Background
The capability of a tug is conventionally expressed by its bollard pull rating, the maximum static thrust it can develop pulling against a fixed bollard during a standardised trial. Bollard pull is the principal commercial parameter for tug selection, the basis for charter rates, and the input to engineering calculations of tug requirements for berthing, escort towing, and ocean tow planning. The certification of bollard pull, however, is a more nuanced engineering exercise than the headline number suggests, and the relationship between certified bollard pull and useful thrust at non-zero speed is critical to operational planning.
This article describes tug fundamentals, bollard pull definition and the certification process, the principal tug types from conventional twin-screw to azimuth stern drive (ASD), tractor, and escort tugs, the role of escort towing for tankers and LNG carriers, the contractual framework provided by BIMCO TOWCON and TOWHIRE 2021, the principal pieces of towing equipment and their failure modes, and the persistent risk of girting and capsize in close-quarters work.
Tug Fundamentals
A tug must combine high installed power, manoeuvrability, low-speed thrust efficiency, and stability under tow line loads. These requirements drive a distinctive hull form: relatively short and beamy with a low freeboard aft, a high foredeck for crew protection from green water, and a strong superstructure for navigational visibility. Modern tugs typically carry crews of 4-8 depending on size and trading area, with a master, chief engineer, and deckhands.
Tug power is delivered through propulsion systems that have evolved substantially since the 1970s. The conventional single or twin fixed-pitch propeller arrangement, dominant for most of the 20th century, has been progressively replaced by azimuth thrusters, Voith Schneider cycloidal propellers, and other steerable propulsion systems that allow thrust to be directed in any direction without the tug having to rotate. The choice of propulsion type defines the tug’s tactical capabilities and is the basis for the tug type classification described below.
Bollard Pull Definition and Certification
Bollard pull is the maximum sustained pull that a tug can develop, measured at the bollard or pull point during a static trial in deep, calm water. The certified bollard pull is normally a dynamometer-measured force averaged over a specified duration with the tug pulling against a shore bollard via a wire of sufficient length to remove wake effects. The IACS Recommendation 16 (Towing and Mooring Arrangements) and various class society procedural guides set out the trial requirements.
The principal parameters in a bollard pull trial are:
- Wire length, typically at least three times the tug’s length, to ensure thrust is exerted in line.
- Water depth, normally at least twice the tug’s draft and clear of bottom interference.
- Wind, normally not exceeding Beaufort 3-4 to avoid spurious readings.
- Trial duration, with continuous bollard pull (peak pull sustained for typically 5-10 minutes) and maximum bollard pull (5-30 second peak) reported separately.
Continuous bollard pull is the figure used for design and contractual purposes. Maximum bollard pull is a useful indicator of short-term peak capability but should not be relied upon for sustained operations.
A critical practical point is that bollard pull is a static measurement. As the tug builds speed through the water, useful thrust falls because of propeller efficiency limits and hull resistance. At 5 knots through the water, a tug may deliver only 70-80% of its bollard pull as net useful tow force. At 10 knots, the figure may be 40-50%. Escort towing operations specifically design around the speed-thrust relationship.
Tug Types
The principal tug types in commercial service are described below.
Conventional Tugs
Conventional tugs use one or two fixed-pitch propellers in fixed nozzles or open water, driven through reduction gearing from a medium-speed diesel. Steering is by a conventional rudder. Conventional tugs are mechanically simple, robust, and inexpensive, but they have significant limitations: they must rotate the hull to change thrust direction, they have poor manoeuvrability at low speed, and they are vulnerable to girting (see below). Most pre-1990 harbour fleets were conventional, and many remain in service in smaller ports and developing countries.
Azimuth Stern Drive (ASD) Tugs
ASD tugs use two azimuth thrusters mounted under the stern, each capable of rotating through 360 degrees. The thrusters can vector thrust in any direction, allowing the tug to push, pull, or move sideways without rotating the hull. ASD tugs typically work bow-first when pushing and stern-first when pulling, with the towing point on the foredeck (a “stern-drive forward-tow” configuration). ASD tugs are now the dominant new-build harbour tug type worldwide.
Tractor Tugs
Tractor tugs place their propulsion at the bow (or amidships in some designs) rather than the stern. Voith Schneider tractor tugs use vertical-axis cycloidal propellers mounted under the forward hull. ASD tractor tugs use forward-mounted azimuth thrusters. The advantage of the tractor configuration is that the propulsion is forward of the towing point on the after deck, eliminating the girting risk that conventional and ASD tugs face when working alongside a moving ship. Tractor tugs are particularly favoured for escort work and for handling very large vessels.
Articulated Tug-Barges (ATB)
ATBs are tug-barge combinations where the tug is pin-connected to the stern notch of a dedicated barge. ATBs are predominantly North American, used in domestic petroleum and dry cargo trades, where Jones Act economics favour the ATB over a self-propelled ship. The tug pushes the barge, and the combination operates as a single unit underway with manoeuvring characteristics intermediate between a ship and a tug-and-tow.
Escort Tugs
Escort tugs are specialised vessels designed to perform escort towing, where the tug remains attached to a moving ship (typically a laden tanker or LNG carrier) and is positioned to apply braking or steering force in the event of a steering or propulsion failure. Escort tugs use either tractor or ASD-with-tractor capabilities, with high-strength towing winches, large skegs to generate hydrodynamic side force, and high installed power. The largest escort tugs (around 95 tonnes bollard pull, 100+ tonnes static escort force) are deployed at LNG and crude terminals where the consequences of an uncontrolled grounding are extreme.
Escort Towing Operations
Escort towing developed in the wake of the Exxon Valdez grounding (1989) as part of the regulatory response to tanker casualties in restricted waters. The principle is that a laden tanker passing through a sensitive area is escorted by one or more dedicated tugs that can intervene immediately if the tanker loses steering or propulsion.
In an “indirect” escort mode, the tug is connected to the tanker by a long towing pennant and is towed astern. The tug uses its skeg and rudder to generate side force at high speed (10-12 knots), creating a steering or braking moment on the tanker without using its own propulsion to develop the force. Indirect escort can develop static-equivalent forces of 100-150 tonnes from a tug whose nominal bollard pull is 70-90 tonnes, because the hydrodynamic forces on the skeg supplement the propulsion thrust.
In “direct” escort mode, the tug pulls or pushes directly against the tanker hull using its propulsion thrust. This is used at lower speeds where indirect escort is ineffective.
Major escort terminals operate written escort tug requirements specifying the minimum bollard pull, the type and number of tugs, the escort distance, and the escort speed. Examples include Prince William Sound (Alaska) following Exxon Valdez, the Strait of Juan de Fuca (Washington/British Columbia), the Sound (Denmark), and most major LNG receiving terminals.
Towage During Berthing and Unberthing
Harbour tugs assist in berthing and unberthing of large vessels by providing the forces and moments that the vessel’s own engines and thrusters cannot deliver. The number of tugs and their bollard pull is determined by:
- Vessel size and windage area.
- Wind strength and direction.
- Current strength and direction.
- Berth geometry (open quay, dolphin berth, slip).
- Vessel’s own thruster capability.
A 14,000 TEU container vessel berthing in 15 m/s onshore wind may require three or four tugs of 70-80 tonnes bollard pull each to control approach. A laden VLCC berthing in light wind may require two tugs of similar size. OCIMF guidance gives indicative tug requirements for tanker terminals; container line operating manuals give equivalent guidance for the box trades.
The tug is typically attached to the vessel using either a ship’s rope (a synthetic line passed from the vessel to the tug, often over the bullnose for the bow tug) or the tug’s own line. Modern operations increasingly use the ship’s line because it gives the pilot cleaner control: the line is bared to the tug winch which can render or take in as needed under the master’s or pilot’s instruction relayed by VHF.
Salvage Tugs
Salvage tugs are specialised vessels designed for ocean rescue, casualty intervention, and oceangoing tow of disabled vessels. They are larger than harbour tugs, typically 60-80 metres in length, with installed power of 12,000-25,000 kW and bollard pull of 150-300 tonnes. They carry extensive firefighting equipment (FiFi 1 or 2 class, capable of throwing 7,200-9,600 cubic metres per hour of seawater), salvage pumps, towing wires of 2,000-3,000 metres, and accommodation for the casualty crew if required.
The salvage tug industry consolidated dramatically in the late 20th century. The historic operators (Smit, Wijsmuller, Bugsier, Tsavliris, Titan) have been merged or acquired into a smaller number of major groups. Boskalis Smit, T&T Salvage, Resolve Marine, Donjon, and a handful of others operate the major modern fleet. The smaller fleet means that response to large casualties is often co-ordinated between operators under Lloyd’s Open Form terms.
BIMCO TOWCON and TOWHIRE 2021
The two principal contracts for non-salvage ocean towage are BIMCO TOWCON and BIMCO TOWHIRE, both revised most recently in 2021. They were originally drafted in the late 1980s by BIMCO in consultation with the international towage industry as an alternative to the older UK Standard Towing Conditions and various ad hoc forms.
TOWCON 2021 is a lump-sum contract for a defined tow from departure point to destination. The tug owner is paid a fixed sum (with delay and adverse weather provisions) regardless of the time taken. Risk of weather delay falls on the tug owner subject to specific exceptions. TOWCON is commonly used for offshore unit moves, dry-tow shipments where the tug element is contracted separately, and known-route deliveries.
TOWHIRE 2021 is a daily-hire contract where the tow is paid by the day at a stated rate, with risk of delay generally on the hirer (the tow). TOWHIRE is used where the tow time is uncertain, for example salvage station-keeping, escort missions, or delivery to a contract destination on hire.
Both forms allocate liability for damage to tug, tow, and third parties through a “knock-for-knock” regime: each party bears its own losses and indemnifies the other regardless of fault, subject to specific exceptions. This is intended to avoid expensive cross-claims and to keep insurance simple. The 2021 revisions updated the sanctions, BIMCO Cyber Security, and force majeure clauses, reflecting modern commercial expectations.
BIMCO Salvage 2014 is the related form for non-LOF salvage, used where the parties wish to fix terms in advance rather than relying on LOF arbitration.
BIMCO SUPPLYTIME 2017 covers offshore supply vessel charters, which are similar to towage in many respects but include accommodation and stores supply functions in addition to towing.
Towing Equipment
The principal towing equipment items on a modern tug are:
- The towing winch, a high-line-pull powered winch with constant-tension capability for ocean tows. Modern winches typically have line pull capacity equal to or exceeding the bollard pull, with brake holding capacity 1.25 to 1.5 times the line pull.
- The towing wire or pennant, of high-grade steel wire or, increasingly, ultra-high-molecular-weight polyethylene (HMPE) such as Dyneema. HMPE lines are lighter, easier to handle, and have higher strength-to-weight ratio, but they require careful chafe protection.
- The gob rope (or “gob eye”), a short pennant with a fairlead that constrains the towing wire to lead from a defined point near the tug’s centre of rotation. The gob rope is critical to preventing girting (see next section).
- The towing hook, used in some configurations as an emergency quick-release for the tow line.
- Stern rollers and fairleads, to guide the wire over the deck without snagging.
Girting and Capsize Risk
Girting (also called “tripping” or “girding”) is the catastrophic failure mode where a tug working alongside a moving ship is pulled over by the towline force exceeding her transverse stability. Girting occurs when the line of action of the tow force passes outside the tug’s metacentric stability envelope, capsizing the tug typically in seconds.
The classical girting scenario involves a conventional or ASD tug working close to a moving ship, with the towline angle steepening as the ship moves forward and outward relative to the tug. If the tug cannot match the ship’s speed or break the line in time, the line forces the tug onto her side and she capsizes. The crew, often working on the after deck adjacent to the tow connection, may have only seconds to escape.
The 2010 capsize of the Italian tug Charlotte off Genoa, the 2007 capsize of Berto off Tobago, and the 2018 capsize of the Sea Sun in the Bosphorus illustrate the persistent nature of the risk. Investigation reports typically identify a combination of factors: ship speed too high for tug response, towline angle outside safe envelope, gob rope absent or improperly positioned, and crew not familiar with quick-release procedures.
Tractor tugs are inherently less prone to girting because their towing point is aft of the propulsion, so transverse forces do not act through a stability-limiting moment arm. ASD tugs working with proper gob rope and trained crews can largely manage the risk, but the capability of a tractor or escort tug to escape girting is materially better. This is one reason why major tanker and LNG terminals specify escort tugs of tractor or ASD-tractor configuration.
Related Wiki Articles
- Berthing Operations and Fender Selection
- Pilotage Operations
- Towage and Salvage Operations
- Marine Mooring Equipment and Winches
- Marine Anchor and Anchor Handling Equipment
- Marine Engine Performance Monitoring
- SOLAS Convention
- Heavy Weather Operations
See also
Calculators
- AHTS - Bollard Pull Test
- Tug Bollard Pull - Port Selection
- Bollard Pull - BHP Conversion
- Tug Bollard Pull Required
Formula references
Related wiki articles
References
- BIMCO TOWCON 2021 and TOWHIRE 2021
- BIMCO Salvage 2014
- BIMCO SUPPLYTIME 2017
- Lloyd’s Open Form (LOF 2020)
- IACS Recommendation 16, Towing and Mooring Arrangements
- IMO Resolution MSC.255(84) and MSC.96(72) on tug operations
- OCIMF Mooring Equipment Guidelines (MEG4), 4th Edition
- OCIMF Guidelines for the Design and Construction of Offshore Supply Vessels
- ABS Guide for Building and Classing Tugs
- DNV Rules for Classification of Ships, Pt 5, Ch 10
- Lloyd’s Register Rules for Special Service Craft
- ICS Bridge Procedures Guide, current edition
- The Nautical Institute publications on tug operations and escort towing