Tug Operations and Bollard Pull

Tugs concentrate diesel or diesel-electric power into short, beamy hulls built for low-speed thrust. A 35-metre ASD harbour tug of 4,500 kW delivers about 60 to 70 tonnes of bollard pull; a large ocean escort tug of 18,000 kW may exceed 250 tonnes. That headline number, bollard pull, is the commercial currency of the towage industry: it determines charter rates, underpins engineering calculations for berthing, and appears on the class-society certificate that certifies the tug fit for its stated duty. But the number requires interpretation. It’s a static figure measured at zero speed, and the useful thrust a tug delivers at 8 knots is typically 40 to 60 percent of what it produces alongside a bollard. Escort tug dynamics, girting risk, propulsion-type selection, and towage-contract structure all depend on understanding what the certified figure actually means.

The bollard pull calculator allows selection of minimum tug requirements for a range of vessel sizes and environmental conditions. For BHP-to-bollard-pull conversion, the Bollard Pull BHP Conversion calculator applies the empirical rules described in this article. The companion article on towage and salvage operations covers the contractual and legal framework for ocean towage under BIMCO TOWCON, TOWHIRE 2021, and Lloyd’s Open Form; this article concentrates on the equipment, certification, and operational side.

Bollard pull: definition and certification

What the static bollard pull trial measures

Bollard pull is the maximum sustained pulling force a tug can develop when pulling against a fixed shore bollard in calm, deep water with all machinery at full power. The result is not a theoretical calculation; it is a measured quantity obtained under controlled conditions with a calibrated dynamometer interposed between the towing wire and the bollard.

The trial conditions matter because water depth, wire angle, currents, and weather all affect the result. Class society procedures (DNV Rules Pt 5 Ch 10, ABS Guide for Building and Classing Tugs, Lloyd’s Register Rules for Ships, Bureau Veritas Rules) converge on common requirements: water depth at least twice the tug’s maximum draft and not less than 10 metres, open water clear of the bottom’s backflow effect, wind not exceeding Beaufort 3 (about 5.4 m/s), and a wire length of at least three times the tug’s overall length (typically 75 to 150 m) so the pull is effectively horizontal and any wake behind the tug doesn’t shorten the effective thrust column.

The trial distinguishes two figures:

• Continuous bollard pull: the mean dynamometer reading sustained over at least 10 continuous minutes at full power. This is the certified figure used for commercial and engineering purposes.
• Maximum bollard pull: the highest reading achieved during the trial, typically a peak lasting 15 to 30 seconds. It appears on many certificates but should not be used in design calculations; it reflects the momentary contribution of propeller water-column inertia rather than steady-state engine output.

Most modern class procedures require at least two trial runs per direction of operation (some tugs pull better stern-first, others bow-first depending on propulsion layout) and average the results. For ASD tugs the trial is typically conducted with both thrusters oriented in the same direction, the configuration that maximises static pull; the certification reflects that specific orientation.

IACS Recommendation No. 16 (Towing and Mooring Arrangements) provides the inter-class alignment framework. The relevant clause specifies that the certificate must identify the propulsion mode, the direction of pull, the wire length used, and the date of the trial. Certificates are typically valid for the life of the tug subject to re-trial if a major propulsion change is made.

Wire-length corrections and depth corrections

Two corrections that sometimes appear in older certificates or in contractual disputes deserve attention.

Wire-length correction accounts for the catenary weight of a heavy steel wire under tension. A wire of 120 metres and 50 kg/m (a 76 mm dia. steel tow wire) develops a catenary sag at lower tensions that reduces the horizontal component of pull. At higher tensions the catenary flattens and the horizontal component approaches the total tension. Modern HMPE (high-modulus polyethylene) tow lines of similar strength weigh only 8 to 12 kg/m; their catenary effect is negligible. For steel wire trials, the correction shifts certified bollard pull by 1 to 3 percent depending on wire weight and tension.

Water-depth correction matters when the trial is conducted in less than ideal depth. In shallow water the propeller inflow column is disturbed by bottom interaction, increasing bollard pull relative to deep-water conditions. Class procedures set minimum depths partly to prevent this inflation. Where a trial is conducted in depth less than the ideal but above the minimum, a correction factor may be applied.

Escort tug dynamic certification

Standard bollard pull certification doesn’t capture what an escort tug does. An escort tug in service applies steering and braking forces to a laden tanker moving at 6 to 12 knots, and the forces it can generate at those speeds are different from, and often larger than, its static bollard pull. OCIMF Guidelines for the Design, Testing and Operational Use of Escort Vessels specify a dynamic trial in which the tug is connected to a test bollard via a long pennant while being towed at the design escort speed. The dynamometer measures transverse (steering) force and longitudinal (braking) force separately.

The dynamic steering force (also called the escort force or indirect force) derives from two contributions: the hydrodynamic side force generated by the hull, skeg, and rudder as the tug takes a large angle of attack to the flow, and the propulsion thrust directed transversely. DNV’s Rules for Escort Tugs (Pt 5 Ch 10 Sec 2) require that the escort capability be stated on the certificate as both the static bollard pull and the dynamic steering and braking forces at the certified escort speed, in kN or tonnes. Most escort tugs at Prince William Sound and at LNG terminals carry certificates showing both parameters.

Power-to-bollard-pull relationships

The bollard pull achievable from a given installed power depends on propulsion type, propeller loading, and whether a nozzle (kort nozzle or flanking rudder nozzle) is fitted. The relationship is normally stated as a rule-of-thumb coefficient $k$ in the expression:

where $BP$ is bollard pull in kN and $P$ is shaft power in kW. This cube-root scaling comes from the actuator-disk theory of propeller thrust: at static conditions, thrust is proportional to the square root of the power-times-disk-area product, and for geometrically similar propellers, disk area scales with power to roughly the two-thirds.

For practical tug sizing, a simpler linear approximation is used:

where $k_{\text{lin}}$ is in kN/kW. Representative values by propulsion type:

The linear approximation becomes unreliable outside 3,000 to 10,000 kW; below that range propeller blade-area limits dominate, and above it hull resistance during the trial starts to reduce net pull.

The nozzle effect is worth understanding in isolation. A kort nozzle accelerates the inflow into the propeller, increasing thrust at zero or low advance speeds by 25 to 30 percent relative to an open propeller of the same diameter and power. At speeds above about 8 knots, the nozzle’s inlet drag reverses this advantage and bare propellers are faster. Tugs are static-thrust machines; nozzles are standard on any tug optimised for bollard pull.

For the numerical conversion between engine BHP, shaft power, and bollard pull, the Bollard Pull BHP Conversion calculator applies these coefficients with user-adjustable propulsion-type selection. The AHTS Bollard Pull Test calculator covers the offshore anchor-handling variant of the trial.

Tug propulsion types and configurations

Conventional tugs

Conventional tugs drive one or two fixed-pitch propellers from a medium-speed diesel through a gearbox, and steer with a conventional rudder. Until the 1980s this arrangement was universal. The main limitation is directional: to change the thrust vector, the hull must turn, and turning takes time and sea room. At low speed in confined harbour approaches, conventional tugs have poor directional control relative to a vessel they’re pushing or pulling.

Conventional tugs are mechanically simple and inexpensive to maintain. Many remain in service in smaller ports and in developing-country fleets where ASD spares and training infrastructure are limited. For straightforward deep-water bollard operations they’re perfectly adequate; the problems emerge in close-quarters ship-handling where speed and precision matter.

Azimuth stern drive (ASD) tugs

ASD tugs mount two azimuth thrusters under the stern, each rotating 360 degrees on a vertical axis. Each thruster drives a propeller (in nozzle or open) that can be directed in any horizontal direction. The tug doesn’t need to rotate to change thrust direction; it can push, pull, or translate sideways without turning. In harbour work, an ASD tug typically works bow-first when pushing and stern-first when pulling, the tow point being on the foredeck.

ASD tugs dominate new-build harbour tug orders globally. The Robert Allan, Damen, Sanmar, and Uzmar catalogues are almost exclusively ASD designs. The 2,000 to 5,000 kW range of harbour ASD tug is now mature enough that builders quote from standard hulls with customised topside arrangements.

One configuration consequence: an ASD tug has its propulsion aft of the tow connection on the foredeck. When pulling with a line led aft, the tug is in the conventional-tug configuration and faces the girting risk described below. Managing this with a gob rope is standard practice, but it’s an operational constraint that tractor designs don’t share.

Tractor tugs (VSP and forward ASD)

Tractor tugs place their propulsion forward of the midship tow point, which is on the after deck or a stern towing bracket. The classic tractor propulsion is the Voith Schneider propeller (VSP), a vertical-axis cycloidal propeller of three to eight blades that rotates on a vertical shaft and controls blade pitch to produce thrust in any horizontal direction without rotating the unit itself. VSP tugs are extremely agile at low speed; their weakness is lower peak static thrust per kilowatt compared with azimuth thrusters in nozzles.

Forward-mounted ASD tractor tugs (sometimes called “rotor tugs” in the marketing literature, though RotorTug is technically a specific three-unit design from Robert Allan) use one or two azimuth thrusters forward plus a single stern-mounted thruster or rudder. The forward propulsion keeps the thrust vector ahead of the tow point, eliminating the girting moment.

The principal operational advantage of the tractor layout is that the towline always runs from the after tow point toward the ship, with the propulsion working against the tow load from ahead. The tug can’t be pulled over stern-first in the way an ASD tug can because the physics don’t permit the destabilising geometry to develop.

RotorTug

The RotorTug is a patented design from Robert Allan / RotorTug BV that mounts three azimuth thrusters: two forward and one aft. The forward pair handles most of the propulsion and manoeuvring load; the aft thruster provides the third point of a stable propulsion triangle that eliminates the girting hazard by making either end of the tug functionally a tractor. RotorTugs are in service at Rotterdam, Singapore, and several major container terminals. Their static bollard pull at 3,000 to 4,200 kW is among the highest achieved by production tugs of that power; the 2,940 kW RotorTug “Multratug 9” delivered 52 tonnes bollard pull in trials.

Escort tugs

Escort tugs are a performance category that overlaps with configuration. An escort tug can be any configuration that meets OCIMF’s escort performance requirements: the ability to apply adequate steering and braking force to a laden tanker or gas carrier transiting at 6 to 12 knots. In practice, modern escort tugs are either tractor-VSP, tractor-ASD, or RotorTug configurations, because those eliminate or reduce the girting risk that would be unacceptable when attached to a moving large vessel.

Escort tugs carry large skegs (a fixed fin below the hull aft) that generate hydrodynamic side force when the tug takes an angle of attack to the tow direction. The skeg force amplifies the effective towing force in indirect mode. OCIMF’s guidelines require escort tugs to demonstrate their dynamic steering and braking forces at the terminal’s design escort speed (commonly 8 or 10 knots) in a full-scale trial, and the resulting figures are stated on the tug’s escort certificate.

At Prince William Sound (Alaska), the regulatory requirement following the 1989 Exxon Valdez grounding mandates a minimum of two escort tugs of at least 4,550 kW and documented escort capability for any laden tanker transiting the Sound. OCIMF’s 2002 guidelines for escort tug use at oil terminals formed the basis for similar requirements at most major VLCC and LNG berths.

Escort towing modes

Direct mode

In direct mode, the escort tug pulls or pushes directly against the assisted vessel using its propulsion thrust. The tug is connected to the tanker (usually via a pennant to a midship chock or aft towing point) and applies force to steer or brake the tanker by propelling itself transversely or against the tanker’s direction of travel. Direct mode is effective at low speeds, typically below 4 to 5 knots, where hydrodynamic forces on the tug hull are small and propulsion thrust dominates. The effective force in direct mode is approximately equal to the tug’s static bollard pull, reduced by the tug’s own hull resistance at the operating speed.

Indirect mode

Indirect mode is the defining capability of modern escort tugs and the reason a 75-tonne bollard-pull tug can apply 100 to 150 tonnes of effective force to a moving tanker. The tug is connected aft of the tanker via a long pennant and is towed along by the tanker while oriented at a large angle to the flow (30 to 70 degrees). At this angle, the flow over the hull and the large stern skeg generates a hydrodynamic side force analogous to a ship’s keel resisting leeway. The tug’s propulsion is directed to increase this angle of attack, not to pull directly on the pennant.

The effective escort force $F_e$ in indirect mode is approximately:

where $F_{hydro}$ is the hydrodynamic skeg force (dependent on tug speed, skeg area, and angle of attack), $F_{prop}$ is the propulsion thrust, and $\theta$ is the angle between the thrust vector and the pennant direction. At design escort speeds, $F_{hydro}$ is typically two to three times $F_{prop}$, so indirect mode fundamentally changes the force budget.

The Escort Tug Operating Speed Window calculator models the speed range over which an escort tug can maintain both effective indirect force and adequate stability margin.

Indirect mode imposes a high load on the towing gear. Pennant tensions in indirect mode at 10 knots may exceed three times the static bollard pull for the same tug. Equipment ratings (winch brake, pennant breaking strength, tow point structural strength) must be sized for indirect-mode dynamic loads, not just static bollard pull.

Transition and capsize risk during escort

Transitioning between indirect and direct mode, and between different escort positions (from port quarter to starboard quarter, for example), is one of the highest-risk manoeuvres in escort towing. During the transition the tug may pass through configurations where both the hydrodynamic force and the propulsion thrust are directed to pull the tug onto its side. OCIMF guidance requires that escort tug crews be trained on simulators or in live exercises specifically in transition manoeuvres, and that the master of the assisted vessel agree transition sequences in advance.

Ship-handling operations

Berthing and unberthing

Harbour tugs assist in the berthing and unberthing of large vessels by providing the forces and moments the vessel’s own engines and thrusters can’t deliver economically or quickly enough. The number of tugs and the required bollard pull depend on:

• Vessel displacement and windage area (length times superstructure height above waterline is a workable proxy for sail area).
• Wind speed and direction; beam wind loading on a fully loaded container ship of 400 m creates substantial heel and leeway forces.
• Current speed and direction at the berth approach.
• Berth geometry: swing room, dolphins, current shear across the berth.
• The vessel’s own lateral thruster power, if any.

OCIMF’s current and wind load tables (integrated into its Mooring Equipment Guidelines, MEG4, and into earlier single-vessel guidance documents) give the standard method for computing these loads. For tanker terminals, the OCIMF current load calculator and OCIMF wind load calculator implement the MEG4 methodology directly.

A 400 m, 14,000 TEU containership berthing in 18 m/s beam wind can develop a total wind load of 400 to 600 kN on the hull and stack. Three tugs of 60 tonnes bollard pull each provide around 1,800 kN combined; with reasonable allowance for line angles and dynamic loading, that margin is workable but tight. Most major container-port operators pre-arrange tug requirements as part of their pre-arrival planning, specifying minimum bollard pull per tug and the number of tugs as a function of vessel LOA and local wind limits.

The pilot manages the tug deployment. Communication between pilot and tug masters is typically in English on an agreed VHF channel, with the pilot giving directional instructions and the tug masters managing their own line tension and position. The physical attachment: for the bow tug, a synthetic line (often the ship’s own forward spring) is passed through the bullnose and down to the tug, which makes it fast to the bow cleat or tow hook. Midship and stern tugs may use either the ship’s lines or their own tow wires.

Push-pull operations

Modern harbour tug work distinguishes push-pull operations depending on whether the tug is pushing against the hull (no line, physical contact through tug fendering) or pulling via a line. Pushing is mechanically simpler: the tug presses its fender against the hull and vectors thrust toward the desired motion. It’s fast to initiate and easy to break off. But for large vessels in open water, the tug’s thrust at the ship’s curved hull creates a moment, not just a force; the contact point’s position relative to the ship’s turning centre matters for the net effect.

Pulling via a line gives the tug the ability to apply force in the direction away from the ship, which is otherwise impossible in push mode. It also allows the tug to work at a distance from the hull, reducing the risk of contact damage. The disadvantage is the time to pass and make fast the line, and the risk that the line fouls or the connection fails at a critical moment.

Tug line force and pivot point

The effective moment a tug applies to a vessel depends not only on the tug’s bollard pull but on the distance from the tug connection point to the vessel’s pivot point (the instantaneous centre of rotation) and the angle between the tow line and the vessel’s centreline. The Tug Line Force Pivot Moment calculator calculates the turning moment contributed by each tug given its position, line angle, and applied force.

Towing equipment

Towing winches

The towing winch is the primary piece of towing equipment on any tug beyond the simplest harbour work. Modern winches are rated for line pull (the force they can haul in at rated speed), brake holding capacity (the force the drum brake can hold statically), and wire capacity (the length and diameter of wire the drum can accommodate).

For harbour tugs, brake holding capacity is the critical parameter: the winch doesn’t typically heave in during a berthing, but it must hold the line under full bollard pull without rendering. Class rules require brake holding capacity of at least 1.5 times the continuous bollard pull. For ocean tow and escort tugs, the constant-tension function matters: the winch automatically pays out or heaves in to hold a set line tension as the tow surges, preventing shock loads that could part the wire.

The Marine Mooring Equipment and Winches article covers winch design in the mooring context; the same mechanical principles apply to tow winches, with higher load ratings.

Tow wires and pennants

Steel tow wire (IWRC, six-strand, galvanised) has been the standard for ocean towage for a century. A 76 mm (3-inch) steel tow wire has a breaking load around 3,700 kN and weighs about 22 kg/m; a standard 500-metre drum on a 200-tonne bollard-pull salvage tug weighs over 11 tonnes. The weight and handling difficulty of steel wire drives its replacement in many applications by HMPE (high-modulus polyethylene), primarily Dyneema or Spectra. A Dyneema wire of equivalent breaking load weighs 85 to 90 percent less, floats in water (eliminating catenary complications), and doesn’t cause the spring-back injury risk of a parting steel wire.

HMPE’s weakness is susceptibility to heat (melting point around 135 to 150°C for UHMWPE), abrasion on rough metalwork, and creep under sustained high load. Chafe sleeves at wear points and stainless-steel or polymer-coated fairleads are mandatory for HMPE on working tugs.

Escort pennants, which must survive indirect-mode dynamic loads (potentially three times bollard pull at design speed), are typically steel wire or steel-core HMPE composite. The pennant shackle, swivel, and tow hook are rated individually and are inspected under class supervision as part of the escort certification.

The gob rope (gog rope)

The gob rope is a short constrained line, typically chain or heavy rope, that runs from a strongpoint on the tug’s deck to a fairlead low on the tug’s side, constraining the tow wire so it cannot lead too far athwartship. Its purpose is to limit the heeling moment a transverse tow load can impose on the tug by keeping the line’s lead angle within the stability envelope.

On an ASD tug, the gob rope typically leads from the tow line (picked up by a strop looped over the main wire) down to a low fairlead at the tug’s beam, set about 3 to 4 metres from the centreline. When the tow wire tries to swing outboard, the gob rope pulls the strop and constrains the wire lead angle. The exact setting of the gob rope’s length and fairlead position determines the effective protection; a gob rope set too long doesn’t constrain the angle adequately, and one set too short imposes a permanent downward load on the winch drum.

Investigation reports into tug capsizes (the 2010 Charlotte capsize off Genoa, the 2018 Sea Sun capsize in the Bosphorus) consistently find absent or improperly set gob ropes among the contributing factors. The Australian Transport Safety Bureau review of tug capsizes in Australia (ATSB MAB-2010-001) identified inadequate gob rope use as a systemic issue.

Girting and capsize

Girting (or girding) is the capsize failure mode unique to tugs working alongside a moving ship. It occurs when a tug’s towline takes a transverse load large enough to heel the tug beyond the angle where its restoring moment can recover, typically 30 to 45 degrees, and the capsize follows within seconds. The mechanics: a tug pulling toward a ship has the tow line angled from the ship’s hull to the tug’s tow point. If the ship surges forward and the tug can’t match the speed, the line sweeps aft along the tug’s side. The moment the line is pulling transversely amidships, the tug is in a configuration where its metacentric height ($GM$) provides no restoring moment against the tow load, because the towline force acts at the waterline level and is entirely transverse. At a sufficiently high tow force, the tug heels to capsize.

The geometry is unforgiving because it develops quickly. A tug at 2 knots relative to a ship accelerating to 4 knots covers this transition in 10 to 15 seconds. Crew on the after deck have no realistic escape window unless they are clear of the deck before the event. The towing hook’s quick-release function is the last resort; it must be operable from a safe position and must actually be used in the moment, which requires trained and alert crew.

The propulsion-configuration effect on girting risk is decisive:

ASD tugs working in harbour with a properly set gob rope and trained crews can reduce their girting risk to near-tractor levels. The gob rope is not a backup; it’s a mandatory part of every ASD tug’s harbour-operating procedure.

The British Ports Association and the Nautical Institute’s publication “Tug Use in Port” (6th edition) gives the standard guidance on gob-rope setting, speed limits for tug attachment, and the conditions under which a pilot should hold the tug’s line rather than keep it belayed. Major ports publish local tug-operating-procedures that specify maximum ship speed before line must be dropped and the required configuration of each tug in the fleet.

BIMCO TOWCON and TOWHIRE 2021

The two BIMCO forms for commercial ocean towage set the contractual framework that governs the deployment and remuneration of ocean-going tugs. They operate as a foundation layer on which individual negotiations layer additional terms; both use the knock-for-knock liability allocation (each party bears its own losses, the tug owner indemnifies the hirer for tug loss, the hirer indemnifies the tug owner for tow loss), which keeps cross-claims and insurance simple.

TOWCON 2021 is a lump-sum form: the tug owner is paid a fixed fee for the complete tow from departure to destination, regardless of time elapsed (subject to weather deviation provisions). Weather risk falls on the tug owner. TOWCON suits known-route deliveries: offshore platform towage from a shipyard to an installation site, delivery of a floating dock, or a standard port-to-port tow.

TOWHIRE 2021 is a daily-hire form: the hirer pays a stated daily rate, and weather delays run at the hirer’s account. TOWHIRE is used where tow duration is uncertain, including station-keeping, standby, or delivery where weather routing can materially change voyage time.

The 2021 revisions to both forms updated the sanctions clause (extending to secondary sanctions coverage), added the BIMCO Cyber Security Clause (2019 version), and modernised the force majeure language. Neither form allocates liability for pollution beyond the knock-for-knock foundation; P&I Club cover for third-party pollution liability supplements the contractual allocation.

Full treatment of ocean towage contracts, Lloyd’s Open Form (LOF 2020), the Salvage Convention 1989, SCOPIC, and salvage award calculation is in the companion article Towage and Salvage Operations.

Harbour tug fleet planning

Major container and oil terminals develop minimum tug-requirement matrices specifying the number of tugs, minimum bollard pull per tug, and required tug type (conventional acceptable, ASD required, escort required) as a function of vessel LOA, beam, deadweight, wind speed, and current speed. These matrices are used for pre-arrival planning and are normally embedded in the port’s harbour master’s standing orders.

A worked example: for a laden VLCC of 300,000 DWT approaching a berth in 12 m/s beam wind:

• Wind load on hull and superstructure: approximately 450 to 600 kN using MEG4 coefficients.
• Current load at 0.5 knot (0.25 m/s) beam current: approximately 200 kN.
• Combined environmental load: approximately 650 to 800 kN.
• Required bollard pull (applying a 1.5 safety factor and accounting for line angle losses of ~10%): roughly 1,100 to 1,350 kN.
• Tug allocation at 600 kN (60 t) per tug: three tugs.

The Required Bollard Pull calculator implements this method, with wind area inputs, current loading from the MEG4 tables, and a user-adjustable safety factor.

For the tug’s own fuel and emissions estimation during harbour or escort operations, the Harbour/Escort Tug Fuel and CO2 calculator estimates fuel burn and CO2 output as a function of power output and operating hours.

Salvage tugs

Salvage tugs are purpose-built (or converted harbour tugs of high capability) deployed to respond to casualty situations: a vessel adrift, aground, or on fire. They carry firefighting monitors (FiFi 1 or FiFi 2 classification: FiFi 1 = 7,200 m3/h water capacity, FiFi 2 = 9,600 m3/h), salvage pumps, diving support, portable generators, and towing gear including both HMPE and steel wire in large reel quantities (2,000 to 4,000 m).

The salvage tug fleet has consolidated. The historic names (Smit International, Wijsmuller, Bugsier, Tsavliris) are now largely absorbed into Boskalis SMIT, T&T Salvage, Resolve Marine, and Donjon Marine. These operators hold emergency response contracts (Emergency Response and Salvage Vessels, ERSVs) at major tanker terminals and in heavy-traffic straits. Response times to casualty zones are contracted in hours; the UK SOSREP regime (Secretary of State’s Representative for Maritime Salvage and Intervention) has powers under the Merchant Shipping Act 1995 to direct salvage operations to prevent pollution.

The legal and contractual dimension of salvage (LOF 2020, Article 13 rewards, Article 14 / SCOPIC special compensation) is covered in detail in Towage and Salvage Operations. The Salvage Convention 1989 and SCOPIC article covers the legal text directly.

Limitations of bollard pull as a tug capability measure

Bollard pull is the maritime industry’s most widely cited tug metric, but it has structural limitations that require awareness.

Speed reduction. Bollard pull is a zero-speed figure. At 5 knots, net effective tow force is typically 65 to 75 percent of static bollard pull for a conventional or ASD tug; at 10 knots, 40 to 55 percent. Ocean towage planning that assumes static bollard pull at tow speed overestimates available force, sometimes badly. Weather routing calculations for an ocean tow must use effective pull at the estimated tow speed, not the certified bollard pull.

Propulsion mode and direction. An ASD tug’s bollard pull is measured in the configuration that maximises it: both thrusters pointing in the same direction. In harbour work, where the thrusters are constantly redirecting, average instantaneous thrust is lower. The difference is material in confined berth approach manoeuvres where thrusters are feathering constantly.

Trial conditions vs. operational conditions. Trials are conducted in calm, deep water. Operational conditions include swell, shallow water, cross-currents, and wind on the tug itself. A tug working in 1.5-metre swell develops intermittent cavitation on its propellers and peak-to-trough thrust variation that averaged numbers don’t capture.

Certification currency. A bollard pull certificate issued 15 years ago reflects the tug’s condition at that time. Propeller erosion, nozzle wear, and gearbox efficiency losses accumulate. Re-trials are not routinely required unless a major propulsion overhaul occurs. For large-contract towage, owners and hirers should consider requesting a recent trial.

Indirect escort force vs. bollard pull. As described, indirect mode can generate much larger forces than bollard pull, but only at speed and only in the escort tug’s plane of maximum skeg force. Bollard pull is an inadequate specification for an escort tug; the dynamic escort certificate is the relevant measure.

See also

Related calculators

• Tug Bollard Pull Selection
• Bollard Pull BHP Conversion
• Required Bollard Pull
• AHTS Bollard Pull Test
• Escort Tug Operating Speed Window
• Tug Line Force Pivot Moment
• Harbour/Escort Tug Fuel and CO2
• OCIMF Wind Load
• OCIMF Current Load

Related wiki articles

• Berthing Operations and Fender Selection
• Towage and Salvage Operations
• Salvage Convention 1989 and SCOPIC
• Pilotage Operations
• Marine Mooring Equipment and Winches
• Mooring Forces and Station Keeping
• Marine Anchor and Anchor Handling Equipment