anchors
Mooring lines and the anchor are used at the right time, with a correct reading of the scene — chiefly while approaching the berth or casting off and manoeuvring away. Their detailed use is treated in the chapters on approaching the berth and on berthing and mooring.
7. Extreme Manoeuvres at Very Slow Speed
At some point almost every ship must be turned very tightly, inbound or outbound, to enter or leave safely. That is why most ports keep a turning basin within harbour limits, its diameter usually a little over the length of the largest vessel handled — for a full-service port, 500 m or more. The ship is preferably turned when in light condition. In such tight turns the assistance of tugs is inevitable.
7.1 Turning with one tug
When one tug pushes at the stern (or bow), the centre of the turn is the pivot point (P), so turning on the spot within a radius of ½L is not possible. The radius of the area required is:
where $k$ is the turning radius of the moment of inertia about the vertical axis through the centre of gravity ($k = 0.35L$); $P$ is the pivot point; $G$ the centre of gravity; and $C$ the point at which the tug acts on the vessel.
7.2 Turning within 1L using two tugs under wind
Simulations for a 255 m container vessel turning on the spot (ship's speed nil) with two tugs, under a 20 kt wind 4 points on the starboard bow (draft 11 m, $A = 880$, $B = 6200$), gave a wind force of 29 tons that rises to about 41 tons when the wind comes abeam (the 43% increase). Although it varies with hull shape and type, the ship needs a turning basin of double her length (R = 2L) to make a full turn with two tugs and no engine — she drifts roughly one extra ship-length under 20 kts of wind.
7.3 Short round turn at slow speed
Turning in a tight basin at very slow speed, where the engine cannot run for long, follows a repeated cycle:
| Step | Action |
|---|---|
| a | Give wheel hard over before running ahead. |
| b | Run engines ahead; she starts swinging to the chosen side. Hold until the speed just begins to build. |
| c | At the tolerance limit, stop the engine; the swing continues and speed falls slowly. |
| d | Run engines astern — speed drops while she keeps turning. |
| e | When speed is well down, stop, give wheel hard over again and run ahead to add swing. Repeat to complete the turn. |
With a right-handed propeller the astern phases keep her turning to starboard (transverse thrust); with a left-handed propeller, to port. Where the turning side is optional, plan the turn to match the propeller type, so the speed stays at an absolute minimum through a large turn.
7.4 Turning while making sternway
When the ship is falling astern before being turned, the turning circle is about 1.5 times her length, the wind under 20 kts, with two tugs of 55 tons bollard pull — one on the bow, one on the quarter.
| Position | What happens |
|---|---|
| Pos-1 | She falls astern with the stern tug pulling to swing the stern to port and the bow to starboard. As the bow clears obstructions and enters the basin, the forward tug and bow thruster push the bow, keeping her turning to starboard. The high ROT bleeds the astern speed; if it will not fall, engines ahead and starboard hard-over helm may be added. |
| Pos-2 | The stern reaches the basin limits and the astern speed is killed to zero or a minimum; care is taken that she does not gather ahead way. As she keeps turning to starboard the ahead speed rises marginally, so astern is ordered to neutralise it until the turn completes. |
| Pos-3 | The turn is complete and she makes her next course. Because of the lateral traverse of the stern it keeps moving off the new course; opposite helm with higher RPM controls it, or the stern tug pushes/pulls if the speed is too low. Once bow and stern are under control and speed is sufficient, the tugs are released. |
7.5 Changing the pivot point
At low speed, using the engine to reverse the ship's motion from ahead to astern (or the reverse) moves the pivot point substantially from forward to aft. Shifting it away from an external force increases that force's effectiveness in swinging the bow or stern; shifting it towards the force reduces the force's effect. Running the engine astern brings the pivot point aft.
