AN Ch. 2 — Types of Harbour Tug

1. Classification of basic harbour tug types (2.1)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 2 — Types of Harbour Tug.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 2 — Types of tugs · Anexo 2-A, Área II, item 26.2 (Tipos de rebocadores portuários), com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores no Brasil).

Tugs are named after their main characteristic — the type of propulsion, the propulsion manufacturer, or the location of the propulsion or steering system. Hence names such as conventional, Voith-Schneider, Z-peller, Kort nozzle and tractor tugs. There is no uniform naming system, and that is confusing: a "Z-peller tug" might have azimuth propellers forward or aft, and for tug performance that difference matters greatly. Hensen therefore argues it is better to classify tugs by their location of propulsion and towing point, since assistance is what tugs exist for.

Some history. Azimuth thrusters are not a 20th-century invention: the principle was patented as "steering propellers" in the early 1870s. Colonel WH Mallory patented a twin-propeller azimuth thruster in 1881 (Fig. 2A.1) to balance torque reactions. The Voith-Schneider also had a forerunner — the USS Alarm (1874) carried a Fowler vertical-axis propeller (Fig. 2A.2).

The two basic groups

Through tug development a large number of types emerged, grouped into three families in Table 2A.1: basic tug types, related tug types and special tugs. Classifying the basic types by thruster and towing point location gives two main groups:

Some types can be classed as either, depending on how they operate:

• Reverse-tractor (pusher) tugs — increasingly also called ASD-tugs: azimuth propellers aft, towing point forward, built to work mainly over the tug's bow (common in Japan, Hong Kong, Taiwan). A reverse-tractor keeps the tractor relationship (towing point towards the ship, propellers away) but lies in the reverse direction.
• ASD (Azimuth Stern Drive) tugs — multi-purpose tugs with azimuth propellers aft, built to work over the bow like a reverse-tractor and over the stern like a conventional. Usually a winch forward and a winch or hook aft.
• Combi-tugs — older tugs modernised with a 360° steerable bow thruster and an extra after towing point; can work as a conventional or as a tractor. See combi-tug.

So the basic types are: conventional; tractor tugs (azimuth or Voith); ASD-tugs; reverse-tractor tugs; combi-tugs. The related types (Table 2A.1) — Rotortug, Z-tech and RSD — have much in common with the basic ones: the Rotortug has three azimuth thrusters; the Z-tech and RSD have two under one of the tug's ends. A more recent family is the FAST (Forward-Aft Single Thruster) tugs — SDM, EDDY, Giano and RAVE — with a thruster (azimuth or Voith) at each end.

2. General requirements for good tug performance (2.2)

2.1 Performance, wheelhouse and decision support

Several factors decide whether a harbour tug performs safely and well. The first is a short response time: the tug must react in a minimum of time, both when assisting and when making fast. Because ship crews keep shrinking, making fast takes longer, so fast and easy handling of towing equipment is increasingly important.

Effectiveness depends not only on manoeuvrability but also on bollard pull and underwater shape — large container ships stacked six high need powerful tugs in strong winds. When the ship is underway, loss of the tug's effectiveness due to ship speed and towing direction should be as small as possible. Effectiveness and safety also depend on the tug's stability and suitable towing equipment. The manoeuvring space required should be as small as possible — achieved by a suitable tug type, good manoeuvrability, limited dimensions and proper towing gear.

Wheelhouse construction and layout (2.2.2)

From the manoeuvring station the captain must have a good view of the tug's ends and sides, the towline, the working deck, the contact area with the ship, the assisted ship, other tugs and the direction of operation — a field of view as close to 360° as possible, plus small upward-facing windows for making fast to high forecastles or freeboards.

The essential outside information comes from three things: the towline(s) — direction and tension; the assisted ship — relative heading, speed, distance off and how it reacts; and the combined ship/tug direction relative to channel boundaries, traffic and banks. Depending on tug type and method, this information may come from opposite directions and may change during one trip. A reverse-tractor working over the bow gets nearly all of it from forward, so one forward-facing station suffices. A tractor used for push-pull works over the stern and needs an aft-facing panel; free-sailing it needs a forward one — hence central or multiple panels. Controls must be logical: pushing a lever forward in the direction faced should increase movement that way, and a wheel/joystick to the left should turn the tug left, regardless of ahead or astern. Modern tugs often have one central panel in a small cockpit-like wheelhouse, with the radar, communication and quick-release systems within hand reach.

Decision support systems are a welcome development, but the right questions must be asked: does the system reduce the tug master's workload, keep his eye out of the window, warn him in critical situations (speed, rate of turn, distances off), and inform him about stability and safe heeling limits? Examples include the Damen Human Machine Interface (with Electronic Stability Protection), Raymarine DockSense Alert (stereo-vision hazard mapping), the FlyingView bird's-eye view (four fish-eye cameras), and Augmented Reality bridge systems. Cameras have a drawback — they fail in reduced visibility, dust or smog — so 3D sonar is a better alternative.

2.2 Superstructure, fendering, hazardous areas and eco

Superstructure and underwater design (2.2.3)

Because tugs work near a ship's flared bow or overhanging stern, the superstructure must be set well inboard of the deck edge to avoid damage. The underwater design must keep the propulsion units from hitting the ship's hull when rolling alongside — important when assisting submarines, whose hull the tug's propellers could strike.

Fendering (2.2.4)

Good fendering protects both ship and tug and reduces sliding when pushing at an angle. Fenders are rubber or synthetic rubber, chosen for resistance to polluted water, ozone, UV and temperature extremes. The choice of bow/stern fendering depends on the assisting method, the tug's size and power, the contact-area size, the ship type (large flare or overhanging stern may need extra fendering on top of the bow), the environmental conditions, and the tug's construction. Submarines and aircraft carriers may need underwater fendering and fendering at the top of the wheelhouse.

Tyres are often added to enlarge the contact area but are not designed as fenders, so mounting them is problematic. Fender material needs a high coefficient of friction to hold position when pushing at an angle — rubber-to-steel is about 0.8, with friction force F = c × P. Where no grip is wanted (e.g. side fendering), a low-friction top layer of UHMW polyethylene (friction coefficient 0.15) is used; some fenders add water lubrication. PIANC gives permissible hull pressures that fall with ship size — for example, general cargo of 20,000 dwt and less, 400–700 kN/m²; oil tankers over 60,000 dwt < 350 kN/m²; VLCCs 150–200 kN/m²; gas and bulk carriers < 200 kN/m². ASD and reverse-tractor tugs handling submarines may add fendered steel sponsons on the quarters so the azimuth nozzles never touch the submarine.

Hazardous areas and eco (2.2.5–2.2.6)

Tugs handling gas carriers near LNG/LPG terminals, and tugs working in ice, are subject to additional specific requirements dealt with later in the chapter. The need for more environmentally friendly tugs — eco-tugs, E-tugs, hybrids — is widespread; two points matter most for shiphandling: changing to full power must be simple so no mistakes are made under stress, and full power must be available without delay when needed.

3. Conventional tugs (2.3)

Conventional tugs are still built — in decreasing numbers — and used for push-pull, alongside towing and, especially in European ports, for towing on a line. The simplest is a single-screw tug with a single plate rudder; because of the towing-point location these tugs have performance and safety limits. Towing on a line, the main risk is girting, lowered (but not eliminated) by a quick-release winch and hook. Astern power is generally low. The towing point lies about 0.45 × LWL from aft (further aft on American tugs, which allows a longer deckhouse but limits effectiveness when towing on a line at speed). New small designs include the Smart tug (20.4 m, BP 19 t) and the Container tug (6 m, BP max 1.5 t, transportable in a 20 ft container).

3.1 Propulsion, nozzles and rudders (2.3.2)

Nearly all tugs use diesel engines (high or medium speed, with reduction gearing), though hybrids — diesel-direct, diesel-electric and battery — are spreading. Diesel-electric propulsion is easily controlled, delivers any shaft speed ahead and astern without delay, and is more efficient than diesel-direct under the varying loads typical of tugs; power is available faster because generators already run at full speed. On fixed-pitch propellers thrust is reversed by reverse-reduction gear; on CPP by changing pitch.

Going full astern, an open fixed-pitch propeller develops about 60% of maximum ahead thrust; an open CPP only 40–45%, because a CPP's blades, set for astern, take a smaller pitch at the lower part than the top.

Nozzles raise efficiency but cut steering, so special rudder systems are used. Steerable nozzles give very good astern steering (see figure 2B.5). Most tugs have balanced, semi-balanced or spade rudders; single plate rudders lose efficiency through flow separation. Manoeuvrability can be increased by:

• Movable flap rudders (Becker the most used) (see figure 2B.6): a flap of 20–30% of the rudder area turns further than the main rudder (main up to 45–65°, flap to 90–110°). Max lift, reached around 30°, is 60–70% higher than a plain rudder; side thrust up to 50% of ahead thrust; Becker claims 95–97% of propeller thrust is captured.
• Schilling (fishtail) rudders (see figure 2B.8): no moving parts, wedge "fishtail" end; 30–40% more lift, max at ~40°, usable to 70° where the slipstream deflects 90°. Twin Schilling VecTwin rudders vector thrust through 360°, giving up to 70% side thrust and eliminating shaft reversal.
• Flanking rudders: fitted ahead of the propeller (with fixed nozzles) to improve astern steering; amidships when going ahead (see figure 2B.10).
• Towmaster: a shutter system of several rudders behind (and ahead of) each nozzle, good steering ahead and astern at the cost of complexity (see figure 2B.11).

Conventional tugs can be single, twin or even triple screw — twin/triple being more manoeuvrable. Twin-screw propellers are often inward-turning for higher efficiency; with inward-turning fixed-pitch propellers a tug can flank (move sideways) by setting the inboard propeller astern, the outboard ahead and the helm towards the desired side (see figure 2B.13). A tunnel bow thruster loses ~50% effectiveness at only two knots ahead; a retractable 360° azimuth bow thruster (the combi-tug feature) is far more effective in any direction.

3.2 Manoeuvring and ship handling (2.3.3–2.3.4)

A single-screw tug is governed by the aft location of rudder and propeller, the transverse (paddle-wheel) effect when going astern, and low astern power. With ahead thrust and helm, the stern swings opposite to the intended turn (unlike a tractor). With a right-handed propeller set astern, the wash hits the starboard quarter, the stern goes to port and the bow to starboard. Twin-screw tugs are far more manoeuvrable — they turn on the spot and back straight by reversing one propeller and setting the other ahead with helm.

Conventional tugs serve all assisting methods but not equally well (see figure 2B.14). Towing on a line they are effective; as a stern tug they are severely limited by the midships towing point — above about three knots they can assist on only one side, cannot shift sides, and cannot control the ship's speed, with a constant risk of girting. When towing on a line they cannot easily change to pushing without releasing, because pushing with the stern puts the propellers too close to the hull (low efficiency) and the stern fendering is not designed for it. A single-screw tug cannot pull at right angles (transverse effect) nor hold that angle in cross-current/wind. Capability can be raised by a gob rope, a carrousel system, or an azimuth bow thruster (the combi-tug).

4. Combi-tugs (2.4)

A combi-tug is a conventional single-screw tug fitted with a 360° steerable (azimuth) bow thruster and an extra after towing point (see figure 2B.15). The first appeared in the early 1960s. With main propulsion plus bow thruster the tug can turn on the spot, sail straight astern at a fair speed and move sideways (see figure 2B.16); set in line with the propulsion the thruster adds bollard pull ahead and astern and raises top speed. Unlike a tunnel thruster, an azimuth bow thruster with a nozzle below the keel stays effective in any direction even at speed.

In ship handling, a combi-tug tows on a line forward (like a conventional tug, but faster, more manoeuvrable, with less girting risk) or aft. As a stern tug it shines: using the after towing point (via a gob rope or a stern fairlead) it can take a ship still doing seven to eight knots, control its speed by setting propulsion and thruster the same way, or assist steering by sheering out with main propulsion astern and the thruster working sideways (see figure 2B.17). It can shift behind the ship's stern faster than a normal conventional tug. Its advantages are greatest as a stern tug on a line; at the ship's side it keeps most of a conventional tug's disadvantages, the low-power bow thruster only partly helping.

Posições da figura, explicadas no texto (4)

1. 1. The combi-tug makes fast aft and approaches stern first to the stern of the ship to pass the towline (see figure 2B.17 position 1*)*.
2. 2 e 3. In positions 2 and 3 the incoming water flow creates lift forces on the tug and consequently a force in the towline.
   When the ship's speed reduces, the effect of the tug in position 2 and 3 will become less due to the reduced lift forces.
3. 4. The original towing point is then in use again and the tug can operate again as a normal conventional tug (position 4).
   In circumstances where there are strong cross winds and/ or currents, and much effort is required from the tug to compensate for those forces, the tug is more effective when it proceeds with the assisted ship as a normal conventional tug (position 4) and thus can use its full ahead power.
4. 5. As soon as the towline has been secured and the aft towing point is in use by means of a gob rope or fairlead, the combi-tug can control the vessel's speed (position 5) or assist in steering (positions

5. Tractor tugs with cycloidal (Voith) propellers (2.5)

Tractor tugs have their propulsion under the forebody. Those with a vertical-blade (cycloidal) system are Voith-Schneider or Voith tugs (VS tugs) — the "Voith Water Tractor" concept put the cycloidal propulsion under the forebody and the towing gear on the after deck, overcoming many conventional-tug limits (see figure 2B.18). The engine runs at constant rpm; thrust magnitude and direction are regulated from the wheelhouse, so the system is effectively a controllable-pitch propeller.

Posições da figura, explicadas no texto (6)

1. 1. Voith-Schneider propeller
2. 2. Propeller guard plate
3. 3. Skeg
4. 4. Towing staple
5. 5. Second towing position
6. 6. Wheelhouse with specific Voith controls

Two VS units sit side by side; their pitch and thrust direction adjust uniformly through 360° without delay. A protection plate guards the blades and acts like a nozzle; the tug docks on these plates and on the large skeg (see figure 2B.19). The skeg is typical of tractor tugs: it gives course stability and moves the centre of hydrodynamic pressure further aft, which helps safety and towing performance — especially as a stern tug at speed. The towing staple lies far aft, usually right above the middle of the skeg. The hull is wide and flat to house the units; heavy stern fendering is fitted because VS tugs push with the stern. The units sit at about 0.25–0.30 × LWL from forward and the towing point at 0.1–0.2 × LWL from aft.

Posições da figura, explicadas no texto (3)

1. 1. In sketch 1 there is no thrust; the propellers are 'idling'.
2. 2. In sketch 2 the steering centre is moved by one hydraulic cylinder to port.
3. 3. In sketch 3 the steering point N is moved by the two hydraulic cylinders to port and forward, which gives thrust in the indicated direction S.

VS tractor tugs are highly manoeuvrable: they turn on the spot (see figure 2B.23), give high thrust in any direction and sail straight astern at speed, with astern thrust nearly equal to ahead. They can apply side thrust (see figure 2B.24), so they are safer making fast near the bow and can compensate interaction forces — advantages the single-screw conventional lacks. Transverse thrust is controlled by the wheel and longitudinal thrust by pitch levers (see figure 2B.21); transverse has priority, so full 100% thrust cannot be applied in every direction at once (see figure 2B.22). VS propulsion produces little wash, useful when skimming oil or working close to deep-loaded lighters. A known issue is course stability on tugs with a small length-to-beam ratio: Voith solved it with baffles or vertical strips on the stern (StRAke stabilisers, with Robert Allan Ltd).

In ship handling, course control runs by the indirect method at high speed (forces can far exceed bollard pull) or the direct method at low speed (see figure 2B.25A). A VS tug can change from towing on a line to pushing without releasing the towline — a forward tug up to about two knots — which is handy berthing (see figure 2B.25C). They suit push-pull and, though not the best forward tug on a line at speed, are excellent after tugs for course and speed control, working to port and starboard (see figure 2B.25), with ship speed up to about five knots when making fast directly.

6. Tractor tugs with azimuth propellers (2.6)

An azimuth tractor tug (ATT/ATD) has two 360° steerable thrusters under the forebody. The first azimuth propellers entered service in the 1960s; the first tug so fitted was the German Janus (1967). Thrusters may be fixed-pitch (often Niigata) or controllable-pitch; fixed-pitch revolutions can be modulated steplessly from zero by a slipping clutch, which largely removes the need for CPP and is cheaper. CPP, however, allows faster acceleration and an easier fire-fighting drive. Thrusters are fitted in nozzles for efficiency, with protection or docking plates against grounding.

The hull resembles a VS tractor, but an azimuth tractor of the same power has less draft (lighter units, less stiffening). The towing point lies about 0.1 × LWL from aft and the propellers at 0.30–0.35 × LWL from forward; thrusters set further forward increase effectiveness. Thrust is about equal in any direction, with astern perhaps 5% less; producing side thrust the units interact, so they are set at a small angle to each other.

Manoeuvring is broadly like a VS tractor: safe, highly manoeuvrable, turning on the spot, moving sideways, with astern bollard pull near ahead. Thanks to the shallower draft, another skeg design and almost 100% thrust in any direction, azimuth tractors are more effective at speed when direct towing as a stern tug and as a forward tug on a line — provided the towing point is well placed — but slightly less effective in the indirect mode at high speed than a well-designed VS tractor with the correct skeg and towing point (see figure 2B.32).

7. Reverse-tractor tugs & the Japanese tug concept (2.7–2.8)

Reverse-tractor tugs (2.7)

Reverse-tractor tugs (formerly pusher tugs) have two azimuth propellers under the stern and are designed for working over the tug's bow, as in many West Pacific ports. They carry a large towing winch forward and only small gear aft (often just a hook) — the after towing point usually lies too far aft to tow on a line at speed like a conventional tug. The units sit about 0.1 × LWL from aft; draft is less than a comparable real tractor; the wheelhouse is arranged for an unobstructed forward view.

Control is the same as an azimuth tractor. With two thrusters and a forward towing point these tugs are highly manoeuvrable and safe — they turn on the spot and move sideways (see figure 2B.44); astern bollard pull is about 5–10% below ahead. The name says it: they work like tractors but heading the reverse way (towing point towards the ship, propellers away). As a forward tug on a line they are not very effective in steering ships with headway (the towing point is at the forwardmost end); as a stern tug they are very suitable for steering and speed control (see figure 2B.35), in indirect or direct mode — somewhat less effective indirectly than a similar VS tug, but possibly more effective in the direct mode thanks to the lesser draft.

Japanese tug concept (2.8)

A specific reverse-tractor, adopted also by Taiwanese, Chinese and South Korean designers, the Japanese tug differs markedly from European/American ASD-tugs: a large flared bow hung with tyres, a transom stern extending below the waterline, pronounced sheer, a long deckhouse, the winch relatively far aft, a relatively small beam and draft, and a high free-sailing speed of 15 knots or more. Two thrusters aft (often Niigata) (see figure 2B.40), no skeg but a small one before the propellers, often bilge keels and no funnels (exhaust through the transom for an all-round view) (see figure 2B.41).

The fine, often bulbous bow reduces wave-making resistance and lengthens the waterline (with the transom stern) for higher free-sailing speed — needed for escorting large or hazardous-cargo ships in inland routes — while a widely opening, big-flare bow deck stays dry at high speed (see figure 2B.37). There are mainly two kinds: berthing/unberthing tugs (push-pull and direct towing at the bow) and escort-capable tugs (powerful, operating even in storms; they do not make fast during escort). Around Japan escorting is compulsory for hazardous cargoes or certain lengths, and is done by tugboats with FiFi. The wide-radius flared bow reduces pushing forces on hulls; the after-positioned forward fairlead lets these tugs work with the steep towlines common in dockyards (see figure 2B.36).

8. Azimuth Stern Drive (ASD) tugs & Uni-lever system (2.9–2.10)

An ASD-tug is nearly the same as a reverse-tractor but designed to operate both like a reverse-tractor (over the bow) and like a conventional tug (over the stern), combining the advantages of both. It has a towing winch forward and a winch (sometimes optional) or hook aft; some have a single winch serving both ends through a trunk. The after towing point sits at a good location for towing on a line — 0.35–0.4 × LWL from the stern — while the two azimuth propellers sit about 0.1 × LWL from the stern. Skegs vary by design (see figure 2B.43a); draft is less than a comparable tractor. ASD-tugs may add a tunnel or azimuth bow thruster for position-keeping and extra bollard pull. There is great interest in this type because of its manoeuvrability and multi-purpose ability; bollard pulls range widely (e.g. Azistern 60–120 t).

Towing forward on a line like a conventional tug (see figure 2B.45), the ASD is very effective (girting risk minimised by a reliable quick-release). As a stern tug on a line it works over the bow — good for speed and course control to both sides; effectiveness in the indirect mode is somewhat less than a VS tractor, but it may be a little more effective direct-pulling. Like a reverse-tractor it changes easily from towing on a line to push-pull without releasing the towline, and is very suitable at the ship's side thanks to high reversing power and 360° thrusters. Maximum stern thrust is about 5–10% below ahead.

Posições da figura, explicadas no texto (4)

1. 1. When towing forward on a line like a conventional tug (see figure 2B.45, 1, opposite page) the ASD-tug is very effective, although the risk of girting exists.
2. 1 e 2. As a stern tug on a line an ASD-tug works over the bow (situation 1 and 2).
   Effectiveness when assisting in indirect mode (situation 2) is generally somewhat less when compared to VS tractor tugs, but ASD-tugs may be somewhat more effective when direct pulling (situation 1).
3. 2. The forward ASD-tug should then assist like a reverse-tractor tug (situation 2) which is very often the case.
4. 3. Like reverse-tractor tugs, ASD-tugs can also easily change from towing on a line to push-pull without releasing or changing the towline position (situation 3).

Uni-lever system (2.10)

Although most azimuth tugs have separate controls, many — especially in Western Pacific ports (Korea, Japan, China, Taiwan) — use a Uni-lever (master-pilot) system, where a single handle sets both thrusters automatically for the requested movement (see figure 2B.47), with a separate engine-speed handle. A well-regarded tug-master instructor notes that any combination achievable with separate controls is achievable on a Uni-lever — but it is harder to find the optimal combination, so quality training matters. The Uni-lever is intuitive for simple work but can override the master, can get "stuck ahead" if pulled from full ahead to full astern too fast, and may take a couple of minutes to recover. Separate controls generally do only what is asked (see figure 2B.48). The conclusion: a master must know his control system's limitations and be trained to handle them safely.

9. Related tug types (2.11–2.16)

The related tug types (Table 2A.1) — Rotortug, Z-tech, RSD, Carrousel, DOT and the All-Rounder — earn the name because their capabilities compare largely with the basic types of Part B.

Rotortug (2.11)

A Rotortug is a triple-azimuth-thruster tug, the thrusters in a triangular arrangement, with a towing/escort winch forward and a towing winch aft — essentially a tractor tug whose skeg is replaced by a third thruster. The latest generation are ARTs (Advanced Rotortugs); main types run from ART 65-28 (27.5 m, BP 65 t) to ART 85-32W (31.5 m, BP 85 t). Increasing beam (the W "wide body") increases stability; note that with ASD-tugs the units counteract heel in the indirect mode, whereas with Rotortugs the thrusters increase heel — though heel can be cut by lowering thrust (see figure 2C.3). By mid-2018 there were 44 operational and 12 building.

Thanks to the third thruster (and sometimes a second winch) the Rotortug's capabilities exceed other types: high steering and braking forces in both direct and indirect mode over the full 0–10 knot range (see figure 2C.4), fast reactions, safe work near a ship's bow at speed, and operation in confined spaces such as locks and bridges. A special low-speed manoeuvre, "rotoring", applies steering force within the beam of the assisted ship — forward, with the towline at the two-thruster end, the tug generates up to 70% of bollard pull transverse to the ship's centreline — which helps through locks and brings ships close to the quay (see figure 2C.2), and can reduce the number of tugs needed. Side thrust reaches up to 95% of bollard pull and side-stepping ~6 knots. The drawbacks: three thrusters interact (some loss of effectiveness) and add resistance; the tug is complex, so specific Rotortug training is strongly advised. A variant, the in-field support vessel (ISV) ART100-42, was built for the Prelude FLNG project; a related ART100-46 was built for the ConocoPhillips Bayu Undan field.

Z-tech tug (2.12)

The Z-tech (PSA Marine, Singapore; designed by Robert Allan Ltd, first in 2003) combines the best of tractor and ASD types in one design, to work closely under the extreme flares of large container ships. The wheelhouse sits well aft (towards the thrusters), giving a large, anchor-free working deck forward and a wide, heavily fendered bow (the skeg end). Speed and bollard pull astern are at least 95% of ahead. Operationally it compares best with an azimuth tractor (see figure 2C.10)(or a reverse-tractor); it sails "skeg first" or "thrusters first" as needed. By mid-2017 about 70 were in service worldwide.

RSD tug (2.13)

The RSD (Reverse Stern Drive) tug (Damen) combines ASD and ATD advantages, like the original Z-tech idea, with the thruster end forward. Its key feature is that bow and stern are of equal height, so the tug can safely be a bow tug (like a real tractor and like an ASD over the bow) or a stern tug on a line — without a low stern facing the incoming flow and waves; "it has two bows." A twin fin makes it a highly manoeuvrable, compact, good sea-keeping tug; with the topmast down it works under overhanging flares. The first is the RSD 2513 (24.75 m, BP ahead 70 t, astern 67 t) — considered a safe replacement for an ASD working over the bow.

Carrousel, DOT and All-Rounder (2.14–2.16)

The carrousel tug uses a full-circle radial system — an arc became a complete circle of roughly the tug's beam — around the superstructure, on which the towing point can travel. Its advantages: it copes safely with large towline forces from hydrodynamic lift with smaller heel (capsizing by athwartships towline force is not possible); it lets the tug turn freely without the towline fouling the superstructure; and it can turn the whole hull (with efficient shaft propellers) in line with the towline, so a stern tug can steer to both sides of a ship with headway — exactly what conventional tugs lack. The first was Multratug 12 (2002, a converted combi-tug) (see figure 2C.13); model and full-scale tests confirmed high towline forces (see figure 2C.14) (see figure 2C.15). The carrousel suits transit/escort work, since forces grow with speed, but for low-speed harbour work bollard pull also matters.

The DOT (Dynamic Oval Towing) tug is a conventional tug with a movable towing point on a heavy rail around the superstructure, letting the tug pull over bow or stern without releasing and preventing capsize at right angles — essentially the carrousel principle, mostly on smaller conventional tugs. The All-Rounder AR360T (24 m, BP up to 65 t) is a high-powered twin-shaft conventional tug with an all-around radial winch support and a new hull shape for effective towing/braking at bow and stern from zero speed to headway, with an automatic overload-release winch travelling on two rails.

10. FAST tugs (2.17–2.21)

FAST tugs (Forward-Aft Single Thruster) carry one thruster — azimuth or Voith — at each end, in the centreline, with the towing point between them: SDM/ATT, EDDY, CRT and Giano. They handle quite differently from other tugs, because each end has at most 50% of the total power, whereas a tractor or reverse-tractor has 100% forward. Near a ship's bow at speed, that matters: a tractor can better compensate suction forces by steering away, while a FAST tug may not veer far enough and could swing around unless the towline is quickly slacked. Redundancy is also smaller, though if one thruster fails the other remains. A carrousel system removes most of these concerns. FAST tugs demand thorough training — for masters and pilots alike.

SDM / ATT (2.18)

The SDM (Ship Docking Module) — also called ATT (Asymmetric Tractor Tug) in Spain — has a shallow flat elliptical bottom, a very wide beam, and an asymmetric layout: one azimuth thruster a quarter of the length from forward and offset to starboard, the other a quarter from aft and offset to port, with a skeg at each end. It was designed as a pure harbour tug for maximum bollard pull in all directions, quick positioning, high side-stepping and work under large flares in confined waters. Mark I/II are 27.4 m, BP 55–60 t; Spanish ATTs reach 74.5 t peak. Free-sailing ~12.5 knots, side-stepping 6.5 knots. A hole in each skeg balances the pressure difference between the forward (intake) and aft (outflow) nozzles, removing a steering correction. Drawbacks (large funnels limiting the athwartships view; water on deck at the low bow) are solved in the ATT2020 design. In the USA they usually work at the ship's side (see figure 2D.11a); in Spain, by the centre lead forward and aft (see figure 2D.12) (see figure 2D.13).

EDDY (2.19)

The EDDY (Efficient Double-ended Dynamic) tug has an in-line thruster at each end on the centreline with the towing point between, sizes 24–37 m and BP 45–100 t. The standard drive is a diesel-electric/diesel-direct hybrid (batteries optional), giving fuel savings of 30–50% versus similar tugs (two-thirds from hull shape and lower displacement, one-third from the hybrid system), low noise and low wake. High freeboard, centreline-protected thrusters and a five-compartment subdivision make it virtually unsinkable. Net pushing force is about 90% of maximum because the flow towards the thrusters is not optimal. It generates towline tension continuously, in any direction — excellent in locks and narrow rivers (see figure 2D.21), able to stay parallel to a lock wall and operate within the ship's beam, single- or twin-towline (see figure 2D.20), with continuous tension even while repositioning (see figure 2D.23).

Carrousel RAVE Tug — CRT (2.20)

The Carrousel RAVE Tug (CRT) joins a carrousel system with a RAVE (Robert Allan / Voith Escort) hull — two Voith units, one forward and one aft on the centreline, with the carrousel winch turning around the superstructure. Multratug 32/33 (31.9 m) have a static BP of 70 t but a dynamic BP at 10 knots of at least 150 t steering and 170 t braking force (see figure 2D.27). The crew may go on deck only with the towline between the bow towing pins (see figure 2D.26); a door alarm warns if anyone opens a door while the winch works. Three working modes are possible: the carrousel (any direction, for maximum dynamic force), the bow pins and the stern pins. The two Voith units are turned 5–10° off the centreline so the forward thrust does not feed the after unit (which would cut bollard pull). Reserve power is kept to change heading quickly and shed excess towline force.

Giano (2.21)

The Giano tug (Naples, 2016), like the EDDY, has one thruster forward and one aft on the centreline, with an escort winch and towing point near each end (its name evokes the two-faced Roman god Janus). It is 25.75 m, BP 55 t in all directions, side-stepping 7 knots, with CPP and an LR escort notation; very wide (L/B ≈ 2), with stability exceeding the IMO Intact Stability Code (escort heeling angle ≤ 15°; the Giano measured 8° static). Two skegs form a kind of tunnel with underwater fendering, letting it push full power sideways at a stopped ship without listing (see figure 2D.31). Escort capability is large, with the forward towing point close to the forward thruster (see figure 2D.33); it can compensate interaction forces and work safely near the bow of a ship with speed.

11. LNG terminal tugs (2.22)

Tugs handling LNG carriers and working near LNG/LPG terminals must meet extra requirements on top of the usual ones (dynamic stability, sea behaviour endurable for the crew, optimum fendering, all-round visibility). These can include: being seaworthy in the terminal's specified wave conditions; enough bollard pull for the largest expected object in the prevailing wind and waves; a powerful escort winch with render-recovery capability; and escort capability.

Further, safety-specific requirements may apply: gas-detection alarms in the wheelhouse and gas sensors around the deckhouse, engine-room intakes and accommodation ventilation; explosion-proof air-intake fan motors; main engines with a "rigsaver" safety stop if gas is drawn in; spark arresters; remotely operated ventilation dampers; Fire-Fighting 1 or above with water spray; fendering with water lubrication and very low contact pressure (e.g. not more than 14 t/m²); anti-spark fendering; certified electrical equipment for the LNG zone; anti-static synthetic towlines; and additional FiFi. Clear instructions are essential for the master and crew if gas leaks or an explosion develops — whether to release the towline and run to safety, or pull the ship away, depends on the gas source.

FiFi classes (DNV GL) range from Fire Fighter I/II/III by number of monitors, monitor capacity (e.g. 1,200–3,600 m³/h), throw length (120–180 m) and height (50–110 m), and fuel endurance. Terminal tugs working in open water usually operate over the bow and back into the waves, so their after deck needs much sheer and smooth up-going lines to make the stern climb out of the water — a need shared by all ASD tugs operating bow-to-bow at a ship with speed.

12. Eco-tugs (2.23)

Eco-tugs are designed to operate effectively while reducing fossil-fuel use and/or air pollution. Most measures on air emissions — CO₂, SOx, NOx and particulate matter — come through MARPOL Annex VI (IMO Tier I–III standards) and apply to tugs too. Three basic systems are addressed: (A) reducing fuel use by hybrid technique; (B) using cleaner fuel; (C) fully electric tugs.

A — Hybrid (reduced fuel consumption)

Because tugs need only about 20% of maximum power in transit, mechanical (diesel-direct) propulsion — efficient only near design speed — has poor fuel economy and high emissions at part load (see figure 2E.2). Electric or hybrid propulsion improves this (see figure 2E.3). Systems range from mechanical and electric propulsion to several hybrid variants:

• Hybrid propulsion: a mechanical drive for high speed/efficiency plus an electromotor on the same shaft for low speeds, avoiding inefficient part-load running (see figure 2E.4).
• Electric propulsion with hybrid power supply: two or more power sources (diesel generators, fuel cells, energy storage) (see figure 2E.5). Batteries enable switching off engines at part load, recharging when emissions are lower, peak-shaving and backup power — temporarily sailing with no emissions, noise or vibration.
• Hybrid propulsion with hybrid power supply: combines the efficiency of direct mechanical drive with combustion power from the prime mover(s) and stored power from energy storage for electric supply (see figure 2E.6).
• Electric propulsion with DC hybrid power supply: a DC architecture that lets the diesel engine run at variable speed, further cutting fuel consumption, emissions, noise and engine loading (see figure 2E.7).

Operating modes include PTI (power take in — booster, fully electric, or diesel-electric) and PTO (power take off — parallel, transit, shore-connection) (see figure 2E.9). Five aspects are crucial for an eco-tug: full power available in minimum time, preferably without extra button-pressing; full power available for at least several hours; simple switching between drives; full reliability; and no negative effect on stability.

B — Cleaner fuel

Beyond exhaust treatment, options include dual-fuel engines (diesel + LNG or hydrogen); LNG, hydrogen or ammonia only; CNG (easier to store than LNG); and studies on methanol, ammonia, lignin oil and even iron powder. LNG is a growing market but must be kept at –162°C in specific tanks with roughly twice the volume of diesel for the same energy, kept clear of the side shell, bottom and engine room and not so high as to harm stability (see figure 2E.11); the IGF Code (in force 1 Jan 2017) sets safety rules for low-flashpoint fuels. Hydrogen is clean and abundant but expensive, hard to store and highly flammable (the Antwerp Hydrotug is the first hydrogen-powered tug). Ammonia is liquid at –34°C but needs ~3× the volume of conventional fuel. As a final note, for tugs' fluctuating loads, diesel-electric (or LNG-electric) matches revolutions and power to optimum efficiency better than diesel-direct.

C — Fully electric tugs

Fully electric tugs have almost zero emissions and are quiet on board and underwater. The first operational one is the Turkish Zeetug Gisas Power (18.7 m, BP 32 t), its lithium-ion batteries charged in one hour at a quick station, with two redundant battery rooms. The Damen RSD-E 2513 Sparky (Port of Auckland) has four independent battery packs, can push-pull at 70 t for at least 30 minutes, then recharges in about two hours, with enough capacity for at least two berthing operations. An all-electric tug also removes the engine-room oxygen demand, making work around LNG terminals safer. Care with lithium batteries is essential — a 2012 battery fire on the Campbell Foss led to its return to diesel and to structural and fire-suppression changes; DNV found that ventilation alone cannot prevent an explosion if a very large number of modules (≥ 4,000 amp-hours) fail at once, so the design must contain fire and gas to as small a part as possible.

13. Ice tugs (2.24)

Selecting or designing an ice tug begins with the local ice conditions: type of ice, salinity and snow; ice thickness in normal and extreme situations; ice coverage; ridging and ridge consolidation; ice drift and pressure; land-fast and grounded ice — plus wind and current that affect them. Key design issues are the required ice-breaking capacity versus other capacities, tug type (ASD or conventional, both most suitable for severe ice), main engine and propulsion (the propeller is the weakest point, the engine the strongest; the line must tolerate rapid load variations), nozzle vs open propellers, torsion and vibration from propeller-ice contact, engine cooling, internal heating and noise, ice-removal from deck and superstructure, towing-gear operability against freezing, and excellent searchlights and visibility (window heating, demisting).

Stability and ice class. Stability needs special attention because ice accretion on superstructure, mast and winch can build quickly in freezing temperatures and rapidly reduce stability — it must be a stability factor. An ice-class hull is thicker, with more scantlings, more watertight bulkheads, rudder/propeller protection (two rudder pintles, strengthened propeller tips) and heating for vital tanks; sea chests and sea bays may need rearranging against ice blocking.

By tug type, an ASD-tug is generally the most versatile in ice, and ASD-tugs with CPP are optimal — CPP recovers thrust very fast after ice blockage and even reduces nozzle blockage. For Voith tugs in ice, experience differs: some find they block easily in anything but the lightest ice (the coupled control limits clearing each unit independently, and the tractor concept has no hull to plough ice away), but a study at the Aker Arctic laboratory found VS tugs highly effective icebreakers — the deeply immersed, closely spaced, outward-rotating VSPs push ice aside and flush the channel wide, confirmed by masters in Rostock and Swedish waters. Open-propeller tugs are favoured in very hard ice (blocked nozzles give zero thrust), though their bollard pull per installed power is significantly lower; ice work often uses over 80% of MCR for over 80% of the time. A purpose-built example is the Russian Yuribey (Sabetta, 39.54 m, BP ahead 97 t, Azipod propulsion, Ice Class ARC6, designed for –50°C). A removable ice-breaking bow can convert a pusher tug for inland ice work.

14. Research & tug performance (2.25–2.26)

Research (2.25)

Research is essential to ever-better tugs — for normal assistance, work in waves, escorting and crew living conditions — covering hull form, skegs, ropes, deck equipment, liveability, automation and sensors. Beyond model and tank tests, CFD (computational fluid dynamics) simulates how fluid flows around the tug and affects it. Around 2000 only potential-flow calculations were possible; from 2010 viscous-flow calculations let designers study turbulent, separated flows around full-bodied tug hulls — even hulls with appendages in indirect modes — though every condition is costly in computer power (see figure 2E.21). CFD gives high-quality flow information, but needs good designers and a procedure of verification and validation: as the saying goes, "garbage in, garbage out." Published research feeds the whole industry — for example, the SafeTug JIP at MARIN (2010) underpinned Bureau Veritas's Safety Guidelines for Design, Construction and Operation of Tugs (2014).

Tug performance (2.26)

Two basic principles help judge performance. First, when the propeller wash runs with the water flow (e.g. a bow tug pulling a ship with headway) the propeller is in positive flow; against it (e.g. a stern tug braking) it is in negative flow. Negative flow produces greater thrust but much higher torque on propeller and engine, and unstable flow with fluctuating loads and vibration.

Bollard pull is measured by an electronic load cell in series with the towline, with engines ahead (and increasingly astern, for azimuth tugs), at the manufacturer's continuous rating (MCR), with sufficient underkeel clearance and line length so the wash does not distort the result. A MARIN Joint Industry Project (2016) found no performance degradation for most tugs when water depth is about 6× the propeller immersion and line length 50× the propeller diameter; below those thresholds, degradation is expected. Higher water density gives higher thrust for the same power. The certified continuous bollard pull is the force held without declining for at least 5 minutes for harbour tugs, recorded electronically at 1 Hz or better.

The relationship also varies with the amount of power: a conventional tug of 700 bhp with a fixed propeller can reach 2 tons/100 bhp, while a 6,000 bhp conventional with nozzles may give under 1.3 tons/100 bhp. Performance at zero speed in different directions is shown in thrust vector diagrams (about 1.1 t/100 bhp ahead for a VS tug, 1.4 t for azimuth, 1.5 t for a conventional with nozzled propellers) (see figure 2E.23); over a range of speeds, polar diagrams (Chapter 4) are used. Simulated diagrams should, as far as possible, be validated in full-scale trials.