AN Ch. 3 — Assisting Methods

1. Tug assistance required in ports (3.1)

Source: HENSEN, Henk. Tug Use in Port: A Practical Guide. 4th ed. Rotterdam: STC Publishing, 2021. Chapter 3 — Assisting Methods.

Edital: Anexo 2-B, Área II (Arte Naval / Shiphandling), item 9 (Hensen) → Chapter 3 — Assisting Methods · Anexo 2-A, Área II, item 26.3 (Métodos de assistência), com itens 4 (Emprego de rebocadores na manobrabilidade) e 31 (Métodos de utilização de rebocadores empregados no Brasil).

In a port a tug renders one of two services: assistance during a transit to or from a berth (including mooring and unmooring), or assistance only during mooring and unmooring. How much assistance is needed depends on particulars of three things: the ship (type, size, draft, length/width ratio, loading condition, windage and manoeuvrability); the berth and nearby manoeuvring area (type, size, alignment, berthing and manoeuvring space, turning-circle size, water depth, current, wind and availability of mooring boats); and the transit route (width, length, depth, bends, maximum allowable speed, expected traffic, moored ships to pass, plus current, wind, waves, shallow water and banks).

The key difference between assistance during mooring/unmooring and during a transit lies in the ship's speed, which is the major factor for selecting the most appropriate tug type and assisting method.

3.1.1 What assistance during a transit involves

Assistance during a transit may comprise passage through a river or channel, entry manoeuvres into a harbour or turning basin, passage through narrow harbour basins, and passing narrow bridges or locks. Over most of a transit route a vessel's speed is usually in the range of about 3–6 knots and sometimes higher. At these relatively low speeds the influence of wind, current and waves is more pronounced — the larger drift angle widens the required path, and steering ability is poorer. On the other hand, speeds up to 6 knots can become rather high for effective tug assistance.

Where tugs are used mainly for mooring/unmooring, assistance comprises the approach phase towards the turning basin or berth, turning in a turning basin, and the mooring/unmooring itself. The ship's speed during these manoeuvres is normally very low or zero, so the influence of ship's speed on the different tug types is less predominant.

3.1.2 What the assisting tugs must be capable of

Hensen reduces the requirement to a few capabilities the assisting tugs, with the method applied, must be able to deliver:

• Steering assistance and speed control — steering while the ship has headway, needed in narrow passages, passing bridges or sharp bends, or entering harbour or turning basins under varying current and wind. Controlling heading and speed may be needed when approaching a basin or entering a lock.
• Compensating for wind and current during transit while the ship has speed — drift can be met by steering a drift angle or by higher speed, but higher speed is usually impossible in confined areas, and narrow passages allow only small drift angles, so tug assistance is then needed.
• Controlling transverse speed towards a berth while compensating for wind and current during mooring/unmooring — here the ship's longitudinal ground speed is practically zero, so mainly crosswise pushing and/or pulling forces are applied.

2. Assisting methods in use (3.2.1)

The many ways ships are handled by tugs around the world trace back mainly to large differences in local circumstances. Assessment of methods in use worldwide shows only two markedly different methods: tugs towing on a line, and tugs operating at a ship's side. Almost all tug types can apply both, though for some tugs effective towing on a line is problematic. In European ports towing on a line predominates; in the USA and West Pacific ports tugs usually operate at a ship's side. There is a general tendency towards more flexible tug types, which is reshaping the methods used.

In some ports combinations are used, or the normal method is changed for a specific situation. A port where tugs normally work alongside may have them tow on a line to pass narrow bridges or enter a dry dock. At seaside terminals affected by waves, a crew that assists alongside in calm weather may switch to towing on a line as conditions deteriorate, to avoid parting towlines and losing control.

Based on 1996 research into worldwide practice, and assuming two tugs assist a vessel, the methods are generally applied in the following ways.

2.1 Tugs alongside during approach; pushing or push-pull while mooring

This is the normal method in most ports of the USA, Canada, Australia, Malaysia and South Africa, and at large oil terminals in Norway. The method is similar across these ports but the tug type differs, and how the tug is secured depends mainly on that type (see push-pull). With omnidirectional propulsion the tugs are made fast at the forward and aft shoulder — generally one bow line for ASD / reverse-tractor tugs, and a stern line for tractor tugs (Fig. 3.3).

In the USA, tugs may be secured alongside by one, two or three lines. A conventional tug normally uses two or three (Fig. 3.4): the forward line is the tug's backing line; a spring may come from the forward winch through the most-forward bow chock; the third stern line lets the tug work at right angles without falling alongside when the ship moves through the water, and compensates the transverse effect of the tug's own propeller going astern and the ship's propeller wash. Twin-screw tugs or tugs with steerable nozzles, being more manoeuvrable, normally use fewer lines — usually one or two.

Other USA variants: tugs may work stem to stem — a ship moving astern is steered by a tug pushing at its bow (push the port bow to swing the ship to starboard, the starboard bow to swing to port, Fig. 3.10). In some USA ports and the Panama Canal a rudder (steering) tug is used at the stern (Fig. 3.8): a rudder tug controls speed, and a conventional tug steers by giving forward thrust with starboard or port rudder.

2.2 Forward tug alongside, aft tug on a line during approach; push-pull while mooring

This method — not very different from the above — is mainly found in the ports of Japan, Taiwan and Hong Kong (Fig. 3.11). The after tug is made fast by its bow line amidships or at the quarter and follows the ship, used for steering and speed control; the forward tug is made fast at the forward shoulder. During berthing the tugs change over to push-pull. The tugs there are of similar design, more or less purpose-built for this operation — reverse-tractor tugs (sometimes ASD-tugs) with 360° steerable thrusters under the stern, made fast from the tug's forward winch. For specific manoeuvres (entering dry or floating docks) these tugs must tow on a line. The method also appears elsewhere with reverse-tractor or tractor tugs, and sometimes with conventional tugs (the stern tug then operating like a rudder tug).

3. Towing on a line & combinations (3.2.1 cont.)

3.1 Towing on a line during transit and while mooring

This is the method used specifically in Europe, most often with conventional tugs, though other types are also used; it is applied in many other ports working with conventional tugs (Fig. 3.12). Ships are assisted during the whole transit towards the berth — on the river, from the river into the harbour and through harbour basins up to the berth. Its advantage is that it works in narrow waters, so it is also used passing narrow bridges or entering locks and dry-docks. There the forward tug sometimes has two towlines — "cross lines" or "gate lines", or both from a double bow winch on some reverse-tractor tugs — so the tug reacts very quickly and needs little manoeuvring space (Fig. 3.13).

The original tugs for this were conventional tugs with a small engine and a streamlined underwater body, very effective when the ship had some speed by using the tug's mass and the hydrodynamic forces on its hull. As ships grew, more powerful tugs followed; modern conventional tugs are more manoeuvrable, with more power and a smaller length/width ratio, and remain effective at speed. Because of conventional tugs' limits, azimuth tugs and VS tugs are now also used for towing on a line. When more than two tugs assist during berthing, the forward and aft tug usually stay on the towline to control approach speed while the others push at the ship's side.

3.2 Towing on a line during approach; push-pull while mooring

This is becoming common practice where towing on a line is carried out with highly manoeuvrable tractor, reverse-tractor or ASD-tugs (Fig. 3.14). The more familiar pilots and tug captains become with these tugs' capabilities, the better they apply them to shiphandling.

3.3 Combinations of the above systems

In many ports various tug types operate, and assisting a larger ship often needs more than two tugs; entry or berthing manoeuvres can be so complicated that not one method but a combination is used. Figure 3.15 shows an Australian example for large bulk carriers entering harbour: reverse-tractor or ASD-tugs alongside and on a line aft, with a conventional tug forward — good for steering and where the stopping distance is short; nearer the berth one alongside tug shifts to the other side to push.

3.4 Speed control by a stern tug

Omni-directional stern tugs are sometimes used to brake the ship's speed, or to hold it at a low level (e.g. 3–4 knots, below the ship's Dead Slow Ahead speed) while the engine keeps running so the ship can steer. Braking can be needed after an engine breakdown, or approaching the turning circle; it also avoids the transverse effect of the ship's propeller going astern, and the pull is usually short. Holding the speed is used approaching a lock (Fig. 3.16), passing a narrow bridge in wind with a high-windage ship, or passing moored ships at low speed — and where Dead Slow Ahead is too high (more than 6–7 knots) for forward tugs to make fast safely and stopping the engine is not an option; the stern tug then pulls for much longer. Speed control is becoming more common, but vibration on board the tug can be tremendous, so tug masters may dislike it over long periods.

4. Push-pull versus towing on a line; locks; tug–method relationship (3.2.2–3.2.4)

4.1 Push-pull versus towing on a line (3.2.2)

Tugs working alongside in push-pull act quickly with short response times, important during berthing, and avoid the interaction forces and turning moments of working close to the bow of a ship at speed. But they have drawbacks: towlines are often very short, a problem in waves; pulling with a short towline loses effectiveness because the tug's wash hits the ship's hull at right angles (lengthening the line, if a winch allows, reduces this); a conventional tug pulls weakly because of low astern propeller performance; the combined width of ship plus tugs is a problem in locks and bridges; tugs must be on the side opposite the berth, so there is no flexibility in berthing side once fastened; the turning lever is short (only the distance between the two tugs); with a high-windage ship and tugs fastened on the lee side, strong gusts can jam the tug between ship and quay; and the tug master has little insight into the pilot's manoeuvres.

4.2 Locks and tug assistance (3.2.3)

Large ships entering locks are assisted in various ways, in most cases with just one towline (Figs. 3.16, 3.18). It is also possible to use two towlines from two independent winches, as in the new locks of the Panama Canal (Figs. 3.9, 3.17): two lines make the tug effective in the limited lock width and cut response times, but securing two towlines is harder with the trend to minimum crew. Modern Rotortugs and one-thruster-forward-one-aft tugs have specific capabilities to assist within the ship's width.

4.3 Relationship between tug type and assisting method (3.2.4)

There is a clear relationship between tug type and method: the essential factor is whether a tug suits operating at a ship's side, towing on a line, or both — and the most suitable tug is not always available or used. In Japan, Taiwan and Hong Kong there is one method and mainly one tug — the reverse-tractor tug, whose omnidirectional stern propulsion suits working on a line at the stern and alongside at the forward shoulder; ASD-tugs are also used, and the reverse-tractor is expected to dominate future orders there (the VS tugs once in Yokohama have already been replaced by the Japanese reverse-tractor type).

In Europe towing on a line is general practice, originally with conventional tugs, but their limits have brought in omni-directional types and more flexible methods. In the USA tugs work at a ship's side most of the time and the conventional tug was for years almost the only type — its limited manoeuvrability and low astern power offset by extra towlines, high engine power and specific rudder configurations — but USA and Canadian ports now also tend towards tractor, reverse-tractor and ASD-tugs. Australian, New Zealand and South African ports mainly work at a ship's side, with fleets already mostly omnidirectional. The growing variety of tug types lets a port select the most suitable tug for its particulars, methods and future developments.

5. Tug assistance in ice — introduction, ship types, tugs (3.3.1–3.3.5)

5.1 Introduction and ship types (3.3.1–3.3.2)

In winter, traffic to several ports is impeded by ice. Icebreakers keep ports open; where ice is not too thick a ship may break it herself, otherwise an icebreaker or tugs are required. Crucially, all they can do before arrival is break the ice — they cannot completely remove it from a berth — so special berthing and unberthing procedures are needed. Mooring in ice is usually time-consuming; the methods here are based on experience in one of the largest Baltic ports, where ice impedes shipping for several months each year.

Whether a ship can berth or unberth with or without tugs depends on her size, strength, engine power, berth location and ice conditions. A vessel in ice should be ballasted and trimmed so the propeller and rudder are fully submerged; if blades are exposed at or near the surface, the risk of propeller damage from striking ice is greatly increased, and such vessels (and those that may damage propellers or rudders with sternway, and light-draft vessels with bronze propellers that cannot be trimmed enough) require tug assistance. For berthing, ships fall into two groups: those that can work their engine on Dead Slow on a spring line without parting it (small vessels, and ships with controllable-pitch propellers); and ships with large engines and high starting power that cannot work at Dead Slow without parting even a doubled spring line.

5.2 Preparation, tugs and tug assistance (3.3.3–3.3.4)

Before mooring, a berth must be prepared by an icebreaker or by tugs: ice is broken near the berth and an approach route made; before departure, ice is broken around the ship and a departure route made. How ships are handled in ice depends largely on the tug type. Tugs with light draft and propellers in nozzles have very limited capability — moving astern, the nozzles immediately fill with ice (it can happen even going ahead), so the tug must stop and clear them by repeatedly reversing thrust. Such tugs, and others with ice problems, should not tow on a line: the assisted ship might not react or stop fast enough to avoid collision.

Towing on a line in ice is risky in general, acceptable only when the ship moves at a very controlled low speed on a straight course (or easy bends) and during berthing/unberthing. Assistance in ice on arrival and departure is mainly by pushing, including breaking ice and sweeping it from between ship and berth — without tugs it is almost impossible to remove ice from there. Towing wires and ropes should keep their strength in low temperatures but should never enter icy water (they become very hard to handle). The most reliable tugs in ice are ice-strengthened conventional tugs with open propellers — twin-screw preferred for manoeuvring. Shallow-draft nozzled tugs are very limited (nozzles block with ice) but not worthless: they can create a surface stream for moving ice. The best types are conventional twin-screw tugs with open propellers and ASD-tugs (Voith tugs sometimes too).

5.3 Ice blockage and nozzle clearing (3.3.5)

Performance of azimuth thrusters in nozzles in ice improves with adequate hull-thruster clearance and short reaction times for pitch changes or turning, but thrusters will still block now and then. The signal of blockage is vibration of one or both propellers (also visible on the meters). The tug master must recognise it quickly and react: a conventional fixed-pitch propeller in a nozzle reverses revolutions to clear; a controllable-pitch propeller reverses pitch. With ASD-tugs the options are greater — if the tug still has speed, one thruster is declutched and turned to wash the ice out, then clutched back in with rpm increased; this may be needed three times in five minutes.

Two clearing procedures are used, summarised below. Ice can also trap between two blades of a CPP, preventing pitch adjustment and risking damage — again the fixed-pitch method applies. Despite good procedures, nozzles and propellers often get damaged in ice, or a nozzle may be lost; thruster removal at a yard is not simple.

6. Berthing, unberthing and safety in ice (3.3.6–3.3.9)

6.1 Berthing in ice (3.3.6)

A berth is approached at a small angle. Once the forward spring is secured, the engine is set Dead Slow Ahead and revolutions or pitch raised gradually, just avoiding breaking the spring (it is best to double the spring); the rudder swings the stern in and out, away from the dockside, so the propeller flow forces loose ice out from between ship and dock and washes it astern. With weak ice this clears it completely; with dense, thick ice tugs are required. Some berths can be approached parallel to the dock (Fig. 3.21): ice is pushed away by the bow, and unbroken ice on the offshore side presses the ship to the berth and stops her swinging out — but only with young, weak ice.

Where parallel approach is impossible, tugs are used. In general, the bow is kept as close as possible to the berth with a tug pushing at the bow (Fig. 3.22 A, B); after the forward spring is secured the tug breaks ice outside the ship and washes it away from between ship and dock (Fig. 3.22 C, D). A tug just ahead of the ship, stern directed at the ship's bow, can also sweep ice away by going ahead — provided the ship passes no head lines (Fig. 3.24). Since bow ice is squeezed between bow and dock, having 20–30 m of free berth ahead helps: the ship approaches ahead of her planned position, ice is broken and swept, then she is brought alongside and moved astern while the tug keeps pushing the bow to the dock (Fig. 3.25).

A bow thruster can be very effective (Fig. 3.26): approaching at an angle, with forward springs and head lines ashore, the stern is taken out by rudder and engine and the bow thruster set to take the bow off, creating a water flow that sweeps ice from between ship and dock while ropes and the pushing tug hold the bow in. Another good method is moving the ship astern to moor starboard side alongside (Fig. 3.27): after a small-angle approach and securing the back spring, the engine goes astern — the strong propeller stream moves ice in the bow direction; this suits larger vessels because astern thrust is lower than ahead, so spring-line tension is less. These procedures using engine and spring lines do not suit ships with large engines and high power on Dead Slow; for them all ice work is done by tugs (Fig. 3.28) — one stern tug on a line takes the stern off, a second pushes the stern to the berth and clears the ice, the ship's wash is not used, and berthing takes a long time.

6.2 Unberthing in ice (3.3.7)

Before unberthing, tugs break ice around the ship and out to about 20–40 m from bow and stern. Some vessels can be taken off by the stern with a stern tug towing on a line (Fig. 3.32): bow ice keeps the ship off the berth, and the stern tug drifts ice between ship and dock, preventing the ship coming too close when moving astern. If the ship must unberth bow-first (Fig. 3.33), a second tug may break ice near the stern and keep it off the berth, and a third may crush ice on the outer side. A ship to be swung after unberthing must do so in an area or channel prepared in the ice by large tugs or icebreakers beforehand.

6.3 Safety of tugs in ice (3.3.8)

Tugs are at great risk towing on a line through an ice channel: if the tug must stop because of nozzle blockage, the ship must stop immediately; the tug may enter dense ice and lose speed very quickly. The assisted ship must therefore use engines with the utmost care. For these reasons the safest way to tow on a line is moving the ship astern (Fig. 3.35), with the engine always ready to go ahead so the ship can be stopped at once.

6.4 Finally (3.3.9)

Assisting in ice takes much time and effort, and engine or rudder failures may happen as ships proceed through ice, so the available tugs must also be able to assist in that case (Fig. 3.38). Tugs may carry out specific ice duties such as scraping ice from dock and lock walls — some St Lawrence Seaway tugs have bow ice-scrapers (Fig. 3.39). Further useful guidance is in Handling Ships in Ice (Capt. J Buysse) and Polar Ship Operations (Capt. Duke Snider), both The Nautical Institute.

7. Assisting navy ships — submarines (3.4 / 3.4.1)

Navy ships visit commercial ports regularly. Naval ports have purpose-designed tugs and trained crews; commercial ports do not, and because visits are infrequent the knowledge and the tug fendering for handling these ships are limited. Before a large navy ship arrives, berth and procedures are agreed among navy officials, port authorities, towing companies and pilots, and tug masters are briefed. Many navy ships are handled like merchant ships (with attention to the allowable hull force when pushing), but two types need special attention: submarines and aircraft carriers.

7.1 General — what makes a submarine different

Submarines vary in size: diesel-electric attack boats are about 60–90 m long with ~8 m draught; nuclear attack boats (SSNs) about 90–120 m with ~10 m; nuclear ballistic-missile boats (SSBNs) about 100–180 m with up to ~12 m. The essential point is that a submarine is like an iceberg — about 90% is below the waterline, so for a fairly small vessel the draft is very large and the widest part is underwater. They must be approached with care. Most are single-screw, with good steerage from about two knots; turning circles are naturally large.

7.2 Protruding parts and equipment to be aware of

Several parts demand attention (Fig. 3.40). The sail (USA) or fin (Europe), also called the fairwater, is an observation platform and entrance on the surface and a vertical stabiliser underwater; it may carry diving planes (fairwater planes). Hydroplanes (vanes / diving planes) control vertical motion — fore planes (sometimes on the sail) and, just abreast of the top rudder blade, the after planes, which are underwater and cannot be seen, so tugs must take the utmost care approaching the after part; rudder and after planes are often combined in an X-shaped crossway system.

The propeller is called the propulsor and is the utmost aft end — unlike surface ships it sits behind the rudder, so a kick ahead with hard rudder has no turning effect; tugs must keep clear of the propulsor and its strong wash. The hull is covered with anechoic tiles that absorb active sonar and damp the boat's own sounds; they are costly and easily damaged, so tugs must keep them intact. Deck fittings differ from merchant ships: bollards are stowed below a smooth deck and lifted for use; bull rings (a kind of panama fairlead) are at the forward and aft ends; a tow-box and anchor light may sit near the stern rudder and must not be damaged by a towline. There are "No Push" areas marked on the casing above the waterline where sensitive equipment lies, and the pilot must pass this in the pilot/tug-master exchange.

7.3 Tug assistance and recommended methods

In commercial ports tugs lack the underwater fendering of naval-port tugs, so the assistance must be such that the submarine is not damaged. The best approach is towing on a line, with tugs fastened fore and aft via the bull ring on the submarine's bollards, combined with push-pull. ASD-tugs are preferred because they push with the bow; if the bow fendering is not suitable, extra bow fendering is required. Fastening tugs alongside should be avoided unless the tug has underwater fendering (as in naval ports and the Panama Canal). When the submarine has no propulsion, the same method is used, often with a tug forward on a long line. With propulsion, going through Panama Canal locks with locomotives, tugs only escort and assist if the pilot judges it necessary. In Hong Kong tugs never went alongside — submarines tied up to a mooring buoy and were escorted by a tug with a towline, giving only a gentle pull to get the bow to the buoy.

8. Assisting navy ships — aircraft carriers (3.4.2)

There are about 45 aircraft carriers, roughly 180–335 m long and up to 76.8 m wide (flight-deck width), with up to four propellers; several are nuclear-powered. Everyone handling a carrier must be carefully briefed on tug locations, procedures and manoeuvres well in advance, and the pilot holds a pilot/tug-master exchange on boarding to confirm intentions. As with submarines, how a carrier is handled in a commercial port depends on the local situation, fairway width and alignment, environment and available tugs.

8.1 Tug requirements and the masts problem

Carriers generally have enough bollards and fairleads on the mooring deck and sunken bollards to secure towing lines; towing companies must know the SWL of the bollards and the maximum allowable hull pressure. If the carrier anchors within the port, two suitable powerful escorting tugs are recommended; if she berths, generally four tugs are recommended for commercial ports, with extra fenders depending on tug and carrier design. The one basic requirement is that, because of the carrier's large overhanging flight deck, tugs must be able to lower their mast — especially where they work alongside (Figs. 3.50, 3.51). Recommended types are Voith tugs and tugs with azimuth propulsion, all with proper fendering. Carriers need much manoeuvring room: for the Nimitz class (the largest), the entrance channel should be about 240 m wide outside the breakwater and 180 m inside, and the optimal turning-circle diameter is 670 m — twice the ship's length.

8.2 Summary of carrier assistance

Tug assistance for aircraft carriers depends on port layout, available tugs, local procedures and wind. The recommendation is usually four suitable tugs, employed as two forward and two aft on a line via fairleads on or near the centre line, or one forward and one aft on a line plus two alongside, or four alongside; when the carrier anchors in port, two passive escorting tugs. Tugs assisting alongside must be able to lower their masts, with extra fenders if needed; the wind condition must be taken into account for the required bollard pull; carriers require much manoeuvring room; and the SWL of bollards and fairleads, and the maximum allowable hull pressure or pushing points, must be known.