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The right tugboat size and bollard pull for a port cannot be selected from vessel length or a single horsepower figure. The defensible answer is a local, berth-specific towage risk assessment that matches the assisted ship, tug arrangement, fairway and berth geometry, environmental limits, and the safe working load of port fittings.
Bollard pull is a measured indicator of towing force, not a complete description of a tug’s suitability. Use published or measured bollard pull to compare available capability, then confirm the number, position, propulsion, operating method, and environmental limits in the port’s current rules and operating plan.
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What is the right tug size for a port?
There is no globally correct tug length, engine rating, or bollard-pull value for an arbitrary port. A tug that is adequate for a sheltered berth with generous stopping distance may be unsuitable in a constrained entrance, a shallow turning basin, a strong cross-current, or a berth with low-rated bollards.
The starting point is the port’s operating envelope. A harbour authority should identify the ships and manoeuvres it intends to accept, assess the hazards at each berth, and set minimum tug numbers and capability for defined vessel and environmental categories. The UK Department for Transport’s guide to good practice on port and marine facilities states: “Berthing and un-berthing operations using tugs should be risk assessed by harbour authorities.”
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That assessment can produce a practical local matrix, but the matrix is an output of the analysis, not a substitute for it. It should explain when a vessel needs one or more tugs, where they are positioned, what minimum individual and total bollard pull is acceptable, and which weather, visibility, draft, or equipment conditions require a different plan.
Why is bollard pull not enough to choose a tug?
Bollard pull is the static towing force a tug can exert under a defined test condition. It is useful evidence when comparing tug capability, but it does not describe how effectively the tug can apply that force during a particular ship-assist manoeuvre.
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Two tugs with similar bollard-pull ratings may behave differently because of their propulsion and steering arrangements, hull form, ability to work at the bow or stern, line-handling equipment, escort capability, and response to the available water depth. The assisted vessel may also need lateral control, braking, directional force, or precise positioning rather than maximum straight-line pull.
For each operation, compare the following rather than treating one number as decisive:
| Decision area | Questions to answer | Why it matters |
|---|---|---|
| Total and individual bollard pull | What force is required, and how much must each tug contribute? | A high total can still leave a critical bow or stern task underpowered. |
| Tug number and position | Is assistance needed at the bow, stern, alongside, or in a mixed arrangement? | Force must be available where it controls yaw, speed, and lateral movement. |
| Propulsion and manoeuvring | Can the tug generate and direct force in the planned position and water depth? | Capability depends on controllability as well as static pull. |
| Assisted ship | What are its length, beam, draft, displacement, windage, trim, propulsion, rudder, thrusters, and manoeuvrability? | The same tug plan may be safe for one ship and inadequate for another. |
| Method of assistance | Will the tug work on a line, alongside, or in a mixed mode? | Line angles, contact forces, speed, and release procedures change the demand. |
| Port geometry | What are the fairway width, entrance restrictions, turning area, berth approach, depth, speed limit, and stopping distance? | Confinement and limited water increase the consequences of lost control. |
| Environment | What wind, current, waves, visibility, ice, and forecast limits apply? | Environmental force can dominate the tug requirement. |
| Port fittings | What safe working load is available on bollards, hooks, leads, and other fittings? | The fitting or line may become the limiting component before the tug does. |
How do I define the port’s operating envelope?
Map the locations and conditions in which tug assistance will actually occur. The IMO Guidelines on Safety of Towing Operations, section 5.2, identify port particulars, the assisted ship, environmental conditions, and the method of tug assistance as the main factors affecting total bollard pull.
- Divide the port into manoeuvring areas. Treat the entrance, fairway, turning circle, anchor or waiting area, berth approach, and berth itself as separate operating zones. A tug requirement that is adequate in open water may not provide enough control in a narrow approach.
- Record physical limits. Document channel and berth depth, under-keel-clearance policy, width, bends, obstructions, overhead restrictions, turning diameter, speed limits, and the distance available to stop or recover from an aborted approach.
- Describe the berth. Include fender layout, dolphin and bollard locations, leads, mooring arrangement, approach direction, exposure to wind and current, and any restrictions on where a tug can safely come alongside.
- Identify traffic and interaction. Consider passing ships, one-way or two-way traffic, crossing movements, restricted visibility, nearby structures, and whether a tug can move clear if the manoeuvre is discontinued.
- Set operating boundaries. Define wind, current, wave, visibility, and ice limits for each berth and ship category. State who can suspend or alter the operation when limits are approached.
- Analyse abnormal conditions. Include loss of a tug, a failed thruster or rudder, propulsion defects, steering problems, communication failure, parted towline, fouled line, blackout, and delayed release.
- Check available stopping space. Stopping distance is not an abstract design value: it must be related to actual speed, water depth, ship response, tug position, and the ability to apply force without exceeding line or fitting limits.
- Document assumptions. Record the ship data, weather limit, tug characteristics, line arrangement, personnel, communications, and equipment condition used in the assessment so that the plan can be revised when any assumption changes.
How should the assisted ship be assessed?
Start with the ship that will be handled, not the tug that happens to be available. Create a ship-assist profile for each recurring vessel class and identify the departures from that profile that require a new assessment.
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- Dimensions and displacement: length overall, beam, draft, displacement, freeboard, and the vertical area exposed to wind. A large windage area can create a major lateral demand even when displacement is modest.
- Draft and under-keel clearance: record the actual sailing draft and the minimum clearance at every stage. Shallow water can alter steering response and restrict the tug’s safe operating position.
- Trim and loading: note trim, ballast condition, deck cargo, open or enclosed cargo spaces, and any unusual loading that changes windage or manoeuvrability.
- Propulsion and steering: identify the main propulsion type, rudder arrangement, thrusters, controllability at low speed, stopping performance, and known limitations. A ship with effective thrusters may need a different tug arrangement from one that cannot generate useful transverse force.
- Defects and operational status: establish whether the main engine, steering gear, thrusters, anchors, winches, fairleads, and mooring fittings are fully operational. The UK guidance specifically says local towage guidance should be modified for ship defects and the tug capability available that day.
- Bridge-team information: make sure the master, pilot, and tug masters share the ship’s particulars, intended manoeuvre, communication method, abort points, line plan, and expected forces.
Do not use a vessel’s length alone as a proxy for difficulty. Beam, draft, windage, propulsion response, and the available water and stopping distance can change the tug requirement substantially.
How do I choose the number and position of tugs?
The number of tugs follows from the manoeuvre and the failure consequences, not from a universal length threshold. A plan may require one tug for a controlled movement in a protected berth, two tugs for independent bow and stern control, or additional capability where the approach is constrained or environmental forces are high.
For every planned tug, specify:
- the task: braking, steering, checking a drift, holding off, pushing, pulling, or emergency recovery;
- the position: bow, stern, alongside, or another approved location;
- the operating mode: line, alongside, or a mixed method;
- the direction and timing of force application;
- the minimum individual bollard pull and reserve expected from that tug;
- the communication, line-handling, release, and abort procedure; and
- the consequence and response if that tug becomes unavailable.
A large tug positioned where it cannot safely apply force may be less useful than a smaller, more manoeuvrable tug in the correct location. Conversely, several low-capability tugs may not provide the controllable force or redundancy needed for a heavy ship in a confined approach. Compare both total and individual pull, and verify that the arrangement remains workable if one tug is delayed or has a defect.
Which environmental conditions change the tug requirement?
Wind, current, waves, visibility, and ice should be treated as operating variables, not footnotes. Their effect depends on direction, duration, water depth, the ship’s exposed area, and the stage of the manoeuvre.
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- Wind: assess direction and gusts against the ship’s projected area, particularly for high-sided ships, empty or partially loaded vessels, and ships with deck cargo.
- Current: evaluate set and drift through the entrance, turning area, and berth. A current that is manageable during the approach may create a different demand during a stern swing or final alignment.
- Waves and swell: consider tug line dynamics, tug-ship contact, fender loads, and the ability to keep a line safely made fast.
- Visibility: fog, rain, darkness, and glare affect communication, lookout, line handling, and the margin for an aborted manoeuvre.
- Ice: account for resistance, pressure, damage risk, and whether the tug’s propulsion and hull are suitable for the expected ice condition.
Set separate limits for normal, marginal, and stop-work conditions. A plan should state what happens when the forecast changes, when observed conditions exceed the limit, or when the ship’s actual draft, trim, or equipment condition differs from the submitted information.
How can bollard-pull data and estimates be used responsibly?
Use a tug’s measured bollard pull as evidence of available force, with the test conditions and date understood. Transport Canada’s TP 15491E guidance says measured bollard pull can help assess a tug’s ability to handle a tow when environmental conditions are considered, and that the value should be available to the master and crew.
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Where more specific information is absent, the same guidance records this empirical estimate:
BP = Δ2/3 × V3 / (120 × 60) + 0.06 × B × D1 × K
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|---|---|
| BP | Required bollard pull, in tons |
| Δ | Full displacement of the towed vessel, in tons |
| V | Tow speed, in knots |
| B | Breadth, in metres |
| D1 | Exposed transverse depth above the waterline, including deck cargo |
| K | Factor reflecting potential weather and sea conditions |
Transport Canada gives K ranges of 1.5–2.0 for exposed coastal tows, 0.5–1.5 for sheltered coastal tows, and 0.5 for protected-water tows. Those ranges illustrate how environment changes the estimate; they do not create a universal harbour-assist sizing rule. A port should not transfer a towage estimate directly to berthing without checking the manoeuvre, geometry, ship response, tug method, and fitting loads.
Likewise, do not add individual bollard-pull ratings mechanically and assume the total is available in the desired direction. Line angle, speed, interaction between tugs, water depth, propulsion limits, and the timing of force application all affect usable capability.
How should a port turn the assessment into local rules?
Write rules that a pilot, master, tug master, and harbour control can apply consistently, while preserving a route for professional judgement when conditions fall outside the matrix.
- Define vessel categories. Use dimensions and operating characteristics that matter locally, such as length, beam, draft, displacement, windage, propulsion, and manoeuvrability.
- Define berth and traffic-control zones. A vessel category may have different requirements at different berths or approaches.
- Set minimum arrangements. Specify tug number, bow or stern position, individual and total bollard pull, acceptable propulsion or manoeuvring capability, and the approved assistance method.
- Add environmental modifiers. State when wind, current, waves, visibility, ice, darkness, or forecast uncertainty requires extra tug capability, a different position, a speed restriction, or suspension.
- Check fittings and lines. Ensure the bollards, hooks, fairleads, lines, and fenders can tolerate the planned loads with an appropriate margin and that the line plan is physically possible.
- Set exception and escalation rules. Require a new review for defects, unusual cargo, changed draft or trim, restricted visibility, unavailable tugs, or operations outside the published matrix.
- Review after incidents and changes. Update the guidance when the berth, channel, fleet, tug fleet, traffic pattern, or environmental information changes.
What does a real local tug matrix look like?
Local rules can vary sharply even between ports handling similar ships. The Vancouver Fraser Port Authority’s January 2024 Port Information Guide, for example, includes a TCZ-3 matrix that varies tug number and bollard-pull requirements with vessel length, beam, and draft.
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In one displayed category, ships under 200 m length overall and under 35 m beam with draft under 8 m have a 20-tonne bow and 30-tonne stern requirement. Adjacent draft bands and larger vessel categories specify different combinations. These are Vancouver requirements for the named traffic-control zone and the dated guide, not global minimums. Anyone relying on the matrix should confirm the live revision, geographic scope, definitions, exemptions, and any subsequent port notice before using a threshold operationally.
This example demonstrates why a port should publish its own categories rather than copy a number from another jurisdiction. The underlying assessment must explain the local waterway, berth, traffic, environmental limits, ship population, and tug fleet behind each requirement.
How do port fittings and towlines limit the plan?
The tug may be capable of producing more force than the ship, berth, or line system can safely accept. Check the anticipated safe working load of bollards and other fittings, the rated strength and condition of towlines, fairleads, hooks, winches, and tug push knees or fenders.
Review load direction as well as magnitude. A fitting may have different limitations for a direct pull, a side lead, a high line angle, or a dynamic load caused by waves and surge. Confirm that the planned tug position does not create an unsafe lead, trap personnel, damage the ship’s shell plating, or prevent rapid release.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsInclude a clear stop-and-release protocol. If a line parts, a bollard moves, a winch slips, or a fitting shows damage, the bridge team and tug masters need an agreed response that does not depend on improvisation in a confined area.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should a towage risk assessment be maintained?
A towage plan is a controlled operating document, not a one-time calculation. Assign responsibility for reviewing it and identify the information that triggers an immediate update.
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- Before each operation: confirm vessel particulars, draft, trim, defects, tug availability, measured or current capability data, berth status, forecast, observed conditions, traffic, and communications.
- During the briefing: agree the sequence, tug positions, line method, speeds, headings, abort points, stopping options, and signals for emergency release.
- At the decision point: compare actual conditions with the approved limits. If assumptions are no longer valid, pause and obtain a revised plan rather than silently proceeding.
- After an abnormal event: preserve the facts, review forces and decisions, and determine whether a matrix, berth restriction, equipment inspection, or crew briefing needs to change.
- At scheduled review: reassess when channel dimensions, berth fittings, tug fleet, ship mix, traffic rules, survey data, or regulatory guidance changes.
The assessment should be prepared and accepted by competent port, pilotage, towage, and marine-engineering stakeholders under the applicable jurisdiction. Exact certificates, minimums, manning, and operating limits are local regulatory questions and cannot be inferred from an international guideline alone.
What are the most common tug-selection mistakes?
- Choosing by ship length alone. Correct it by adding beam, draft, windage, displacement, propulsion, manoeuvrability, and the actual berth constraints.
- Treating bollard pull as horsepower. Bollard pull is a force measurement; it does not reveal how a tug can direct force during the manoeuvre.
- Adding tug ratings without checking position. A total number is meaningless if the required bow or stern task cannot be performed.
- Copying another port’s table. Use external matrices as examples only and build categories from the local risk assessment.
- Ignoring safe working loads. Verify the ship and berth fittings before specifying a stronger tug or higher line load.
- Using a calm-weather plan in marginal weather. Put explicit environmental modifiers and stop-work thresholds in the local rules.
- Assuming the submitted ship data is current. Reconfirm draft, trim, defects, thruster status, and cargo or deck configuration before the manoeuvre.
- Failing to plan for loss of capability. State what happens if a tug, line, engine, steering system, or communication link fails.
What should be on a port’s tug-selection checklist?
- Berth, fairway, entrance, turning-area, depth, speed, and stopping-distance data recorded.
- Ship length, beam, draft, displacement, under-keel clearance, trim, windage, propulsion, rudder, thrusters, and manoeuvrability confirmed.
- Wind, current, waves, visibility, ice, traffic, and forecast limits defined.
- Tug number, bow/stern or alongside position, assistance method, propulsion capability, individual pull, and total pull specified.
- Port bollards, hooks, fairleads, lines, winches, fenders, and their safe working loads checked.
- Defects, equipment outages, unavailable tugs, abnormal cargo, and changed ship data covered by exception rules.
- Communications, signals, line-handling responsibilities, abort points, emergency release, and recovery actions agreed.
- Current local rules, notices, certificates, and matrix revision verified.
- Competent harbour, pilotage, towage, and engineering stakeholders identified as approvers.
The result should be a documented, reviewable decision: not “this ship needs a 50-tonne tug,” but “for this vessel category, at this berth, in these conditions, this tug arrangement provides the required controllability and force without exceeding the limits of the ship, lines, fittings, or waterway.”
How were the selection criteria developed?
The criteria above follow the factors identified in IMO towing guidance, UK port-facility guidance, Transport Canada’s tug guidance, and the port-specific example published by the Vancouver Fraser Port Authority. A 2024 paper in the Journal of Marine Science and Engineering also describes an estimation methodology that combines ship and tug particulars, berth parameters, wind and current, waves, towline parameters, port depth, and towing schemes when estimating bollard pull and tug numbers. That methodology supports multi-factor analysis; it is not presented here as a universal or regulator-approved formula.
Sources and scope
This guide addresses port planning where the jurisdiction and port have not been specified. It does not prescribe a universal tug size, minimum number, accepted certificate, or operating limit. The current local port rules and a qualified, berth-specific towage risk assessment remain controlling.
Frequently Asked Questions
Can a tug with higher bollard pull always replace two smaller tugs?
No. A single tug may not provide the independent bow and stern control, redundancy, or positioning needed for the manoeuvre. The replacement must be assessed against force direction, failure consequences, berth geometry, line arrangements, and local rules—not just the sum of rated pull.
What information should a ship agent send before requesting tug assistance?
Provide current length, beam, draft, displacement or loading condition, trim, windage or deck cargo, propulsion and thruster status, steering defects, berth and arrival details, and the intended manoeuvre. Include any restrictions on mooring fittings or lines and report changes before the pilotage briefing.
How often should a port’s tug matrix be reviewed?
Review it on a defined schedule and whenever a berth, channel, tug fleet, traffic pattern, ship mix, survey, regulation, or incident changes. A single operation outside the matrix should trigger an immediate check of whether the assumptions remain valid.
Does the Transport Canada bollard-pull equation size harbour-assist tugs?
It is an empirical estimate recorded for situations where more specific information is absent, with a weather-and-sea-state factor. It should not be treated as a stand-alone harbour-assist sizing rule because berthing geometry, tug method, ship manoeuvrability, local fittings, and stopping requirements may dominate.
