AN Ch. 4 — Tug Capabilities and Limitations

1. Introduction & basic principles (4.1–4.2)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 4 — Tug capabilities and limitations.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 4 — Tug capabilities and limitations · Anexo 2-A, Área II, item 26 (Utilização de rebocadores portuários) com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores empregados no Brasil).

With the assisting methods and tug types now introduced, Chapter 4 turns to the practical heart of the book: effective ship handling with tugs. The starting observation is decisive. When a ship is stopped in the water — no speed through the water — a 30-tonne bollard-pull tug delivers the same effect whatever its type, provided it operates in the most effective way. Differences between tug types only appear once the ship has speed through the water. The whole chapter therefore concentrates on tug performance while assisting ships under way.

Besides the essential matter of bollard pull, effective ship handling rests on two things: correct tug positioning and the right type of tug. The performance of the basic types is discussed against both the assisting method and the tug's position relative to the ship. Because designs vary so widely within each type, the treatment is general; but grasping the basic principles also illuminates the Related types and the FAST tugs (one thruster forward, one aft) introduced in Chapter 2.

4.2 The definitions you must own first

Tug performance is governed by a small set of points and forces. Section 4.2 fixes them before any capability is judged: the pivot point of the assisted ship; the towing point, pushing point and lateral centre of pressure of the tug; the direct and indirect towing methods; skegs; and tug stability. Everything in section 4.3 — what each tug type can and cannot do — follows from how these points sit relative to one another.

2. The pivot point (4.2.1)

The pivot point is an imaginary floating point, lying somewhere on the centreline between stem and stern, about which a vessel turns when forced to change direction. Its position is not fixed: the form of the submerged body, rudder size and type, trim, underkeel clearance and direction of movement all shift it. For effective assistance, knowing where the assisted ship's pivot point lies is critical, because it sets the lever arm of every tug force.

2.1 How speed and rudder move the pivot point

When a ship is dead in the water and forward thrust is applied with rudder, the pivot point lies far forward. As the ship gathers speed it moves aft. In a steady turn with rudder hard over it settles roughly one-third of the ship's length from the bow. A ship moving astern and turning (e.g. on a bow thruster) has its pivot point somewhere between stern and midships. Beamy, full-bodied ships have a smaller turning diameter and a more forward pivot point than slender ships; a ship trimmed by the head also turns tighter with the pivot point further forward. Turning diameter is independent of speed (so long as revolutions match the speed) but depends on rudder angle, and it grows considerably in shallow water because of the larger opposing hydrodynamic forces.

2.2 Why a forward push does so little at speed — and the Donkey Effect

A force on a ship — tug or rudder — produces a transverse force and a turning moment, giving a lateral velocity and a rate of turn. On a ship with headway, this sideways motion is opposed by hydrodynamic forces centred forward, which themselves create a turning moment. That moment opposes a tug pushing forward and assists a tug pushing aft.

So when a tug pushes the bow in the direction of a turn, the pivot point moves aft, the lever looks long — yet the effect is small, because the forward-centred hydrodynamic force counters the tug, and the tug's own underwater resistance counteracts the turn. This is also why a bow thruster does little at slow-to-moderate ahead speed.

Pushing aft, by contrast, gives a long lever and lets the lateral resistance forward contribute to the swing — the further forward and/or aft of the pivot point a tug works, the longer the lever and the more effective the assistance. A ship dead in the water with one tug forward and one aft of equal bollard pull pivots about midships (on even keel); the rate of turn then depends on bollard pull and on the lever between the tugs. A single tug pushing at the bow or stern of a stopped ship turns it about a point roughly one ship's width from the opposite end. Wind and current are further external forces that move the pivot point, and their relative direction keeps changing as the ship's heading changes during a transit.

3. Towing point, centre of pressure, direct/indirect towing & skegs (4.2.2)

A tug's performance is governed mainly by the relative positions of three resultant forces: the centre of thrust (its propulsion), the towing or pushing point, and the lateral centre of pressure of the incoming water flow. The mutual relationship of these points decides both effectiveness and safety.

3.1 The towing point — and movable towing points

For a tug towing on a line, the towing point is not the hook or winch; it is the point from which the line runs straight to the ship — usually the staple or fairlead. Its location is extremely important for stability, safety and performance. Towing points may be fixed, or movable: the gob-rope system, the radial towing hook, the carrousel and DOT systems, and azimuth friction-free / auto-position-escort / certain staple designs. The last group shifts the towing point to the lower side of a heeling tug, cutting the heeling lever — a strong positive for both stability and performance, the carrousel especially. A gob rope shifts the towing point longitudinally; moved to the tug's after end it improves both performance and safety.

3.2 The lateral centre of pressure

The lateral centre of pressure is the point where the force of the incoming water flow acts. It is non-stationary, depending on the underwater hull and appendages (rudder, propellers, skeg), trim and the angle of attack; rudder and propellers shift it strongly. Tractor and especially VS tugs carry a large skeg aft, placing the centre of pressure well aft. The magnitude of the water-flow force rises with the square of the speed — so speed dominates.

With flow coming from abeam, the centre of pressure lies behind midships, about 0.3–0.4 × LWL from aft — nearer 0.3 LWL for conventional tugs, nearer 0.4 LWL for tractor tugs; reverse-tractor and ASD-tugs may have it further forward. As a tug turns its bow into the flow the centre of pressure moves forward (conventional and tractor tugs generally not forward of midships, 0.5 LWL; ASD / reverse-tractor tugs can go forward of midships). Turning the stern into the flow moves it far aft.

3.3 Forward tug on a line: why a conventional tug beats a tractor tug

Consider a tug moving ahead, towing on a line, assisting a ship at speed (Fig. 4.5). The water-flow force F (centred near amidships, point C) resolves into a lift L and drag D: L adds force to the towline, D must be overcome by thrust. The towing point T lies a little behind C, so towline force plus L makes a turning moment the tug must counter with steering. The smaller the distance T–C, the smaller that moment, the less steering power needed, and the more engine power left for towing.

For a conventional tug the propulsion is aft (Ps); the steering force it applies adds to the towline force. For a tractor tug the propulsion is forward (Pt); the required steering force is in the opposite sense and subtracts from the towline force. As speed rises F and L grow, more steering effort is needed, and the gap between the two types widens. Worse for the tractor tug, its towing point is set further aft (for safety and stern-tug work), making it even less effective as a forward tug. The conclusion: working forward on a line at speed, a conventional tug is more effective than a tractor tug. The better a tractor tug's omnidirectional thrust and the lower its underwater resistance, the smaller the penalty.

3.4 Moving astern through the water — the dangerous towing point

When the tug moves astern through the water (Fig. 4.6) the centre of pressure lies much further aft. For a tractor tug, a forward towing point T is now very dangerous: large heeling moment from the hull force, plus large crosswise steering forces, plus the long vertical lever between propulsion and towing point — at higher speeds or too large an angle of attack this can capsize the tug. The safe answer is to locate the towing point aft, just abaft C; the tug then lines up with the towline when engines stop and needs little steering — the basis of safe indirect towing. Conventional tugs do not operate as in Fig. 4.6 at speed either: at very low speed they may work broadside using a gob rope (shifting the towing point aft from T1 to T2, Fig. 4.7) so the tug steers by going ahead or astern; twin-screw tugs often hold broadside on the propellers instead. Using the ship's engine ahead here is dangerous — the propeller wash hits the tug's hull and adds heeling force.

3.5 Direct and indirect towing

The direct method is used by an after tug on a line at relatively low ship speeds: the tug simply pulls in the required direction to steer and/or control speed. Tractor tugs assist stern-towards-stern; ASD / reverse-tractor tugs bow-towards-stern. The smaller the distance C–T relative to P–T, the better the direct performance.

The indirect method is used by an after tug at speeds above about five to six knots: the tug angles itself so the incoming water flow on its skeg and hull generates the towline force — far in excess of bollard pull. The tractor tug's aft towing point (small T–C distance) means little steering power is needed to hold the most effective position; the ASD / reverse-tractor tug, with a larger T–C distance, needs more crosswise power and so develops a little less towline force. As the ship begins to turn it gains a drift angle and the stern (outside of the turn) speeds up, so the tug's speed rises and steering forces climb higher still. The indirect method is the foundation of escorting (Chapter 9).

3.6 Pushing point and skegs

When pushing at a ship's side, the larger the distance between propulsion (P) and pushing point (Pu) relative to the distance between centre of pressure (C) and Pu, the better the tug can work at right angles. Skegs shape this balance and may help one task while hurting another, so what is expected of the tug must be considered first. Hensen lists the main types:

A pure harbour tug should be most effective below six to seven knots, when the assisted ship is slowing, has stopped its main engine and lost much of its controllability — needing high towing forces in all directions with a short response time and low underwater resistance. A tug that must also operate at higher speeds or escort needs a well-designed underwater body and skeg to generate high indirect forces. The two requirements pull in opposite directions, so the design is a compromise.

4. Stability and the stability rules (4.2.3–4.2.3.1)

Operational stability is one of the basic design requirements and matters enormously for harbour, terminal and escort tugs because of the nature of their work. Conventional tugs (and ASD-tugs towing on a line like a conventional) can experience very large athwartships towline forces; tractor and ASD / reverse-tractor tugs experience them when indirect towing — at high speed, far in excess of bollard pull, and pushed higher still by dynamic effects: irregular engine or tug control, wave motion, and low-stretch towlines such as steel wire or HMPE fibres (Dyneema, Spectra).

4.1 The stability essentials — GM, GZ, down-flooding

A heeled tug's weight (at the centre of gravity G) and buoyancy (centre of buoyancy, shifted by the list) combine to right it; the lever between them is the righting arm GZ. Plotting GZ against heel angle gives the stability curve. Stability is indicated by GM — the distance between the initial metacentre M and G: the higher G, the smaller GM. GM can even become negative (e.g. heavy ice on superstructure and masts — most dangerous). The range of the curve and the regulatory area ratios matter too. Means of raising stability and cutting heeling effects:

• High GM and good dynamic stability. A tug needs residual dynamic stability after a sudden heel. Beam has a large influence on GM — beamier = larger GM and righting moment. Length/width ratios are falling (many modern tugs ~2.8:1 or less; the tractor tug Broward 2.5:1, the reverse-tractor Tiger Sun and the Giano tug 2.0, SDMs even 1.8). A larger beam requires more freeboard too, or the deck edge submerges sooner. Sponsons add deck area and reserve buoyancy without widening the waterline.
• Reducing transverse hull resistance. A smaller underwater lateral area and fewer appendages let the tug be pulled through the water rather than rolled over — but this conflicts with the large lateral area a tug needs to generate indirect towline force, so it is a trade-off.
• Lowering the towing point and shifting it to the low side. Keep the staple/fairlead/hook as low as possible above the centre of pressure; the radial towing hook and carrousel shift the towing point to the heeling side, shortening the heeling lever and lengthening the righting lever (Fig. 4.12).
• Lower pushing point (smaller heeling moment when pushing at an angle), a shock-absorbing towline, and freeboard high enough that the deck edge is not immersed at a small angle.

4.2 Stability rules (4.2.3.1)

Standards used to vary widely between classification societies and national authorities — undesirable for safety. A group led by Bureau Veritas, under the SafeTug Joint Industry Project (MARIN), produced harmonised towing and escort stability regulations, proposed to IMO. On 25 November 2016 IMO adopted amendments to the 2008 Intact Stability (IS) Code including tug criteria; they apply to new tugs (keel-laying) from 1 January 2020. The IS Code applies to ships of 24 m and over in international trade, but in practice most harbour tugs are certified for international voyage and national authorities require compliance even for domestic vessels, so the rules have major impact. The new rules classify tugs in three groups:

• Harbour towage — assisting ships within sheltered waters.
• Coastal / ocean-going towing — outside sheltered waters, where towing forces are often a function of bollard pull.
• Escort operations — steering, braking and controlling the assisted ship, where the forces come from hydrodynamic forces on the tug's hull plus the propulsion thrust.

The rules (recommendations) address two girting situations: self-tripping (the tug overturns itself under the heeling couple of towline plus propeller forces) and tow-tripping (the tug veers off — e.g. on loss of propulsion — and is dragged, towline force opposed by hull resistance). For harbour and coastal/ocean tugs: self-tripping requires the righting energy to equal or exceed the heeling energy; tow-tripping computes a heeling lever from being pulled sideways at 5 knots, the equilibrium angle to be less than the down-flooding angle. (This last is questioned: the method was built for conventional tugs, ignores ASD-tugs over the bow, has no residual-stability requirement, and large container ships may enter port well above 5 knots.) For escort tugs a maximum static heeling angle of 15° applies at equilibrium. A quick release is now mandatory for the towing winch as well.

5. Stability recommendations for tug masters (4.2.3.2)

A tug's stability changes continuously the moment it leaves the berth — fuel is burned from the lower bunkers, tanks go slack, waves put water on deck. These effects are mostly negative and can be cumulative. The master cannot influence all of them, but can manage many, and must know how daily operations affect stability and how to control the negatives.

5.1 Know the tug; keep the stability book

The single most important factor is to know the tug's capabilities and limitations — to avoid situations it cannot handle and to prepare for extreme, unfamiliar ones by gathering information in advance. Most harbour tugs, and all terminal and escort tugs, carry a stability book giving GM and curves for various loading conditions and showing which openings (including engine-room vents) are treated as closed. But these are standard situations; during dynamic towage the real situation shifts continuously and can be worse than the book.

5.2 What raises the centre of gravity (and cuts GM)

• Bunkers. The largest GM is usually with bunkers full; with the lower tanks empty GM falls — in the worst case below class requirements. Ballast water adds weight (larger GM) but a smaller stability range.
• Slack tanks. Negative effect — minimise their number.
• Water on deck. Extra weight high up: G rises, GM falls, plus the same free-surface penalty as slack tanks — worst when also in indirect mode at high speed or in waves.
• Icing. In cold areas minimise ice growth — reduce speed, change course, remove ice at the first opportunity.
• Trim by the head is negative — avoid it.

5.3 Openings, the towline lead and interaction

Large heel comes with the risky moments of tug work — girting, getting under an overhanging bow, coming under the bow or being hit by the bulb at speed. If any such heel lets water into the tug it capsizes quickly. So all openings through which water could enter at large heel must be closed, including wheelhouse doors (often wrongly left open, Fig. 4.17) — a remote control is recommended. Engine-room ventilation is the conundrum: the engine needs so much air that even one inlet closed soon stalls it, yet at critical moments those vents must be closeable; an experienced master's view is that the practical answer to dangerous heel is to release the towline, because such heel comes on too fast and dramatically to rely on closing vents.

Keep the towing point low and the towline free of obstructions; use a gob rope in a considered way for a conventional tug at the stern, and keep ship's speed very low when manoeuvring on it. Any change to towing-point position or towline lead must be checked for its effect on stability and recorded in the stability booklet, approved by class. Beware ship–tug interaction near the bow of a ship at speed (forces rise with the square of speed) — keep speed low, especially when passing a towline. Maintain and regularly test the quick-release system at the highest expected loads; where a line is still belayed on the bitt, keep an axe or sharp knife (fibre lines only — never wires). Check towlines for condition and residual strength; conventional towlines lose stretch and strength with use.

6. Capabilities towing on a line — forward & stern tugs (4.3.1)

Capabilities and limitations rest on the two principal methods: tugs towing on a line and tugs operating at a ship's side. The types considered are those found most in ports — conventional, ASD and tractor tugs; the Related and FAST types can then be judged from this plus Chapter 2. Good co-operation between pilot and tug captain, built on understanding each other's capabilities and limitations, is indispensable. For tugs towing on a line, everything follows from the location of the towing point and the propulsion (Fig. 4.18): in a conventional tug the towing point is forward of the propulsion; in a tractor tug it is aft of it.

6.1 Forward tugs on a line

Any type of forward tug on a line can steer or deliver crosswise force to either side. But response and effectiveness differ. A tractor tug moves quickly side to side (side thrust from forward propulsion) but, lying more in line with the towline, must overcome higher sideways resistance at the expense of towline pull, so it is less effective than a conventional tug on a ship at speed. A conventional tug (or ASD-tug over the stern) turns about its towing point, meets a smaller angle of attack and makes better use of hydrodynamic forces — a more effective pull. In general, with rising ship speed the conventional tug's effectiveness increases and the tractor tug's decreases; the higher the speed the larger the gap. An azimuth tractor tug can work at a larger towing angle than a VS tractor tug thanks to better off-axis thrust.

At very low ship speed a conventional tug forward gives very effective steering (position B1b): its resistance creates high steering forces without increasing ship's speed, and the engine power used to stay clear adds towline force. A good forward conventional tug (or ASD over the stern) can apply rudder to give a sideways force compensating wind while also counteracting the ship's rudder (Fig. 4.20) — greatest at low speed with not-too-large ships; the same trick unberths with one tug (rudder hard over to the berth, dead slow ahead, tug pulling off). Forward steering generally adds a speed-increasing vector, and tugs tend to keep lines tight when idle — also speed-increasing — so pilots often order the towline kept slack when no assistance is needed. Reverse-tractor tugs (and ASD-tugs as reverse-tractor) work as forward tugs much like tractor tugs but bow-towards-the-ship's-bow.

6.2 Stern tugs on a line

As a stern tug the situation is entirely different — whether steering can be given to both sides, and whether ship's speed can be controlled, depends on tug type and ship speed.

• Tractor tug (Fig. 4.19C). At lower speeds it steers by direct towing (position 1a increases speed; position 1b also brakes). It can swing easily from position 1 to position 2 for speed control or to steer to port — even at ~7 knots, thanks to the aft towing point. At higher speeds it uses the indirect method (position 1c): steering to both sides while controlling the ship's speed.
• ASD / reverse-tractor tug. Similar, bow towards the ship's stern; generally a little less effective than the tractor tug in indirect steering at higher speeds.
• Conventional tug (Fig. 4.19D). Can steer to one side only (starboard in the figure), and delivers longitudinal forces that may increase ship's speed. Moving to the other quarter is impossible above 1–2 knots. Above 3 knots it is dangerous to move from position 1 to position 2 to control speed — the tug may come broadside with too-high towline forces and capsize unless released in time (a gob-rope winch lets it swing at somewhat higher speed). At very low speed (≤3 knots) it can lie broadside astern and steer both sides; twin-screw tugs often manage without a gob rope.

So: above about 3 knots a conventional stern tug gives steering to one side only; only at very low speed can it steer both sides and control speed. It is very restricted as a stern tug because of its towing-point location. When a conventional tug is close behind the stern, the ship's propeller must be used with great care — wash can overturn the tug; tractor and ASD / reverse-tractor tugs are at less risk because of where their towing point sits, but a short towline near azimuth propellers can cause heavy vibration (lengthen the line), and a ship's engine set astern with large engines is genuinely dangerous — the pilot must always warn the after tugs before using the engine. Each type has versions of differing capability (a twin-screw conventional tug outperforms a single-screw one), so positions must be planned with wind, current and bends in mind.

7. Operating at a ship's side; other types; summary (4.3.1 cont.–4.3.3)

7.1 Pushing mode (Fig. 4.21)

Efficiency in pushing comes down to how well a tug works at right angles without increasing ship's speed. It depends largely on the ratio a : b — the lever of propulsion (P–Pu) against the lever of hydrodynamic force (C–Pu). The better the tug overcomes the turning moment of the hull's hydrodynamic force with sideways thrust, the more it can stay at right angles and the more power is left for pushing; the vertical position of the centre of pressure, stability and freeboard also count, and fendering must stop the tug sliding along the hull.

• Conventional tug. Its aft-lying centre of pressure makes it hard to reach and hold right angles at speed; its large underwater plane and weaker steering limit it. It may need a stern line (Fig. 3.4) to hold right angles as the ship gathers speed, but excessive speed risks parting the line or capsizing. Effective pushing maxes out at about three to four knots (sometimes less for less manoeuvrable tugs). High-lift rudders or Towmaster/Nautican systems improve it.
• ASD / reverse-tractor tug. Very effective at pushing — efficient steering propellers, far-aft propulsion, more forward centre of pressure. Seaspan (ex-C H Cates) claimed a 90° side push at up to eight knots versus the usual four for conventional tugs.
• Tractor tug. Also much more effective than conventional tugs, thanks to omnidirectional propulsion.

The other factor is the maximum heeling angle: the higher the pushing point, the larger the heeling moment (which grows with speed squared) and the less it can be compensated. Conventional tugs, with their large underwater plane and lower steering forces, struggle; wide-beam tractor and ASD / reverse-tractor tugs compensate much better and hold right angles at higher speed. At high ship speed tugs push at a smaller angle, and lift forces themselves create rather high pushing forces (Fig. 4.22).

7.2 Pulling mode

Tugs at a ship's side need good astern power, about equal to ahead — so omnidirectional propulsion suits push-pull. A conventional tug pulling will swing around and needs a stern line leading forward to pull at right angles (the paddle-wheel effect of its propeller adds to the swing); even then only low ship speed is possible. Tractor and ASD / reverse-tractor tugs perform much better, applying force in the ship's direction of movement. A key loss: a tug's propeller wash hitting the ship's hull can cost as much as its bollard pull, sometimes more — so tractor tugs push and pull with their stern to keep propellers away from the hull, and azimuth tugs angle the thrusters to divert the wash; a longer towline also helps (only when pulling alone). When changing from pulling to pushing, dynamic towline forces — especially with a steep line and in waves — can draw the tug hard against the ship when the engine stops (Fig. 4.23), so astern thrust must be applied carefully.

7.3 Other tug types (4.3.2)

The same factors — towing point, pushing point, centre of pressure, propulsion location/type — assess the Related and FAST types. The Rotortug (two thrusters forward, one aft — a kind of active skeg) generates high towline force in indirect mode (thruster near the towing point) and high pushing force (thruster near the pushing point), and works within a ship's width. The Z-tech and RSD tugs compare to an ASD plus tractor tug combined. The carrousel and DOT tugs have a varying towing-point location that greatly increases capability and safety on a line. The FAST tugs (one thruster forward, one aft) work like the Rotortug in narrow spaces but have only half the total power at each end — a drawback for some methods, such as towing on a line on a ship with headway. The exception is the CRT (Carrousel RAVE Tug), whose carrousel plus fore-and-aft towing pins allow many assisting modes and high towline force.

7.4 Summary by type (4.3.3)

8. Effectiveness, performance diagrams & effective tug position (4.3.4–4.3.6)

8.1 How capabilities are determined (4.3.4)

Tug capabilities come from model tests, full-scale trials and simulation. Most studies cover one specific tug or type (Voith on VS tractor tugs; Aquamaster on azimuth tugs). Desktop and full-mission simulations, verified against full-scale trials and run with pilots and tug captains, give fairly reliable results, but may not capture every factor — ship–tug interaction, the flow field around a ship, water-depth and confinement effects, the ship's wake on braking — so some inaccuracy remains. Real limits are felt only in daily handling, but simulation can verify what the book explains. Performance is often shown by polar diagrams (towline force at various towing angles and ship speeds).

8.2 What the diagrams show

8.3 Effective tug position (4.3.5, Fig. 4.29)

Positioning depends on ship particulars (type, size, draft, windage, manoeuvrability), the environment, the passage, stopping distance, turning-circle size, berth and berthing side, and the number, type and bollard pull of available tugs — all read against the pivot point. For a ship with headway turning to starboard:

• No. 1 (forward, on a line): high crosswise steering force, but limited by the transverse forces near the bow; the more it pulls in line with the heading the more it adds speed. Very flexible for compensating wind/current from either side.
• No. 2 (forward shoulder, pushing): poor — same bow forces to overcome but a much shorter lever, and its resistance opposes the turn; it may even turn the ship the wrong way (Brandner's loaded-tanker case at depth/draft 1.2: pushing the port shoulder, the tanker turned to port and speed increased). In ballast and trimmed by the stern in deep water the same push does turn the ship the right way.
• No. 3 (aft quarter, going astern): a good position for starboard steering assistance — by going astern (with a bow line) it brakes the ship and helps the swing; note that, being pulled crosswise through the water by the ship's stern, its drag opposes the turn somewhat.
• No. 4 (aft shoulder/quarter, pushing): effective — long lever and forward-centred lateral resistance help the swing; speed is hardly affected because the higher rate of turn raises the drift angle.
• No. 5 (aft, on a line): very effective — longest steering lever, forward transverse forces help the swing, and it does not add speed (it also brakes while steering).
• No. 6: as effective as No. 5 for steering but adds ship's speed (as would a rudder tug).

For swinging a stopped ship in a turning circle, tugs 1 and 5/6 are best (longest levers). Which positions to use depends on what is required, the ship, the situation and the berthing side; for mooring large ships up to four tugs may be used, often 3 and 4 pushing and 1 and 5/6 controlling approach speed.

8.4 Towing on a line vs operating alongside (4.3.6)

The two basic methods were weighed in § 3.2.2; here the choice of which side to make fast is added. Towing on a line keeps tugs on the safe side and flexible about berthing side; even in the worst case (wind/current too strong) line tugs can assist to the last moment, minimising damage; omnidirectional tugs can switch to push-pull during berthing without releasing the towline, shortening berthing time and keeping the ship under better control. Tugs alongside are positioned by berthing side and by the forces to compensate — which may be the wrong side for berthing, forcing a shift (during which the ship has little or no assistance and may drift); if wind/current are underestimated and the ship drifts, the alongside tugs may have to escape from between ship and bank, leaving the ship unassisted.

9. Design consequences & environmental limits (4.4–4.5)

9.1 Design consequences (4.4)

Because a VS tractor tug is so effective as a stern tug on a line — operating stern-towards-the-ship with the captain facing aft, the ship's direction of movement — some VS tractor tugs are built with the wheelhouse turned 180° and a higher, raised stern for protection from incoming waves (the terminal tugs Baut, Boris, Boxer; the Redbridge with funnels forward of the wheelhouse for an optimum view aft). Because ASD-tugs mainly work over the bow as reverse-tractor tugs (bow-to-bow), the tractor tug becomes a good alternative, and tugs that can act as either tractor or ASD were developed (Z-tech, RSD). The girting risk of the conventional tug's mid-tug towing point drove the carrousel system; the need to work in narrow spaces drove the SDM. The search for improvement is continuous, also because of new tasks (terminal tugs, escort tugs, ISVs).

9.2 Harbour tugs — fog, waves, swell (4.5.1)

Harbour tugs operate in all conditions of current and wind, but fog in confined areas makes assistance very risky: tug movements are too fast for radar, the tug often lies close to the ship (distorted or blank radar picture), and the master may lose sight of the towline — so many ports restrict assistance below about 0.5 mile visibility. Some ports lie near open sea, or breakwaters no longer keep pace with ship size, forcing tugs to work outside in waves; passing towlines is then difficult, alongside towlines are short and steep with high dynamic forces, so strong, sometimes double, high-stretch fibre lines are used. In waves, tugs can switch to towing on a line (longer lines absorb dynamic force) or work the ship's lee side. Indicative upper limits for typical harbour tugs:

For swell (periods ≥ 8 s) modern manoeuvrable tugs are assumed; it needs long heaving lines, long light HMPE pennants, a long strong messenger, and towlines long enough (e.g. 80–100 m) to avoid peak loads. Experienced masters warn of three dangers: the tug surfing down waves (hard to hold course, especially with a lift-generating skeg); beam swell with a rolling ship risking superstructure or appendage contact as tug and ship roll out of phase; and the ship making frequent large rudder movements. Making fast at the centre lead forward and/or aft is safest, and broad experience in such conditions is essential.

9.3 Terminal & escort tugs; the SAFETUG project (4.5.2)

Growing ship size and high dead-slow speeds (container vessels), plus offshore LNG / FPSO / FLNG / SPM terminals, force more tugs to work in waves. To learn how to design for this, the SAFETUG Joint Industry Project ran at MARIN 2005–2010 with the tug industry, addressing operability envelopes and design requirements for an ASD and a Voith design. Key findings:

• Roll is the limiter. Roll motions limit operation and cause high transverse accelerations that harm the master and the towline. Roll damping is improved by bilge keels (20–40% at low speed), Voith-Schneider active roll damping (~50%), a single hard-chine hull (~40%) and skeg area (~10%); B/T ratio matters less.
• Stability. Both designs behaved safely against capsizing modes (nose-diving, broaching, throwing over, water-on-deck) when built to current standards; bow-first operation with extra bow freeboard is inherently safer; the study fed updated escort-stability rules (Bureau Veritas).
• Significant wave height H_s. H_s is the average of the highest one-third of waves; the largest individual wave is roughly twice H_s — a tug claimed good in 3 m H_s must cope with ~6 m. Wave period and direction matter as much as height: short sea waves (not long-period swell) hurt tug performance.
• Propeller-wash interaction (effective bollard-pull loss, Fig. 4.34). Tugs towing at bow and stern lose little (highest average 12%). A tug pulling at the ship's side at about one tug's length loses on average 43–64% — a critical finding for harbour tugs too; loss grows with smaller underkeel clearance and falls with a longer towline or angled thrusters. In waves the loss can reach almost 90%.

After SAFETUG, tugs designed for waves continued (e.g. the ASD-tug Svitzer Lindsway, RAstar 3400, with much-reduced roll, now asked to berth in 3 m H_s); escort and terminal tugs carry render-recovery (dynamic) winches and, again, bilge keels. An experienced master's caution: wave period and direction often beat height; 1.5–2 m short steep seas are quite difficult on harbour tugs; assisting on the lee side differs greatly from the weather side.

10. Conclusions & other practical aspects (4.6–4.7)

10.1 Conclusions regarding tug types (4.6)

The chapter shows how performance differs between harbour tug types — essential knowledge. But each tug and type must be designed for safety and good performance: stability, towing- and pushing-point location, fendering, skegs and propulsion all need due attention; then the right type can be chosen for a port (the deciding factors were set out in Chapter 1). Tug tasks have grown — escorting and remote-area work add requirements for skeg and hull design, towing winch and fender. Safe operation and good performance are the two governing factors, so the capabilities and limitations of each type must be known. For low risk, the important aspects are:

• The location of the towing point.
• The type and location of the propulsion units.
• Stability and freeboard.
• Then: the type of towing winch; hull, skeg and fender design.
• And: ease of tug handling (plus, for some ports, maximum draft — larger for tractor tugs and others with propulsion under the hull).

Performance can be shown by polar diagrams (towline force at various angles and speeds) and pushing-performance curves. Whatever a tug's capabilities, risk is lowest and performance best when the tug master is well trained and knows what the tug can and cannot do — knowledge equally important for pilot and ship's captain.

10.2 Other practical aspects (4.7)

• Co-operation. Pilots, ship masters and tug captains must know each other's capabilities and limitations; the pilot should keep a close eye on the assisting tugs to act when one does not perform or its safety is at risk.
• Communications. A good radio system is indispensable; tug orders must be clear and single-interpretation, tugs addressed by name or position, orders confirmed and repeated. A worldwide standard vocabulary is hardly feasible (tug captains often speak only the local language, and standard vocabulary cannot cover non-standard situations), but a basic local system, standard for all local pilots and tug captains, is necessary — and the ship's captain must always be informed of intended ship and tug manoeuvres.
• Tug use. Harbour tugs should have a power reserve, react fast and need minimum space. Tug size and power should match ship size — large powerful tugs should not handle small ships, and the bollard pulls of tugs on one ship should not differ too much. Plan tug configuration well in advance; avoid repositioning that means releasing and refastening lines (it costs time, leaves the ship less assisted and risks fouling a propeller).
• Speed. Ship's speed must be carefully controlled to the tugs' limits — generally low. The lower the speed the more effectively and safely tugs operate (interaction and shallow-water effects follow in later chapters).