Bollard Pull and Tug Operations: Trials, Types, Escort

Bollard pull is the horizontal towline force a vessel’s main propulsion develops at zero speed through the water, measured by a calibrated load cell fitted between the towline and a fixed shore strongpoint. It is the sizing number for every harbour towage operation and every ocean tow. Commercial practice quotes it in tonnes-force, class rules in kilonewtons, at 1 tonne-force = 9.80665 kN exactly.

Two things follow from that definition and they carry the rest of this article. The certified figure is not the force a tug delivers in service, because it is measured at zero advance speed in deep still water and the tug works at an angle, at speed, in a current. And an escort tug in indirect mode can exceed its certified figure, because the load is then carried by hydrodynamic lift on the hull and skeg rather than by the propulsors.

There is also no international standard behind the number in the ISO or IMO sense. Three procedures are in use, they set materially different trial conditions, and a certificate is readable only alongside the trial report that says which one was followed.

Bollard pull: what the certified figure is

A bollard pull certificate records a force, a direction, a measured brake power, a mean engine speed and the conditions of the trial. It is a class trial certificate, not a statutory one, and its authority in a dispute is contractual, and what it contains is set out in bollard pull certificate .

The static trial and what it leaves out

The trial fixes the vessel to a shore strongpoint, runs the propulsion up, and reads the tension in the line. Everything that makes towage difficult is deliberately excluded: the tug does not move, the water is deep, the wind and current are held to limits, and the propellers have usually just been polished.

That exclusion is the point. A single reproducible number lets a port write a towage matrix, a marine warranty surveyor approve a tow and a broker fix a tug, and it feeds the tug allocation described in berthing operations and fender selection . It also means the figure is an upper bound rather than a working capability, which is why sea trials and performance testing treat it as one certified point rather than a curve.

Continuous pull, overload pull and the peak nobody certifies

MSC/Circ.884 of 21 December 1998, Appendix A item 16, certifies the towing force maintained without any tendency to decline for not less than 10 minutes. The MARIN standard of July 2019 certifies the highest consecutive 5-minute mean inside a 15-minute run, taking the arithmetic mean of at least 300 consecutive data points sampled at 1 Hz or better.

The second certified figure is overload pull, run at the engine maker’s maximum rating. MSC/Circ.884 item 3 requires it to be maintainable for at least 30 minutes; DNV requires at least an hour. Both allow it to be omitted from the trial.

No published procedure certifies a peak. Yards and brokers sometimes quote the highest instantaneous reading, and DNV’s own rule text defines bollard pull as the maximum continuous pull obtained at a static pull test on sea trial. A peak reading belongs in no contract.

What the certificate states, and how long it lasts

Under the MARIN standard the certificate reads in a fixed form: so many metric tonnes in the ahead or astern pulling direction, at a measured brake power in kW and a mean engine speed in rpm. Draught and trim are stated, and so are the fuel type and its calorific value, because a tug run on a different fuel does not reproduce the figure.

DNV requires the angular position of turnable propulsion devices to be recorded. That is how a class certificate handles thruster interaction without quantifying it, and it is why an azimuth tug’s certificate describes one geometry that harbour work rarely reproduces.

Validity is five years under the MARIN standard as adopted by Bureau Veritas. Certificates issued under other procedures carry no standard validity period at all.

Three trial procedures, not one standard

There is no ISO, IEC or IMO standard for bollard pull trials. There is an IMO circular appendix from 1998, there are individual class procedures, and there is one public cross-industry procedure from 2019 that a single IACS member has written into its rules. The conditions differ enough that the same tug can certify differently at two yards.

MSC/Circ.884 Appendix A, the IMO route

MSC/Circ.884, Guidelines for Safe Ocean Towing, carries a bollard pull test procedure at Appendix A. Engines run at the manufacturer’s recommended maximum torque at MCR, service propellers are fitted, and all engine-driven or shaft-driven auxiliaries stay connected.

The site conditions are absolute numbers. Towline length not less than 300 m between the stern and the test bollard, with twice the vessel length accepted only where 300 m is unobtainable. Water depth not less than 20 m within a radius of 100 m of the vessel, with twice the maximum draught accepted only where 20 m is unobtainable, and the circular states directly that reduced water depth may adversely affect the test results.

The same appendix governs the trial for an anchor handling tug supply vessel . Displacement corresponds to full ballast and half fuel, on even keel or with trim by the stern not exceeding 2 percent of length. Wind is limited to 5 m/s and current to 0.5 m/s in any direction. The load cell sits between the towline eye and the bollard, is approved by a competent body, and is accurate to plus or minus 2 percent, with a continuous read-out and a graphical recorder.

The class route

DNV covers tugs and escort vessels in Pt.5 Ch.10, Vessels for special operations, at Sec.11. The test is carried out before entry into service, DNV witnesses it and issues the certificate, and where the trial exceeds the expected bollard pull by more than 10 percent the design approvals are reconsidered.

DNV sets no minimum towline length and no minimum water depth of its own: those numbers are the IMO circular’s. What it adds is a fair wind limit of 5 m/s, a co-current limit of 1 knot, annual calibration of the load cell, a requirement that the arrangement read a horizontal force with friction and vertical components minimised, and the recording of thruster angular position. The towing hook is load tested to bollard pull and its quick release tested against a towline direction 60 degrees or 45 degrees above the horizontal.

Assertions that Lloyd’s Register, ClassNK and RINA converge on a common set of trial conditions do not survive contact with the documents. A class joint industry project involving Bureau Veritas, Lloyd’s Register and ABS was formed precisely because tug rules differ materially between societies.

The MARIN joint industry standard of July 2019

The Bollard Pull Joint Industry Project, run by MARIN from November 2015 over three years with 31 industry participants, investigated 16 factors affecting bollard pull through model tests, computational fluid dynamics and full-scale validation on nine tugs and five types of load cell. The output is a 31-page document titled International Standard For Bollard Pull Trials, dated July 2019, released free to the public.

It carries no ISO, IEC or ASTM designation and no document number. It is a joint industry project deliverable, not a standards-body standard, and calling it an international standard describes its ambition rather than its status.

Its conditions are expressed as ratios rather than absolutes. Water depth not less than 4 times the propeller immersion depth, maintained over a radius of twice the ship’s length, where immersion depth runs from the water surface to the centre of the propulsion unit and, for a Voith Schneider unit, to the mid-height of the blades. At least 50 propeller diameters between the quay and the centre of the propeller nearest the shore, to keep recirculation out of the reading. No minimum towline length at all: the standard requires a torsion-free line so the load cell reads in direct tension, and requires the length to be recorded.

Environmental limits are looser than the circular’s in wind and tighter in current. Wind not more than 10 m/s, current below 0.5 knots from the bow or sides and below 0.3 knots from astern, significant wave height not more than 0.5 m, and a recommendation of below 45 degrees C air and 32 degrees C water so the engines are not derating. With a side current the vessel is allowed to drift with the water rather than being steered against it, and position must not be fixed by cables or other vessels.

The trial runs at least four power settings between 25 and 100 percent, with 100, 85, 60 and 40 percent recommended, which builds a propeller efficiency curve for later part-load re-evaluation. Stated measurement uncertainty is plus or minus 3 percent of the measured towline force, and the certificate is valid for five years.

Bureau Veritas has adopted it. NR467, Rules for the Classification of Steel Ships, Part E, Chapter 1, Appendix 1, edition January 2026, reproduces the procedure clause for clause: the same 5-minute steady state in a 15-minute window, the same depth and quay-clearance ratios, the same current, wave, wind and temperature limits. BV also created an additional service feature, standardized design bollard pull, mandatory on the service notations tug, salvage tug and escort tug, and fixed an acceptance band requiring the measured figure to be no more than 3 percent below and no more than 1 percent above the design bollard pull.

No other class adoption is established. The honest statement is that one IACS member has ruled the procedure and that elsewhere the certified figure remains procedure-dependent.

Where the three procedures disagree

Three of those rows are the reason the same hull can carry two different numbers: the depth criterion, the towline criterion and the averaging window. A 5-minute mean inside a 15-minute run is not the same measurement as a 10-minute non-declining hold, and a depth of 4 times propeller immersion is not the same water as 20 m within 100 m.

Why no correction is applied

The MARIN standard records water density, depth, trim and sea state and corrects for none of them. That is deliberate: the procedure controls the conditions instead of correcting for them, which is why its site limits are expressed so tightly.

The practical consequence is that a bollard pull figure presented as depth-corrected or density-corrected was not produced under that standard. It also means the trial is only as good as the site, and a yard without deep water cannot correct its way to a valid certificate.

Power to thrust: what bollard pull can be predicted from

Static thrust is set by delivered power and propeller disc area together, not by installed power alone. That single relation explains the design of every modern tug and the spread in the thrust-per-kilowatt figures the market quotes.

Ideal static thrust, and why disc area is the variable

At zero speed of advance the propeller’s own induced velocity is the only inflow, and momentum theory gives the ideal static thrust of an actuator disc directly.

Thrust scales as delivered power to the two-thirds and as disc area to the one-third. Doubling installed power on the same propeller raises static thrust by 2 to the two-thirds, which is 1.59 times, not by 2. Repowering an existing tug therefore returns much less than the power increase suggests, and a linear kilonewtons-per-kilowatt coefficient holds only across a narrow power band on geometrically similar hulls.

Disc area is the free variable a designer can actually buy. At fixed delivered power, thrust rises as diameter to the two-thirds, which is why tug propeller diameters are pushed to the draught limit and why thruster gearing is deep-reduction. It is also why the marine propeller on a tug looks nothing like the propeller on a ship of the same installed power, and why propeller pitch and construction is optimised for a bollard condition rather than a service speed.

Designers normalise a measured figure against the ideal disc value with a static merit coefficient. A value of 1 is the ideal open disc; an open propeller falls below it through blade drag, finite blade number and hub losses; and a ducted unit can exceed 1 on this reference because the duct contributes thrust the bare-disc model does not account for.

Thrust per installed kilowatt by propulsion type

The following ranges come from delivered vessels with published power and trial figures, not from a rule of thumb.

Bureau Veritas offers a preliminary-design estimate for the same purpose, which is useful precisely because it is a rule with a source rather than a market convention.

Neither the table nor the estimate substitutes for a trial. Bureau Veritas allows the measured figure to fall no more than 3 percent below the design figure before the design bollard pull itself is amended, which is a tight band for a prediction of this kind.

The nozzle: what it adds, and where it stops paying

The standard tug duct is the accelerating MARIN 19A profile, described in full in kort nozzle . It accelerates inflow at the disc and generates a forward-directed lift component on its own aerofoil section, and published static gains over an open propeller of the same diameter and power span 20 to 40 percent.

The gain falls as advance ratio rises, because the duct’s thrust contribution decreases and eventually becomes drag. Break-even against an open propeller is commonly quoted near 10 knots. A full-scale case from the era when the comparison was still being made gives the shape: a 93-foot, 250 hp single-screw towboat gained 37 percent at the bollard and 25 percent at its normal towing speed of about 4.5 knots.

Why towing pull falls to zero at free-running speed

Static bollard pull is the point where advance ratio equals zero on the propeller’s thrust curve. As advance ratio rises, blade section angle of attack falls and the thrust coefficient falls with it, and the useful towing pull is what remains of propeller thrust after the tug’s own hull resistance at that speed is paid for.

Pull therefore reaches zero at the tug’s free-running speed, not at the propeller’s zero-thrust advance ratio. For a ducted unit the decline is steeper than for an open propeller of the same diameter, because the duct’s contribution decays while its drag grows with the square of speed. That curve, not a percentage table, is the right way to think about what a tug has left at 6 or 8 knots, and the open-water characteristics behind it are set out in propeller theory .

Tug types and propulsion arrangements

Tug types differ in where the propulsors sit relative to the tow point, and that geometry decides manoeuvrability, the direction in which full thrust is available, and exposure to girting. The 2008 IS Code encodes the same distinction numerically, which makes it the citable form of an argument usually made informally.

Conventional single and twin screw tugs

Fixed-pitch screws in nozzles with rudders and steering systems , no azimuth capability, and a tow point amidships or slightly aft. The 2008 IS Code assigns them a transverse thrust coefficient of 0.5, the lowest in its table, but they are the configuration the casualty record indicts because they cannot reposition quickly under load.

They are not a legacy type confined to small ports. MAIB report 17/2024 records that the tug lost off Greenock on 24 February 2023 was twin screw conventional, and conventional tugs remain in front-line harbour ship-assist work in developed ports.

Azimuth stern drive tugs

Twin steerable azimuth thrusters under the stern, usually ducted, with the tow point on the foredeck. The arrangement gives full thrust in any direction and the ability to change the direction of pull without changing heading, which is why it dominates modern harbour towage. Named units on the vessels above include the Rolls-Royce US 205S, now Kongsberg following the 2019 acquisition of Rolls-Royce Commercial Marine, and the Schottel SRP-610. The transverse-thrust arrangement differs from the tunnel units covered in bow thruster and stern thruster , and the drive train from the arrangement in marine propulsion shafting and stern tube .

An ASD working over the stern is exposed in a way an ASD working over the bow is not, and the IS Code says so: the transverse thrust coefficient is floored at 0.7 for an ASD towing over the stern and at 0.5 for one towing over the bow.

Tractor tugs: Voith Schneider and forward azimuth

A tractor tug carries its propulsion forward of amidships and tows over the stern. The Voith Schneider Propeller is a vertical-axis cyclorotor: a rotating casing carries blades set perpendicular to the disc, and an internal gear varies each blade’s angle of attack in phase with rotation, producing thrust of any magnitude and direction without a rudder and without rotating the unit. Invention is credited to Ernst Schneider, with production from 1926, and Voith states more than 800 Voith Water Tractors in service in more than 120 harbours.

Conventional Voith units carry four, five or six blades. The electric eVSP is an eight-blade design. Forward-mounted azimuth drives produce the same tow-point geometry by a different means, and the IS Code treats both together, flooring the transverse thrust coefficient at 0.7 for a tractor towing over the bow and at 0.5 for one towing over the stern.

The Rotortug and three-thruster arrangements

The Rotortug carries two thrusters forward of the midship frame and one aft, the third replacing the traditional skeg. It was developed and patented at the end of the 1990s by Ton Kooren of KOTUG in Rotterdam, and the patent is held and marketed by Rotortug BV. Robert Allan Ltd is the exclusive worldwide designer, not the patent holder, and produces the Advanced Rotortug series designated by bollard pull in tonnes and length in metres, so an ART 85-35 is 85 t and 35 m.

The claim for the layout is redundancy and near-full bollard pull in any direction. The first vessel, RT Magic, was built by Padmos and took Ship of the Year in 2000.

The escort hull: sponsons, skeg and freeboard

An escort tug is a hull form before it is a propulsion arrangement, and the discipline as a whole is treated in escort towage . DNV requires the hull to provide adequate hydrodynamic lift and drag forces in indirect towing mode, with attention to the balance between hydrodynamic forces, towline pull and propulsion forces, and to sudden loss of thrust. Robert Allan’s RAstar series pairs a sponsoned hull with a foil-shaped escort skeg, and the sponsons and skeg are the stated source of the escort force gain, with more than 100 RAstar 3200 units delivered or under construction across variants running from 70 t to 125 t of bollard pull. Newer harbour tugs increasingly pair that hull with battery hybrid propulsion .

Freeboard is a stability parameter here rather than a comfort one. DNV requires freeboard arranged to avoid excessive trim at higher heel angles and a bulwark all round the exposed weather deck, and the 2008 IS Code Part B requires a minimum stern freeboard of at least 0.005 times Load Line length in all operating conditions, with freeing port area at least meeting ICLL regulation 24 and no shutters where icing is likely.

Escort towage: direct and indirect mode

Escort towage is the active steering, braking and control of a ship that has lost propulsion or steering, by a tug connected to its stern at speed. Bureau Veritas defines escorting in terms of indirect towing mode with the escorted ship’s ahead speed in a typical range of 6 to 10 knots, and the 2008 IS Code Part B footnote gives the same range.

Direct mode and its ceiling

In direct mode the tug pulls with its propulsors, and the force available is its bollard pull less what the hull’s own resistance consumes at the escort speed. That is a falling curve, and by 6 knots a large part of the certified figure has already gone.

Direct mode remains the mode of choice at low speed and for the final approach, and it is the only mode available to a tug without an escort hull form.

Indirect mode: the hull carries the load

In indirect mode the tug is held at a large drift angle to the flow by its towline and its propulsors, and the hull and skeg generate lift and drag in the same way as a wing at incidence. DNV states the principle as a rule requirement: the forces are to be in equilibrium with minimum use of propulsive force except for forward thrust and balancing transverse forces. The propulsors position the hull; the hull does the work.

That is why an escort tug can exceed its own certified bollard pull, and why the gain is a property of the hull form rather than of the installed power. It is also why sudden loss of thrust is treated as a design case: DNV requires that on loss of propulsion the remaining forces balance so that the resulting turning moment turns the tug toward a safer position with reduced heel.

Steering force and braking force are two numbers

One towline tension produces both at once, resolved by the towline angle to the assisted ship’s centreline.

Steering force acts across the assisted ship’s track and braking force along it. Quoting a single escort force without saying which component it is, or at what speed, states nothing.

What an escort notation certifies

Bureau Veritas completes the escort tug service notation with three stated design maxima: braking force, escort speed and steering force, alongside the standardized design bollard pull that all three tug notations carry. Design maximum escort speed is not to be taken above 10 knots, with 12 knots accepted case by case for high-powered escort tugs whose free-running speed exceeds 15 knots. Verification is by full-scale trial, or model testing, or a computer simulation program accepted by the Society, so an escort figure has to be read together with how it was established. Escort tugs are also required to carry a calibrated inclinometer.

DNV states the same information as a rating number of the form (F_S, t, v), where F_S is the maximum transverse steering pull in tonnes, t is the time to shift from a steering position on one side to the mirrored position on the other, and v is the speed at which the pull is attained. Qualifiers F, N and O record whether the value came from a full-scale measurement test, a numerical calculation confirmed by the Society, or another society’s rating, and a parameter established without DNV involvement is shown as a dash. Trials are run at 8 knots, 10 knots or both, and where both are used the rating number has six parts.

The escort winch is a rule item in its own right. DNV requires a hydraulic load-reducing system to prevent overload from dynamic oscillation, holds that normal escort operation is not to rely on the winch brakes, and requires the winch to pay out before the pull reaches 110 percent of the rated towline force.

Published escort forces against certified bollard pull

Read as ratios, a sponsoned escort hull produces braking force of the order of 1.5 times its bollard pull and steering force of the order of 1.0 to 1.1 times, at 10 knots. The Damen 2811 is the counterexample that keeps the generalisation honest: it is not a sponsoned escort hull, and its gain is far smaller.

Where escort towage is mandated

There is no international escort towage requirement. Every mandate below is a national or regional overlay, and the baseline is that a port or a terminal decides.

United States: 33 CFR Part 168

The rule was made under section 4116(c) of the Oil Pollution Act of 1990, as amended by section 711 of the Coast Guard Authorization Act of 2010, and published at 59 FR 42968 on 19 August 1994, with amendments at 70 FR 55730 in 2005 and 78 FR 50339 in 2013.

It applies to laden tankers of 5,000 gross tons or more carrying a petroleum oil listed as a pollution category I cargo in 46 CFR Table 30.25-1, and it is the operative half of the Oil Pollution Act of 1990 escort regime. All single hull tankers are covered in both listed areas; double hull tankers are covered in Prince William Sound only, which is what the 2013 amendment brought in. At least two escort vessels are required in each area: Prince William Sound and its adjoining waters, and Puget Sound and associated waters east of a line connecting New Dungeness Light with Discovery Island Light, including Haro Strait, Rosario Strait, the Strait of Georgia, Hood Canal and the eastern Strait of Juan de Fuca.

The standard is performance-based. Under section 168.50(b) the escorts, acting singly or jointly, must be capable of towing the tanker at 4 knots in calm conditions and holding it in steady position against a 45-knot headwind; holding it on a steady course against a 35-degree locked rudder at 6 knots; and turning it 90 degrees with a free-swinging rudder at 6 knots within the same advance and transfer the tanker could achieve with hard-over rudder. The tanker must not exceed a speed beyond which the escorts can reasonably be expected to bring it under control within the navigational limits of the waterway.

The rule names no tug type, no propulsion configuration and no power rating. An escort vessel is defined at section 168.05 as any vessel assigned and dedicated to a tanker during the escort transit and fendered and outfitted with towing gear as appropriate for its role in an emergency response to a disabled tanker. A pre-escort conference under section 168.60 is mandatory before the transit begins.

Washington State: Rosario Strait

RCW 88.16.190(2)(a)(ii), inserted by ESHB 1578 in 2019 and effective 1 September 2020, requires escort in Rosario Strait and connected waterways to the east for laden oil tankers between 5,000 and 40,000 deadweight tonnes, and for articulated tug barges and towed vessels or barges carrying oil in bulk internal to the hull above 5,000 DWT. The escort tug or tugs must have aggregate shaft horsepower equal to at least 5 percent of the deadweight tonnage of a 40,000 DWT tanker. Bunkering and refuelling vessels are exempt.

This sits on top of a requirement dating from the mid-1970s for escorts for laden oil tankers over 40,000 DWT east of the New Dungeness line. The Board of Pilotage Commissioners defined the geography by interpretive statement on 17 September 2020, covering the connected channels, bays and anchorages east of Rosario Strait and north of 48 degrees 30.0 minutes north.

European Union: market access rather than mandate

Regulation (EU) 2017/352 has applied to all TEN-T maritime ports since 24 March 2019 and names towage as a port service. It sets minimum requirements for providers, rules for limiting the number of providers, public service obligations, transparency of charges and a complaints procedure. Towage charges reach the ship through the port disbursement account . The regulation governs how towage is contracted and priced, not when a tug is required, and it is the reason towage market structure differs between EU ports and ports operating under an exclusive port concession elsewhere.

The escort standards

ASTM F1878-21 , Standard Guide for Escort Vessel Evaluation and Selection, is the method document. It sets out methodologies for predicting the control forces needed to constrain a ship that has lost propulsion or steering within the navigational limits of a waterway, the control forces an escort vessel can be expected to impose given ship design, transit speed, wind, current and sea state, and how to integrate the two into an escort plan for a particular ship and waterway. It sits in ASTM Committee F25, runs to 21 pages, and replaced F1878-98 with its 2004, 2009 and 2015 reapprovals.

On the shipboard side, ISO 4827:2022 specifies technical requirements and test methods for escorting and pull-back systems on tankers of not less than 20,000 DWT, and was adopted in the United Kingdom as BS ISO 4827:2022 on 27 January 2023.

There is no OCIMF escort tug guideline. OCIMF’s towing publication is the Static Towing Assembly Guidelines information paper of 2020, and its mooring publication is the Mooring Equipment Guidelines 4th edition of 2018, which governs the ship’s own mooring equipment and winches rather than towage.

Tug stability and the 2008 IS Code

Tug stability under towline load is governed by the 2008 IS Code, resolution MSC.267(85), as amended. The criteria that matter are numerical, they are specific to towing and escort, and they sit in the recommendatory half of the Code, which is the fact most often stated incorrectly.

What became mandatory in 2020, and what did not

Resolution MSC.413(97), adopted 25 November 2016, and resolution MSC.443(99), adopted 24 May 2018, amended Part A and are to be read together as one instrument. They entered into force on 1 January 2020.

What Part A gained is scope and definitions only: subparagraphs 1.2.7 to 1.2.9 bringing in anchor handling, towing and escort, and lifting operations, and definitions 2.27 to 2.31. Definition 2.31 defines a ship engaged in escort operation as one specifically engaged in steering, braking and otherwise controlling the assisted ship, where the steering and braking forces are generated by hydrodynamic forces on the hull and appendages and by the thrust of the propulsion units. Part A chapters 2 and 3 gained no tug-specific criteria at all.

All the numerical criteria sit in Part B section 2.8, added by resolution MSC.415(97) and effective from 1 January 2020. Part B is recommendatory. The criteria apply to ships whose keel is laid, or which are at a similar stage of construction, on or after 1 January 2020, and to ships converted for towing after that date, so an existing tug is not swept in. The wider treatment of the Code’s mandatory half is in intact stability , and the criteria that came later through a different route are in second generation intact stability criteria . The loading condition a tug master actually works from is the stability booklet and loading computer , and the flooded case is damage stability .

The self-tripping heeling lever

Self-tripping is the tug heeled by the transverse thrust of its own propulsion and steering systems, opposed by the towline pull.

The transverse thrust coefficient is where the Code encodes tug geometry. It is 0.5 for conventional non-azimuth propulsion, and 0.90/(1 + l/L_LL) for azimuth units installed at a single point along the length, floored at 0.7 for an ASD tug towing over the stern or a tractor tug towing over the bow, and at 0.5 for an ASD tug towing over the bow or a tractor tug towing over the stern. The floors say plainly that an arrangement with the propulsors at the opposite end from the tow point carries a higher transverse thrust into the stability calculation.

The criterion at Part B 2.8.4.2 requires the area between the righting lever curve and the heeling lever curve, from the equilibrium angle to the second intersection or the downflooding angle, to exceed the area between the heeling lever curve and the righting lever curve from zero to the equilibrium angle.

The tow-tripping heeling lever

Tow-tripping is the tug dragged sideways through the water by the tow, which is the girting case expressed as a stability calculation. Part B 2.8.2.2 gives the heeling lever as a function of the lateral projected area of the underwater hull and a lateral velocity fixed at 2.57 m/s, which is 5 knots, with coefficients derived from the towing point position, the freeboard and the breadth. The criterion at 2.8.4.3 requires the first intersection between the righting lever and heeling lever curves to occur at a heel angle less than the downflooding angle.

The fixed 5-knot lateral velocity is worth reading against the casualty record. The port speed limits that investigations find breached are of the same order, and the Code’s own design case assumes a value the operational failures exceed.

The escort criteria and how Part B binds

Part B 2.8.3 puts the escort tug in an equilibrium determined by hydrodynamic forces on hull and appendages, thrust and towline force, with the steering force, braking force, heel angle and heeling lever obtained from full-scale trials, model tests or numerical simulations acceptable to the Administration, evaluated across the escort speed range. The criteria at 2.8.4.4 require area A to be at least 1.25 times area B, area C to be at least 1.40 times area D, and the equilibrium heel angle to be no more than 15 degrees.

Recommendatory does not mean optional. DNV requires vessels engaged in escort operations to comply with Part B paragraphs 2.8.4.4 and 2.8.6.2, assuming heeling levers according to paragraph 2.8.3, as a condition of the Escort tug notation, and flag administrations reproduce Part B verbatim as instructions to their surveyors. The criteria bind through class and flag rather than through SOLAS tacit acceptance, which is a different mechanism with the same practical effect. Part B 2.8.5.3 separately requires a means of quick release of the towline.

Girting and tug capsize

Girting occurs when high athwartships towline forces pull a tug sideways through the water. If the tug cannot manoeuvre out of that position, the deck edge and bulwarks submerge, the heel increases and it capsizes. The Transportation Safety Board of Canada calls the same failure girding; tripping is a colloquialism that appears in no official report.

It is fast. Biter sank in less than ten seconds after capsizing off Greenock on 24 February 2023.

What the investigations found

Five investigated cases carry the subject, and they are consistent enough to read as one finding repeated.

• Flying Phantom and Red Jasmine, River Clyde, 19 December 2007. MAIB report 17/2008, published 30 September 2008. Three of four crew died. The tug reported it had grounded, the pilot instructed it to let go, the line came taut and the tug was pulled over. The report found failings in the harbour authority’s safety regime, operational shortcomings by the tug operator, and the absence of an accepted international industry standard for tug towline emergency release systems. An open engine room access door downflooded the machinery space.
• Domingue and CMA CGM Simba, Tulear, 20 September 2016. MAIB report 16/2017, published 19 July 2017. Two of five crew died. The tug was connected to the ship’s port quarter when the master briefly went ahead without the pilot warning the tug. Contributing factors: a less manoeuvrable tug than the port’s normal one, crew inexperienced in ship assist, no gog rope, no emergency release mechanism on the towing point, and doors and hatches open. No recommendations were made.
• George H Ledcor and barge Evco 55, Fraser River, 13 August 2018. TSB Canada report M18P0230, released 2 October 2019. The tug girded and capsized after being overtaken by the loaded gravel barge it was towing; all four crew were rescued from the overturned hull. The report records that between 2005 and 2018 the TSB received reports of 26 girding situations resulting in 21 capsizings, and found as risks that training for tug masters was inadequate and that the industry relies on shiphandling skill and informal practice to manage the hazard.
• Adonis, Gladstone, 11 June 2011. ATSB investigation MO-2011-005. One of four aboard drowned in the wheelhouse. Neither tug master realised Adonis had entered a capsize scenario when it moved abaft the barge’s port bow before the barge slowed, the barge’s speed was not reduced in time, and the crew could not use the towing hook’s quick release before the capsize. Retrofitted H-bitts aft of the towing hook had degraded manoeuvrability and, together with a towing winch, left the tug unstable towing over the stern.
• Biter and Hebridean Princess, off Greenock, 24 February 2023. MAIB report 17/2024, published 13 November 2024, classified a very serious marine casualty. Both crew died. The twin-screw conventional tug could not reverse direction to work directly astern before its weight came onto the towing bridle, and the gob rope then did not prevent it being towed sideways. Recommendations 2024/157 to 2024/166 followed, including that the operator review its safety management system and risk assessments covering gob rope rigging and safe speed, adopt a recognised training scheme for tug masters, and commission an independent review of marine pilot training.

MAIB issued Safety Bulletin 1/2026 in February 2026 with recommendation S2026/122, recording that the Biter investigation was the eighth investigation involving the capsize of a conventional tug since 1998, a series that has cost nine lives. Ship assist failures are not confined to girting: NTSB report MIR-23-15, issued 11 July 2023, found that the tug George M attempted a bow-to-bow makeup on a containership transiting the Houston Ship Channel at 9.7 knots, against the towing company’s 7-knot limit, disabling a Z-drive against the bulbous bow. These cases are catalogued alongside other investigations in flag state casualty investigation reports .

Speed is the controlling variable

MAIB report 17/2024 quantified it: the assisted vessel’s speed put loads on Biter’s towlines two to five times greater than they would have been at the port’s recommended range of 2 to 3 knots. That is the most useful single number in the subject, because it converts an argument about seamanship into an argument about a controllable parameter with a known multiplier.

The pilot sets that parameter. The tug master does not control the assisted vessel’s speed through the water, which is why the master and pilot exchange and the pilot and tug exchange are treated as safety-critical steps rather than courtesies, and why pilotage operations and towage planning cannot be separated.

Gob ropes, quick release and watertight integrity

A gob rope , also written gog rope, leads the towline down and aft to a point on the tug’s centreline, restraining the angle at which the line can lead and so limiting the transverse moment. The joint British Tugowners Association, UK Maritime Pilots Association and Workboat Association advice of November 2025 is explicit that it must be treated as a gog system rather than a rope, using a dedicated line managed like a towing rope, and it covers towing point management, communication protocols, safe speed guidance, stability awareness and rope inspection.

Two limits recur. The gob rope did not prevent Biter being towed sideways, and the excess speed almost certainly caused it to render. On Biter the HMPE gob line melted and fused where it was wrapped around the samson post.

Quick release fittings are required and repeatedly found unusable in the seconds available. Watertight integrity is what decides survivability once the tug is over: an open engine room door on Flying Phantom, open doors and hatches on Domingue, and an open accommodation hatch on Biter that may have prevented air being trapped in the wheelhouse. Three investigations, one finding.

Towing gear: from bollard pull to breaking load

Bollard pull sizes the gear. The chain runs from the certified figure to the towline minimum breaking load, and from the towline MBL to the winch brake holding capacity, and getting the order wrong is the commonest error in the subject.

Towline minimum breaking load

Two instruments state the same rule in different units, and the rope and wire choices that follow from it are in towline and tow wire selection .

DNV’s version at Pt.5 Ch.10 Sec.11 [3.6.3] sets the minimum breaking force as a utility factor times the design force, with the factor 2.5 up to 200 kN, 2.625 minus the design force divided by 1,600 between 200 and 1,000 kN, and 2.0 at and above 1,000 kN. The design force is the towline pull, or the bollard pull where the towline pull is not defined. DNV’s breakpoints of 200 kN and 1,000 kN are 20.4 and 102 tonnes-force, against the circular’s 40 and 90 tonnes, and both run from a factor of 2.5 or 3.0 on a small tug down to 2.0 on a large one.

MSC/Circ.884 adds the rest of the arrangement. Minimum towline length is 1,800 metres times the ratio of bollard pull to towline breaking load, both in the same unit. Shackles, rings and connecting items carry an ultimate load bearing capacity at least 50 percent above the MBL of the towing arrangement. Fibre rope pennants carry an MBL of not less than twice the towline MBL below 50 tonnes of bollard pull and 1.5 times above 100 tonnes, interpolated between, in grommet construction with hard eyes and normally not connected directly to the apex of the towing bridle. Wire rope terminations are hard eyes except at the drum end, all wire ropes in use have the same lay, and a spare towline meeting all main towline requirements is carried.

Winch brake holding capacity and emergency release

DNV sets the towing winch holding capacity at 80 percent of the towline minimum breaking force with the towline in the first layer, and dimensions the components exposed to towline pull with the brake engaged against that figure. MSC/Circ.884 paragraph 12.3 sets it against the documented MBL of the largest towline to be used, calculated for the outermost working layer, and paragraph 12.4 requires the winch and its supports to withstand the breaking load of the main towing wire without permanent deformation.

Because the towline MBL is itself 2.0 to 3.0 times bollard pull, the holding capacity lands at roughly 1.6 to 2.4 times bollard pull. That is why a rule of thumb expressed directly against bollard pull looks plausible and is nonetheless attributed to the wrong quantity.

Emergency release is specified tightly. DNV requires the system to function within a maximum of three seconds after activation, with controlled pay-out and enough resistance to rotation to avoid uncontrolled unwinding, and requires release from the bridge and every other control stand, functional on power-drive failure, with normal brake operation restored immediately afterwards and no automatic restart of the winch motor. MSC/Circ.884 paragraph 12.5 requires the sequence to work during a blackout and the drum end attachment to form a weak link. IACS Unified Requirement M79, Towing winch emergency release systems, was issued in October 2018 with Rev.1 of February 2020 implemented from 1 July 2021, and applies to new tugs. Towline failure itself, rather than girting, is the other recurring injury mechanism, and is covered in towline failure and snapback . The same distinction between rendering and holding applies to anchor handling winches , where the load case is vertical rather than transverse.

The ship’s side of the connection

The tug’s gear is only half the system. On the assisted ship, SOLAS regulation II-1/3-8 as amended by resolution MSC.474(102), in force 1 January 2024, governs towing and mooring equipment, and the detail is in SOLAS regulation II-1/3-8 on towing and mooring .

IACS Unified Requirement A2 Rev.5 of September 2020 covers shipboard fittings and supporting hull structures for towing and mooring on conventional ships. It sets the minimum design load on the supporting hull structure at 1.25 times the intended maximum towing load, taken as the static bollard pull, as shown on the towing and mooring arrangements plan, and requires the safe towing load marked on the fitting not to exceed 80 percent of that design load. Two limits on A2 matter here: it governs the assisted ship rather than the tug, and it expressly excludes escort towing from its scope, directing the reader instead to local escort requirements. The broader class framework is set out in IACS unified requirements .

Emergency towing is a separate regime again. Resolution MSC.549(108), adopted 23 May 2024, extends SOLAS emergency towing arrangement requirements to ships other than tankers of 20,000 gross tonnage and above, with entry into force on 1 January 2028. It is adopted and not yet in force.

Contracts: what a bollard pull figure is worth

Because the certificate is not statutory, its force is contractual. Ocean towage and harbour towage are contracted on different families of form, and the liability allocation in each is what a bollard pull warranty actually attaches to.

BIMCO TOWCON 2021 and TOWHIRE 2021

TOWCON and TOWHIRE are the two BIMCO ocean towage forms: TOWCON a lump sum contract and TOWHIRE a daily hire form. Both were adopted by the BIMCO Documentary Committee on 25 January 2021 as third editions, replacing the 2008 forms, alongside a new edition of BARGEHIRE.

Knock for knock remains the cornerstone of both, with the Tugowner Group and Hirer Group definitions broadened to take in each party’s other contractors and a revised excluded-losses provision. The 2021 editions replace rider clauses with standard clauses for war risks, piracy, and infectious or contagious diseases, and add anti-corruption and sanctions provisions. TOWCON gained a box for the tugowner to state an estimated average towage speed as the basis for compensation for extra time from slow steaming or deviation, a requirement for daily reports, provision for bunkering the tug mid-voyage, and a single delay rate in place of separate at-sea and in-port rates. Knock for knock as a liability structure is treated in knock for knock indemnity .

Adoption of a BIMCO revision runs slowly. BIMCO reported that TOWCON was used 1,040 times in 2021, of which almost 90 percent were on the 2008 form, so a fixture described as being on TOWCON is not necessarily on the 2021 edition.

UK Standard Conditions for Towage 2024

The UK Standard Conditions for Towage 2024, published in November 2024, replaced the 1986 revision. The British Tugowners Association is the custodian, and convened the review panel in 2023 from the BTA, the International Group of P&I Clubs, senior admiralty lawyers, the UK Chamber of Shipping and a Fellow of the Chartered Institute of Arbitrators. The conditions were introduced in the 1920s, comprehensively reviewed in 1974 and updated in 1986.

Clause 3 is the commercially decisive provision. Whilst towing, or whilst rendering any other service at the hirer’s express or implied request, the master and crew of the tug or tender are deemed to be the servants of the hirer and under the hirer’s control, and the hirer is accordingly vicariously liable for any act or omission by any person so deemed. The definition of Towing at clause 1(b)(i) covers holding, pushing, pulling, moving, escorting or guiding, or assisting or standing by the vessel, so escorting falls inside the conditions.

The 2024 revision deleted clause 4(e), which had preserved liability for death or personal injury caused by negligence, on the basis that the Unfair Contract Terms Act 1977 is now better understood and tested and that 4(e) had caused uncertainty and driven some Commonwealth operators back to the 1974 Conditions. Clause 4(b) gained the word expense alongside loss or damage, and the phrase whether direct or indirect. The start and end points of Towing were revised, which matters because they define when the clause 4 indemnity operates.

A towage contract does not extinguish the tug owner’s right to claim salvage where the service rendered goes beyond the contracted towage in circumstances outside the reasonable contemplation of the agreement. The boundary, and the Lloyd’s Open Form interface, belongs with towage and salvage operations and the Salvage Convention 1989 and SCOPIC , where tugs are tariffed by bollard pull band.

Manning and competence

STCW regulation II/3 and table A-II/3 cover masters and officers in charge of a navigational watch on ships of less than 500 gross tonnage, which is where most harbour tugs sit, and allow an administration to issue a certificate restricted to near-coastal service. STCW contains no tug-specific competence, no ship-assist competence and no towing endorsement, and the national routes are compared in tug master training and certification .

That gap is what two investigations independently identified. MAIB recommended in report 17/2024 that a recognised training scheme for tug masters be adopted, and TSB Canada found in M18P0230 that industry relies on masters’ shiphandling skill and informal work practices because initial and recurrent training is inadequate. National regimes fill the gap unevenly: the United States runs towing as a separate credential track with the Towing Officer Assessment Record and inspects towing vessels under 46 CFR Subchapter M, while the United Kingdom routes tug officers through an under-500 gt near-coastal certificate. Neither regime is checked by port State control , which inspects the assisted ship rather than the tug.

Limitations

The certified bollard pull is a single measurement at zero speed through the water, in deep still water, at one recorded thruster orientation, with a clean hull and a stated displacement and fuel. It is the ceiling of what the tug can pull on a line and not a prediction of what it will deliver alongside a moving ship.

Because three trial procedures are in use and they differ on depth, towline length, weather limits and the averaging window, figures from different certificates are not strictly comparable. A certificate should be read with the trial report, and a contract that warrants a bollard pull should say under which procedure it was established.

The thrust-per-kilowatt ranges here are derived from delivered vessels with published power and trial figures. They are indicative of modern practice and are not a substitute for a trial, and they do not extend to configurations for which no sourced data was available, including conventional twin-screw tugs in nozzles and open-propeller azimuth arrangements. The MARIN standard itself states that it was developed and validated for vessels with multiple propulsors and is not validated for single-propulsor towing vessels, for which the minimum depth is likely to be larger.

Escort force multiples are properties of a hull form, not of a tug class. The 1.5-times braking ratio quoted above comes from sponsoned escort hulls, and the Damen ASD 2811 figures show how much smaller the gain is without that hull form. No escort figure transfers between designs.

The 2008 IS Code criteria in Part B section 2.8 are recommendatory and apply only to ships whose keel was laid on or after 1 January 2020 or which were converted for towing after that date. They bind through class notation and flag surveyor instructions rather than through SOLAS, so the applicable requirement depends on the society and the administration.

Port towage requirements, tug numbers, minimum bollard pull and speed limits are set locally by harbour authorities and terminals, and none of the material here overrides the harbour master, the pilot or the marine warranty surveyor on a particular ship in a particular port.

Related Articles

• Towage and salvage operations : where contracted towage ends and salvage begins.
• Harbour towage operations : ship assist as a port service, from make-fast to let-go.
• Escort towage : direct and indirect mode, certified force matrices and where escort is mandated.
• Girting and tug capsize : the failure mode, the casualty record and the controls.
• Bollard pull certificate : what the certificate states, who issues it and how long it lasts.
• Tug stability : the 2008 IS Code Part B section 2.8 criteria in full.
• Pilotage operations : the pilot and tug master interface, and who controls speed.
• Berthing operations and fender selection : the manoeuvre the tugs are there to serve.
• Mooring forces and station keeping : the environmental loads a towage matrix is sized against.
• SOLAS regulation II-1/3-8 on towing and mooring : the assisted ship’s side of the connection.
• Anchor handling tug supply vessel : the offshore variant of the same trial and the same gear.
• Ports and terminals overview : where towage sits among the marine services.