AN Ch. 6 — Interaction and Tug Safety (Hensen)

1. Introduction — why interaction matters (6.1)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 6 — Interaction and tug safety.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 6 — Interaction and tug safety · Anexo 2-A, Área II, item 26 (Utilização de rebocadores portuários — interação navio-rebocador e segurança operacional) com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores empregados no Brasil).

The earlier chapters described what each harbour-tug type can do. This chapter turns to the other side of the coin: the risk a tug runs while rendering assistance. It is an essential point in shiphandling, because what is at stake is not only the tug and her crew but also the safety of the assisted vessel. Tug captains and pilots must be fully aware of the dangers involved.

Many unsafe situations can be traced back to interaction effects — the hydrodynamic forces and turning moments that arise when ship and tug operate close together at speed. So Hensen first explains interaction (and the way shallow water magnifies it and increases the assistance needed), and only then catalogues the many specific risky situations a tug can fall into.

The thread of the chapter. Interaction divides into effects that influence tug performance (how well it can pull or push) and effects that influence tug safety (whether it can avoid being capsized or run down). Shallow water makes both worse. The flow pattern around a ship — governed by Mr Bernoulli — is the key to understanding both, and underlies bank suction, bow cushion, girting and the lethal "tug under the bow" accident.

2. Interaction effects influencing tug performance (6.2.1)

There are several distinct kinds of interaction. Some affect a tug's performance, some affect its safety, some both. Hensen first lists the ones that bear on performance; most were already met in earlier chapters.

2.1 Interactions internal to the tug

Tug-propeller / tug-hull interaction. The astern thrust of a reverse-tractor or ASD-tug is about 5–10 % less than its ahead thrust, because the propeller wash hits the body of the tug forward of the thrusters and so reduces bollard pull when thrusting astern. Conversely, an ASD-tug sailing ahead performs a little better than a tractor tug, because the water speed near its propellers is lower than with a tractor (whose propellers sit under the hull).
Interaction between the two tug propellers. Marked with azimuth thrusters and Voith-Schneider propellers: depending on the thrust directions, the two units of a tractor or ASD/reverse-tractor tug influence each other and so affect performance.
Tug-ship interaction through the fendering. Fender energy-absorption and friction characteristics influence how forces pass between tug and ship — and the same happens when the ship's hull is ice-covered.
Tug-towline interaction. Tug reactions such as list, and therefore performance, depend on towline characteristics — especially its ability to absorb dynamic (shock) loads.

2.2 Interactions between tug and ship

Tug-propeller / ship-hull interaction. Propeller wash from the tug hitting the ship's hull cuts pulling performance; it is worse with small underkeel clearance. Pushing tugs are affected too, the water flow into their propellers being interrupted when close to the hull.
Tug-hull / ship-hull interaction. Most marked when a tug works at the ship's side; strongly influenced by shallow and narrow waters and above all by ship's speed — and this one bears on safety as well.
Ship-propeller / ship-hull → tug interaction. Affects a stern tug working in the propeller slipstream or the ship's wake; the wake effect grows in shallow and narrow waters.

Which ones this chapter develops. Of the list above, the two that endanger safety — tug-hull / ship-hull interaction, and ship-propeller / ship-hull → tug interaction — are the subject of this chapter. The others were treated earlier. Small underkeel clearance amplifies several of them, which is why shallow-water effects are considered next.

3. Shallow-water effects on tug assistance (6.2.2)

Some shallow-water effects were already met under bollard pull: as underkeel clearance falls, more bollard pull is needed to hold a ship against a current or to stop a sideways-moving ship. But shallow water — meaning small underkeel clearance — has several further effects that demand the full attention of pilots and tug captains, and where tug assistance is then very welcome.

3.1 Bank suction and bow cushion

A ship running on one side of a river or channel, close to a bank, is drawn bodily towards the bank (bank suction) and at the same time has its bow pushed away (bow cushion) — the resultant force acting somewhat abaft midships. The ship can hold a stable course parallel to the bank by carrying rudder towards it; but once that balance is upset — an irregular or submerged bank, or careless steering — the ship may sheer away and become very hard to control, even sheering to the far side of the channel.

Speed squared. Bank suction and bow cushion grow with the square of the ship's speed: at four knots the bank attraction and the yawing moment are four times as large as at two knots. They are also stronger with smaller underkeel clearance. Keep well off banks and keep speed down; at lower speed more reserve power is available for a "kick ahead" with rudder hard over to counter a sheer. In shallow water and near banks, tugs should be on the alert to check an unexpected sheer. All of this is, again, a matter of Bernoulli (§ 4).

3.2 Other shallow-water effects

Decreased rudder effect.
Possible increase of the propeller's transverse effect.
Larger turning-circle radius. In shallow water the turning circle is much larger and the initial rate of turn much smaller, so taking a bend is harder; tug assistance may be needed. Lower speed leaves more reserve power and lets tugs work more effectively.
Longer stopping distance from larger virtual mass. In shallow water a ship drags a large body of water with her — up to about 40 % of her displacement when underkeel clearance falls to 20 % of the draught. More astern power, and so more tug power, is then needed to stop her than in deep water.

The "following water" effect (Fig. 6.1). In a channel with little clearance, the large mass of water travelling with the ship takes time to slow when she stops abruptly in a basin at the channel end, and overtakes her — pushing her ahead, increasing her rate of turn, or shoving her sideways while turning. It is often called an "added mass" effect, but more likely it is the water flowing along the channel and filling the gap behind that produces the delayed shove; it has been experienced, for example, with large tankers in a Caribbean port. Its size is directly related to ship's speed — one more reason that low speed is vital in shallow water, where interaction between ships is also enlarged.

4. Flow pattern around a ship — Mr Bernoulli (6.2.3)

The interactions that most endanger a tug happen when ships sail or manoeuvre close to one another. The cause is the water flow around a ship. Whether the ship moves through the water or the water moves past the ship makes no difference — the relative water speed is the same. The actual flow felt by a tug stopped in the water is shown in Fig. 6.2; the flow relative to the ship in Fig. 6.3.

If a ship had no beam and no draught, the water would keep a constant relative speed equal to the ship's speed. But she does have beam and draught, so water is pushed sideways and downwards and must still pass from bow to stern in the same time along a path longer than the ship — so for most of its passage the relative velocity of the flow is increased.

Bernoulli's principle. Daniel Bernoulli (Swiss mathematician-physicist, born in the Netherlands in 1700) established the relationship between water speed and water pressure: an increase in water speed produces a decrease in water pressure, and vice versa, with the change in pressure proportional to the square of the change in speed. So when the water speed doubles, the pressure falls to a quarter.

4.1 The pressure field around a hull

Where the relative flow speeds up, pressure drops. Well ahead of the ship the stream lines are evenly spaced; they wedge apart and are then compressed as they pass round the body, and spread again at the stern to fill the gap astern. Diverging stream lines mean slower water and higher pressure; converging stream lines mean faster water and lower pressure. The result is a high-pressure area at the bow (the bow wave), a low-pressure field around the midbody, and a smaller high-pressure area at the stern.

The stern field is smaller than the bow field because of the boundary layer: viscous skin friction drags a "dead" layer of water along the hull, thickening from bow to stern and continuing astern as the frictional wake; this makes the stern stream lines spread less. In wide-bodied ships the water speeds up around the forward shoulders but less round the after quarters, producing a local wave trough there. In shallow water the flow under the ship is restricted, so more water passes along the sides — higher side speed, larger pressure drop along the sides, and higher pressure at bow and stern; narrow waters between ship and bank confine the flow even more, raising side speed and dropping side pressure further still, with the highest pressure near the bow.

This is also bank suction and bow cushion. A ship on one side of a channel has more confined flow on the bank side, so higher water speed and lower pressure there — and is forced towards that low-pressure side (bank suction). The boundary layer along the bank narrows the gap towards the ship's stern, so the resultant acts abaft midships and gives a yaw moment away from the bank; meanwhile the high bow pressure builds a cushion (bow cushion). A steep bank has a bigger effect than a sloping bank, because a sloping bank lets some water flow in sideways and so reduces the pressure drop. Interaction between two ships, or ship and tug, is generated in exactly the same way as between ship and bank — it depends on the distance off and the relative water speed between them.

5. Tug-ship interaction and tug safety — positions 1–5 (6.2.3)

Fig. 6.4 follows a tug slowly overtaking and passing a bulk carrier, tracing the interaction at each station. (Note 1: the tug shown is an ASD-tug but could be a conventional tug; "port/starboard rudder" can be read as the equivalent thruster setting.) The green areas in the figure are the most critical, because of quickly changing turning moments over short travel and of suction forces.

5.1 What the tug feels, stern to bow

Approaching the stern. The tug meets relatively slow water, so it speeds up; the incoming flow may also push it sideways (here, to starboard).
Abeam of the stern (position 1). Faster water between tug and hull lowers the pressure and sucks the tug towards the ship; since the tug's forepart is nearer the hull than its stern, it feels a turning moment towards the ship. A cross-flow lift force adds to the pull. Reaching the after-quarter trough, the turning effect grows and more power is needed.
At the after quarter (position 2). Still sucked in by the local trough, but as the tug draws parallel it suddenly feels an outward turning moment from its own bow cushion, while its stern, near the trough, is sucked towards the ship.
At the midbody (position 3). Still sucked towards the ship with an outward turning moment — identical to bank suction and bow cushion.
At the forward shoulder (position 4). Higher water speed and the local trough mean more power to hold speed; suction rises rapidly, and as the tug's after end reaches the trough the outward turning moment grows again.
Between positions 4 and 5 (near the bow). The outward moment suddenly reverses to an inward moment: the cross-flow near the ship's bow acts on the tug's rudder/thrusters or skeg as a steering force, and the lift force drives the tug sideways away from the ship.

5.2 How each tug type must react

Two families are distinguished: conventional-type tugs (propulsion and steering aft — including ASD-tugs working over the stern) and tractor-type tugs (steerable propulsion forward — including ASD-tugs working over the bow as reverse-tractor). The key contrast: a conventional tug's steering force is a side force that can add to the suction at the wrong moment, whereas a tractor tug can direct its propulsion away from the ship.

Position 1. A conventional tug needs port rudder to counter the inward moment — but port rudder also pushes it towards the ship, so it should keep well clear. A tractor tug directs propulsion away from the ship and is safer. Towlines are also passed here, so conventional tugs must be especially careful.
Between 1 and 2. The conventional tug must switch quickly from port to starboard rudder, whose side force now points away from the ship. The tractor tug must aim propulsion at the hull to counter the moment — introducing a side force towards the suction, which is not safe.
Positions 3 and 4. Conventional rudder stays to starboard, countering suction; tractor propulsion stays to starboard against the bow-out moment, still towards the suction. Near position 4 the suction (starboard) and moment (port) can be marked.
Between 4 and 5. The conventional tug must abruptly change from starboard to port rudder, or it may swing to starboard and end up under the bow. Power can be reduced as relative speed falls; a tug that keeps power up runs off to starboard with possibly dramatic consequences. Cross-flow on the underwater body also reduces effective stability here.

Conventional vs tractor tug: who is safer where (Fig. 6.4 stations)
Station along the hull	Conventional tug (steering aft)	Tractor tug (steering forward)
Position 1 — abeam the stern	Port rudder counters the moment but its side force adds to suction → keep well clear	Aims propulsion away from the ship → safer
Between 1 and 2	Quick port → starboard rudder; side force now points away	Propulsion aimed at hull adds a side force towards the suction → not safe
Positions 3–4 — mid to forward shoulder	Starboard rudder counters suction	Starboard propulsion still adds towards the suction
Between 4 and 5 — near the bow	Must abruptly reverse to port rudder, or swings under the bow	Sets propulsion to port to keep off the bow
Net safety near the bow	Holds steadier (rudder counters moment and suction together)	Ultimately safer — if too close, steering force points away from the ship

Where towlines are passed. Positions 1 and 4–5 are also the stations where towlines are passed. A conventional tug can hold steadier near the bow, because rudder used against the turning moment also counters the suction and lift. A tractor tug, countering the moment, sets propulsion in the same direction as the suction and lift, so it may come too close to the bow — but it is ultimately safer, because if it does get too near, its steering forces point away from the ship. The smaller the gap, the larger the effects; shallow and narrow water increase them; and above all, interaction grows sharply with speed and is most dangerous near the bow.

Stability and high speeds (Note 2). As tug power has risen, stability has become ever more important. Tugs have grown wider — more beam and freeboard greatly help stability — but a beamy tug tends to be course-unstable: it veers off heading easily and checks yaw poorly, an unwanted trait where tug-ship interaction is in play. Speeds when coming alongside or making fast can be high — five knots is normal, nine or ten not unknown — and interaction is then large, especially for a tug taking a line at the bow, calling for highly manoeuvrable tugs with high free-running speed and very experienced captains.

6. Tug-ship interaction and tug performance (6.2.4)

The same flow pattern also affects how well a tug performs close to the hull, though by how much is hard to say, because the flows generated by ship and tug interact and the picture changes fast as the tug shifts position.

The relative water speed along the hull is higher than the free stream — a ship steaming at three knots may see four knots of relative flow along her side, and more still at the shoulders of a wide-bodied ship. A tug pushing at the side meets this faster water and loses effectiveness, especially near the shoulders (Fig. 6.5, positions 1 and 2); shallow and narrow water speed the flow up further and cut effectiveness more.

For tugs towing on a line the case is more tangled: they work in regions of different interaction effects, they change position and heading often (positions 3 and 4), and the effects vary with hull form, loading and speed — so the net effect on performance is hard to pin down. The practical levers are towline length and distance off: for tug no. 3, a shorter line and closer position mean larger interaction; for tug no. 4, towing effectiveness falls with a short line because the propeller wash hits the hull (worse with propulsion aft).

Use a longer towline, stand farther off. For tugs towing on a line (nos. 3 and 4) it is advisable to use a somewhat longer towline and work at a greater distance from the hull — which is both safer and more effective, reducing interaction and the loss from the tug's own propeller wash on the hull.

Working in the wake and slipstream (position 5). A tractor or ASD/reverse-tractor tug at position 5 works in the ship's wake and propeller slipstream, which act in opposite senses. When braking forces are wanted the ship's propeller is normally stopped or run astern; compared with a free stream, the wake lowers the tug's underwater resistance and braking performance for the same engine power, giving a smaller towline force — and this loss grows in shallow and narrow water, where wake speed falls further. The propeller slipstream has the opposite effect. Overall, the most marked performance losses are felt by tugs pushing at the side and by tugs braking in the wake.

7. Tug safety — alongside, passing a towline, heaving lines (6.3.1–6.3.5)

Beyond interaction, an assisting tug meets many other risky situations; the following are only examples, since no list can be complete. Many relate to towing on a line, because that method puts the tug close to a bow or stern at speed, where interaction forces are large and alternating. Critical situations divide simply into "while passing a towline" and "while the towline is secured".

7.1 Coming alongside and departing (6.3.2)

When coming alongside a ship at speed, it is safest to approach near the midsection, where the flow is more uniform; further forward or aft the effects are larger and less predictable. Getting free of the hull can be the harder problem — for example a twin-screw tug, flat-sided underwater, that has landed near the pilot ladder. Moving far forward or aft does not help. Captains find that applying astern thrust on the inner propeller — against intuition — lets the tug come off: it slows the water between the hulls, so pressure rises and, with the bow cushion, pushes the tug clear. A nice example of Bernoulli's law. Lowering the ship's speed also helps, since suction grows with speed.

Azimuth wash can suck a tug in. Tugs whose azimuth thrusters point slightly outward at low speed can be violently sucked towards a slow ship when the inward propeller's wash speeds up the water between the hulls. It is worse for fixed-pitch azimuth tugs without speed-modulating clutches, whose high minimum propeller speed throws much wash at low tug speed — a cause of contact damage when landing alongside or during (un)berthing, avoidable by proper handling. (Note 3: approaching a stationary or moving ship at an inappropriate speed has caused hull dents, tug damage and even oil spills from mooring-assist tugs penetrating bunker spaces.)

7.2 Passing a towline near the bow (6.3.3)

The bow is the most dangerous place, as the interaction section showed; approach with care whatever the ship's speed (Fig. 6.6 shows tug-masters' methods). A tug waiting ahead, slower than the ship, can be given a turning moment by the overtaking ship's changing flow — anticipated in time, it can be countered by any tug type. With a full-shaped loaded bow at reasonable speed the moment can be large; for a loaded VLCC or ore-carrier at four to five knots with small clearance, the bow pressure wave can even push the waiting tug forward, so that the captain must reverse thrust to close the bow. Near the fore part of the bow the cross-flow pushes the tug aside (positions 4–5 of Fig. 6.4); steering a little inward keeps it from closing, but moving further forward brings the inward turning moment towards the ship — largest at small clearance. (Note 4: bow approaches are detailed in the monograph Bow Tug Operations — Risks and Effectiveness.)

The "tug under the bow" accident (Fig. 6.7B). A conventional tug approaches to take a line at the bow, judges itself too close, and tries to clear with engines full ahead while steering to port. The steering force pushes it against the ship; it moves forward along the bow, fails to get free, comes broadside under the bow and is run down. The only safe escape is "Full Astern" — some tug damage may follow, but not disaster. A tractor type is safer, its steering forces pointing away from the ship.

This has happened repeatedly, often fatally. The German Hamburg Testing Tank, the UK work of Dr Dand, Flanders Hydraulics and MARIN have studied it for almost fifty years — across model tugs, container ships, ferries and tankers. Their findings agree:

Around the bow there are strong, fluctuating interaction forces and turning moments that change direction rapidly and vary by ship type.
The forces increase with the square of the speed and fall with distance off; their severity is most easily reduced by lowering ship's speed.
A conventional tug without a bow thruster cannot turn away once alongside the bow in contact with the tow; even a powerful bow thruster only sometimes clears it; a Voith Water Tractor could clear under virtually any condition.
The most dangerous moment is passing the forward part of the bow, where the out-turning moment changes to an in-turning moment; if the tug is then steering towards the ship, collision is hard to avoid.
The recommended safe speed through the water is not more than 6 knots; the only safe escape, if alongside the bow, is always Full Astern.

Tractor and Voith tugs, with propulsion forward, compensate better: set the units away from the bow and the tug moves away. A conventional tug (or an ASD-tug used as one) steering away from the bow feels a force towards the ship as its stern swings quickly inward — a consequence of the aft rudders and propulsion. The hardest problem in practice is that a tug master often cannot judge when the bow becomes too risky: the margin between safe and unsafe is very small. Good manoeuvrability, the right tug type, and experience are all indispensable; so is the ship's speed, which is the pilot's to control, and a crew standing by forward in good time with proper heaving lines.

7.3 Passing a towline at the stern (6.3.4)

Making fast at or close astern, interaction forces are not as large as at the bow but can still cause accidents. Approaching from astern, the tug is pushed towards the stern. The ship's propeller must be watched at all times: a turning propeller disturbs the water and makes it hard for the tug to hold position. In position 1 of Fig. 6.4 the moment is directed towards the ship — a critical spot if unexpected. The worst case is a sudden astern thrust: a powerful engine going astern creates a deep wave trough close behind the stern that sucks the tug in, risking contact.

Rule at the stern. When tugs make fast at or near the stern, the ship's propeller should be stopped (fixed-pitch) or set to minimum pitch (controllable-pitch). If for some reason the propeller must be used for astern thrust, the tug master must be informed in good time so he can clear the dangerous area.

7.4 Heaving lines (6.3.5)

Safe heaving lines are a necessity for ship and tug safety. The system is old-fashioned but works well when used professionally: before tugs arrive, the ship's crew should stand by with at least two heaving lines of proper length and strength, each ending in a monkey's fist made of rope only, with no added weight. Soft throwing rings (which float) or a small bag of no more than 250 g of sand are safer alternatives.

Weighted heaving lines are "almost a criminal action". Adding metal — shackles, iron balls, stones — to a heaving line has repeatedly landed heavy objects on tug decks, narrowly missing crew (Australian incidents at Hay Point, Newcastle, Dalrymple Bay). The monkey's fist must be made of rope only. Worse, when the crew is not ready with proper lines, the tug master is forced to stay longer, and come closer, in the risky bow zone — as in the 2008 Barta / Magdalena case (ship's rope on the bulb, ~6 knots), where the tug came under the bow and capsized with two drowned (Fig. 6.8).

Safer modern methods are sought. The PLT air-driven line thrower (e.g. PLT 75), common in ship-to-ship work and now used by tug operators in the UK, Norway, Spain and Australia, gives a safe distance of up to 90 m, can be used for free crew training, and uses only compressed air. Whether it works on closed-deck ships (cruise ships, car carriers) is doubtful; drones have been raised too, with the same questions about wind, night and low visibility.

8. Girting, tripping and broadside steering (6.3.6–6.3.12)

8.1 Girting and tripping (6.3.6)

When a tug towing on a line cannot keep in line with the ship and the towline takes high athwartships tension, the tug can be dragged over. With a tug whose towing point is amidships (conventional), this is girting and ends in capsize; with a tug whose towing point is at one end (tractor), the tug swings round and comes alongside — tripping — which is less dangerous if the line is not released in time (Fig. 6.13).

In Fig. 6.13A a tug with propulsion aft assists a turn to starboard; if ship's speed becomes too high (the tug pulling too hard, or the pilot increasing power for rudder effect), the tug is carried abeam of the bow and then further aft with the towline under high tension, and is liable to capsize — made worse because the master's own heavy steering adds to the heel. A reliable quick-release system lets him let go and avoid capsize; alternatively, if the pilot sees it in time, reducing ship's speed slackens the line and lets the tug come back in line.

Girting does not need a bend. Even on a straight course, excessive ship's speed alone causes girting: a forward tug steers off to one side to keep clear of the bow, and if speed keeps rising the same girting/tripping situation develops. Fig. 6.13B shows girting from a misunderstanding — the master, not told the ship must enter a basin head-first, pulls the wrong way; the pilot, seeing the rate of turn drop, adds power and rudder, and the danger develops. A tug waiting right ahead for a slow crew to release the line can also be pushed forward by the bow wave of a beamy ship past its own free-running speed, then fall into girting when it edges aside. The recurring lesson: appropriate speed, good communication, and knowing the tug's limitations — the main cause in the 14 cases of the 2015 Kalmar study was not knowing, or not heeding, those limitations. Doors and deck openings must be closed during towing.

8.2 Conventional tugs steering broadside (6.3.7–6.3.11)

Several port manoeuvres use a conventional tug broadside on a line, acting as a steerable drogue; each carries a capsize risk if the tug's limits are exceeded:

Forward tug steering broadside (6.3.7). Ships entering a basin stern-first can be steered by a forward conventional tug working broadside (often on a gob rope), going ahead or astern to apply side force. Too high a stern speed raises the tug's heel until it capsizes — from the large transverse resistance and from the ship's propeller wash speeding the water on the tug. Care is needed using the engine ahead, lest the ship gather headway and collide with the close-by tug.
Stern tug steering broadside (6.3.8). The mirror case with the ship moving ahead and an after tug steering; the difference is the close ship's propeller. Speed is usually very low, but a sudden "half ahead" sends wild propeller wash onto the tug and can capsize it — this has happened more than once.
Stern tug moving from a quarter to dead astern (6.3.9). Dangerous to a conventional tug above about three knots; risk of girting from high athwartships tension. A capsizing tug is pulled under stern-first (e.g. Voima, Kaskinen, 2001). No problem for tractor/reverse-tractor tugs, or a conventional tug with a reliable gob rope.
Stern tug crossing from port to starboard quarter (6.3.10). Must be done with the ship nearly stopped, because of girting risk and the danger of the ship's propeller if the pilot, unaware, goes ahead while the tug is near position 2.
Tug at the ship's side (6.3.11). A conventional tug at right angles to the side, held by quarter or stern lines (Fig. 6.15), must avoid excessive speed, which can part the line or capsize the tug.

8.3 Low visibility — fog (6.3.12)

Dense fog makes every situation above more risky. Pilots and masters lose the visible cues that give position, speed and heading, and must rely on radar and ECDIS, which come with delay and demand interpretation while the ship keeps moving. ECDIS over-zoom can lose distance references (a second navigator should use the measuring tool; obey the overscale warning); GNSS multipath can give wrong fixes that go unnoticed without a second method. Fog navigation also happens too rarely for experience to build.

Tugs as extra eyes — and their own radar limits. Forward tug masters can often see further, or under a fog layer, than the ship's bridge, and can report distances to critical points; a PPU can help the pilot but not the tugs, so complex fog work may not be feasible. A tug towing on a line struggles to orient itself: a forward tug gets a good radar view ahead but a poor one astern, and the close ship blanks the radar in her direction; a bow-to-bow tug may see the picture "180° wrong" for running astern. So keep ship's speed low, keep tugs informed, and make communication optimal. For fog, a good towing winch with a reliable wheelhouse-and-deck quick-release is safest — towing bitts and quick-release hooks are not recommended unless 100 % reliable, since releasing a line under tension fast may be impossible.

Ports should have fog procedures (e.g. when visibility ≤ 500 m): consultations on whether a ship can be handled; on tug choice and placement, current/tidal limits, safe speeds, and fastening tugs where the engine can be stopped; information on visibility, free berths and topped container cranes; and traffic spacing to avoid concentrations. Regular simulator training of fog navigation — pilots and tug masters together — is recommended.

9. Operating bow-to-bow and skeg design (6.3.13)

Bow-to-bow is the way an ASD-tug works as a bow tug on a ship with headway: the tug runs astern, its bow towards the ship (Fig. 6.18–6.19). Directional stability astern can be poor, especially at higher speed: pulling straight astern may be fine, but as soon as the tug deviates — to give steering assistance — holding position becomes hard and control can be lost. A short towline adds tug-ship interaction (§ 6) that destabilises the position with little time to react. A severe ship-tug collision (River Yarra / tug W.J. Trotter) led to a maximum speed of five knots for bow-to-bow work. Only ASD-tugs with a well-designed underwater body, skeg and stern perform well this way at a ship with headway.

9.1 Why the skeg is decisive

Tractor and ASD-over-the-bow tugs both cope with bow interaction, but they behave differently: the tractor's units sit under the main body, so it moves more or less sideways for a course change; the ASD's thrusters sit right aft, in the light part, so running astern it turns rather than translates, and steers somewhat nervously at low speed. A tractor running with its high bow into the seas also has a seakeeping advantage.

The better the underwater hull and skeg design for running astern, the higher the safe astern speed and the larger the steering force a tug can apply. When the centre of pressure of the hydrodynamic side force lies as far forward as possible, a smaller thruster force is needed to balance it and a larger steering force is left for the ship — so the tug can deviate from a straight course safely at higher speed and return to the ship's track. Low underwater resistance and high engine power help too.

Thruster-skeg interaction (Figs. 6.21–6.24). If an ASD-tug's skeg extends far aft, near the thrusters, part of the thruster wash hits the skeg and creates a force that opposes the intended turn — cutting the tug's ability to hold position or come back in line. A box keel set well forward of the thrusters (Fig. 6.23) largely avoids this; a far-aft skeg (Fig. 6.22) suffers it for almost all inward thruster angles. The interaction is genuinely complex — at a small wash-to-skeg angle the faster water along the skeg can instead assist the turn — so a master must know how his particular tug behaves and use the thrusters in the safest way; far-aft-skeg tugs should do bow-to-bow only at low speed. (Note 6: Figs. 6.21–6.27 except 6.24 by IMC Corporate Licensing.)

9.2 Safe speeds and the key items

All speeds are speed through the water, in calm water:

Recommended safe ship's speeds for ASD-tugs (calm water, speed through the water)
Operation	Less capable ASD-tug	Well-designed ASD-tug
Connecting bow-to-bow	3–4 knots	~6 knots
Assisting, fastened bow-to-bow	4–5 knots or less	~7 knots
Stern-to-bow (over the stern)	~7 knots safe maximum; somewhat more with a good tug/master/co-operation, but not above ~60 % of the tug's free-sailing speed

Release the line at the same safe speeds, and plan release well ahead — departing ships pick up speed quickly. (Note 8: a practical rule — base the safe speed on what a master can drive in a controlled manner astern on one engine, then subtract one knot.)

Items requiring attention for bow-to-bow (summary of 6.3.13). Capabilities differ greatly by design, so each tug's limits must be known. The crucial factor is the specific tug's ability to run astern and steer safely at the speed, which depends mainly on hull and skeg design. Risks 1: capsize risk is minimal when fastened bow-to-bow but real for stern-to-bow. Risks 2: a tug fastened bow-to-bow on the ship's track has almost no time to recover from an engine/thruster failure — failure of both is grave. Other watch-points: low visibility (radar 180° "wrong" astern), waves (running astern into them; need freeing ports; consider switching to stern-to-bow), openings closed (engine-room vents especially), overheating of clutches/engines, and push-pull changeover near the berth. Above 4 knots, bow-to-bow steering force falls fast with speed, whereas stern-to-bow steering may hold or even rise. Training and experience on the specific tug are an absolute necessity.

Making fast a forward tug — points for tug and pilot. While a tug is close to the bow preparing to pass a line: avoid course changes; do not use bow thrusters (warn the tug if they must be used); do not use the autopilot; tell the tug of any drift angle from cross winds; and have two proper heaving lines ready. Optimal communication and co-operation is indispensable — the pilot mostly cannot see the tug right ahead, so towing directions must be clearly agreed.

10. Crew reduction, other practical aspects & summary (6.3.14–6.5)

10.1 Crew reduction (6.3.14)

Whether a two-man crew is safe on omni-directional harbour tugs is essentially an economic question, relevant only in high-wage ports. Some ports allow it under conditions — Hamburg, Rotterdam, and some Canadian and New Zealand ports — typically requiring a qualified captain and engineer, two independent main engines, the ability to assist on one engine, specific wheelhouse and visibility standards, deck-operable winch controls and reliable quick-release systems, and man-overboard provisions. Rotterdam adds a third crew member when visibility is under 300 m, with deck ice, winds of Beaufort 7+, sea state 3–4, or work beyond 5 nm. Requirements are not uniform.

Why two-man crews are generally not recommended. A two-man crew raises the master's workload — he already handles the tug with both hands while answering radio calls and complex winch/eco controls. The decisive risk is incapacitation: if the master collapses while the deckhand is forward passing a line at a ship's bow at speed, or if the deckhand is hurt while the master must keep handling the tug in a critical position, help from another tug may come too late, and a large high-sided ship in wind could run aground. The reasonable conclusion is that two-man operations are generally not recommended; where a port's specifics might allow them, a thorough risk assessment with local masters is needed. In practice, even where allowed (Hamburg, Rotterdam), they are not used except in need.

10.2 Other practical aspects (6.3.15)

Bulbous bows. A submerged or part-submerged bulbous bow cannot be seen; the most dangerous case is the tug's stern touching it at high forward speed. Take great care close to a bulbous bow, especially in fog and darkness.
Releasing tugs. Release only when no further assistance can be expected — a departing ship can suffer an engine or rudder failure after the tugs are let go in a fairway, and grounding may follow; escorting with tethered tugs reduces that risk (as with large bulkers at Port Hedland). If a ship cannot release a heavy wire towline as she speeds up, the dragging line gains tension and becomes almost impossible to free — avoidable only by speed control, an experienced crew and good co-operation. Lower towlines gently to the tug's deck, raise Norman pins, and stop a fixed-pitch propeller when stern lines are released.
Underestimating wind and current. This has caused accidents; a tug at the side can be jammed between ship and shore (Fig. 6.29), especially if secured by towlines. Tug bollard pull against wind and current must be more than sufficient.
Sudden changes in heading or speed. While a tug is close to the hull with the master's attention on line handling, an unwarned change can create a critical situation; engine starts of big container ships badly affect a tug behind the stern. Always inform assisting tugs of intended engine/propeller and course manoeuvres.
Ship design consequences. Tension winches reduce the number and worsen the location of bollards; submarines, carriers and some ro-ros make securing awkward. Bollards and fairleads must be sufficient, well located, in good condition and strong enough for modern powerful tugs — failures have resulted otherwise.
Information exchange. The pilot should give the captain the number, type and bollard pull of tugs, rendezvous, fastening and release plan, transit speeds and intended manoeuvres; the captain should report manoeuvring devices or limitations.
High Dead Slow Ahead speeds. Large container ships can have a Dead Slow Ahead over 7 knots; making a forward tug fast bow-to-bow at that speed is not recommended. If the ship loses steerage when the engine is stopped, fasten the omni-directional stern tug(s) first.
Blocked or damaged thrusters. Thrusters and Voith units can be blocked or damaged by debris (Figs. 6.30–6.32) — serious in bow-to-bow work, so avoid it in waters with debris.

10.3 Summary and conclusions (6.4–6.5)

Several interactions exist — some affecting performance, some safety, some both. The safety-affecting ones occur between ships, or tug and ship, when close together; they are stronger in shallow and narrow water and rise sharply with ship's speed. Because tugs must work close to ships that are often making way, these effects must always be reckoned with. The essentials for safe shiphandling with tugs are:

Experience in recognising risky situations and knowing how to deal with them, and good knowledge of tug limitations.
Appropriate ship's speed, allowing for interaction and tug limits — vital for forward tugs working bow-to-bow.
Careful use of the ship's propellers near the stern; inform tugs in good time before using them.
Optimal communication and co-operation between pilots, captains and tug masters — pilots warning of manoeuvres and watching the tugs; masters reporting developing risks; captains reporting manoeuvring and securing limitations.
Tugs fully appropriate and compliant: properly manned, sufficient bollard pull, high manoeuvrability and free-running speed, good stability and freeboard, suitable towing equipment with a working quick-release, and a good wheelhouse view.
Tugs at the side powerful and secured against being jammed by wind, current or waves; enough crew ready to secure tugs quickly; proper heaving lines ready; towlines lowered gently, not dropped; ships designed so towlines can be safely secured.
Plan ship movements in fog with pilots and tugs; beware debris blocking propulsion (worst bow-to-bow); keep weather-deck openings fitted with watertight doors and closed during towing.

Finally (6.5). The number of tug accidents is unknown, because not all are reported or investigated — even some with fatalities. The book Tug Stability records that in 2010–2015 alone, more than 45 people drowned in known harbour-tug capsizes. In one large South American country a towing institute found that over twenty years its tug founderings were twenty times as frequent as at the largest European companies — the main causes being lack of tug-master training and high ship speeds. The scale of these accidents, and their often tragic consequences, is exactly why tug safety, and the training behind it, matter.