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For most confined-water ship-assist jobs, an Azimuth Stern Drive (ASD) tug is the more versatile choice. Its two independently controlled, 360-degree stern thrusters can direct thrust forward, astern, sideways or around the tug’s axis, which is valuable during berthing, unberthing and push-pull work. A conventional twin-screw tug can be the better fit for straight-line barge towing, utility work and some higher-sea operations where predictable pull and robust towing behavior matter more than rapid thrust-vector changes.
There is no universal winner. The right tug depends on the ship’s size and handling characteristics, berth geometry, depth, tide, wind, weather, required speed, escort forces, tow-point arrangement, fendering and crew procedures. Escorting a tanker or other large ship is a separate design decision from low-speed harbor maneuvering.
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What the two tug configurations are
Azimuth Stern Drive (ASD)
An ASD tug has two independent azimuth propulsion units near the stern. Each unit can rotate through 360 degrees, so thrust direction and magnitude can be changed independently. In normal ship-assist work, the tug approaches and tows over its bow, then pushes with the bow against the assisted ship. It can also work bow-to-bow as a “reverse tractor” when the operation and procedures are designed for it.
The same layout that makes an ASD agile also creates a limitation: an ASD is directionally unstable when operated stern-first. Stern-first running therefore requires a deliberate operating method, suitable speed and clear limits rather than an assumption that the tug behaves like a conventional twin-screw vessel in reverse.
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Conventional twin-screw tug
A conventional tug normally has two engines, each driving a fixed propeller with a rudder aft. The arrangement produces dependable ahead pull and good straight-line towing characteristics. The rudders provide directional control, but they cannot vector propeller thrust through a full circle while the tug is nearly stationary.
Conventional twin-screw tugs are no longer the primary harbor-tug configuration in every port, but they remain common for barge towage and utility duties. A 2023 Maritime New Zealand emergency-towing and salvage study describes the type as providing good straight-line pulling, up to around 30 tonnes of bollard pull for the tug type it examined, and retaining a role in higher-sea conditions. That figure is not a universal limit for every conventional tug.
Which tug is better for each ship-assist task?
Berthing and unberthing in confined water
ASD usually has the advantage. A pilot can ask for a quick change from pushing to pulling, alter the thrust angle without first swinging the hull, or combine opposing unit angles to move the tug laterally or pivot it. Those abilities reduce the time and water space needed to line up with a berth, counter windage and correct a developing approach.
A conventional tug can still berth ships safely when the master, pilot and line handlers understand its turning circle, rudder response and stopping distance. It generally needs more room and more advance planning for a comparable change in force direction, especially at very low speed.
Push-pull work alongside
ASD is generally the stronger all-round configuration. A bow winch and independently controlled azimuth units support direct push-pull modes: the tug can hold the bow against the ship, pull on a line, then reposition without waiting for a conventional propeller-and-rudder combination to build speed. ASD units can also operate in opposing directions to help hold a controlled position while the ship’s engines and thrusters are used.
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Bow-to-bow ASD work, sometimes described as a reverse-tractor mode, can be effective when the tug’s winch, fairlead, fenders, line angles and crew training are designed for it. The maneuver must be covered by a risk assessment because line loads, snap-back zones, contact points and the tug’s directional behavior change with the towing direction.
Straight-line barge and utility towing
A conventional twin-screw tug can fit this mission very well. When the principal task is a long, steady pull on a barge or workboat, a conventional tug’s propulsion and rudder arrangement offers familiar handling and efficient use of its bollard pull. Its operating profile may also suit owners who need general towing rather than rapid ship-assist maneuvers.
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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 problemsAn ASD can tow barges, but its purchase, maintenance and crew-training requirements may not deliver a practical benefit if most jobs involve only straight-ahead towing in open water. The decision should compare the actual duty cycle, fuel use, deck machinery, draft and maintenance support rather than relying on the word “ASD” alone.
Escort of tankers and other ships
Escort is mission-specific; neither configuration wins automatically. At escort speed, the decisive question is how much steering and braking force the tug can generate safely through direct and indirect modes. Hull form, skeg area, tow-point location, stability, winch response, towline geometry and speed envelope can matter more than static bollard pull.
Research by MARIN’s SAFETUG project examined representative ASD and Voith Water Tractor designs in waves during berthing, direct escort and indirect escort. It considered propulsion efficiency, direct and indirect assistance, pushing, dynamic towline forces, winch response, roll, yaw and downtime caused by poor operability. The practical lesson is to specify escort performance in terms of steering force, braking force, stability and safe towline behavior.
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All major tug types can perform active and passive escorting. Large skegs on ASD and Voith designs can generate substantial towing and directional forces in indirect mode, but the resulting loads and stability margins must be demonstrated for the actual tug, ship, speed and sea state. A conventional tug can escort a tanker when its hull, tow point, stability, winch and operating procedures are suitable; the propulsion label alone does not establish that suitability.
Mission-fit comparison
| Criterion | ASD tug | Conventional twin-screw tug |
|---|---|---|
| Low-speed vector control | Two independent 360-degree units provide forward, astern, lateral and pivoting thrust. | Rudders steer propeller wash; changing force direction normally requires hull movement and room. |
| Confined-water berthing | Usually favored for precise push, pull, crab and pivot maneuvers. | Capable with experienced crews, but generally less immediate at very low speed. |
| Alongside push-pull work | Bow winch and azimuth control support rapid transitions between pushing and pulling. | Works effectively, but force-vector changes are less direct. |
| Straight-line barge towing | Effective, although its extra maneuvering capability may not be needed. | Strong traditional fit for steady towing and utility work. |
| Higher-sea general towing | Depends on hull, stability, propulsion protection and operating limits. | The 2023 Maritime New Zealand study records a continuing role in higher-sea conditions. |
| Indirect escort | Can generate large skeg-assisted steering and braking forces when designed and operated for escort. | May be suitable, but must prove force, stability and towline performance for the mission. |
| Stern-first operation | Directionally unstable when operating stern-first; requires specific procedures. | Reverse behavior is familiar but still subject to propeller, rudder and visibility limits. |
| Best starting point | Ports emphasizing ship handling, tight approaches and frequent push-pull jobs. | Operators emphasizing barge towage, utility work and dependable straight pull. |
Why bollard pull alone does not settle the choice
Bollard pull is the static towing force a tug can produce under a defined test condition. It is useful for checking whether a tug has enough reserve, but it does not describe how that force can be applied to a moving ship. A tug with high static pull may still be a poor choice if it cannot generate the required lateral force, braking force or steering moment at the specified speed.
The UK Maritime and Coastguard Agency says the correct choice of tug is influenced by “bollard pull, propulsion type, manoeuvrability, size of vessel” and the towage operation. Singapore’s Maritime and Port Authority likewise bases assignment on vessel size and shiphandling characteristics, berth and approach conditions, tide, wind and weather.
Singapore’s current harbor-tug framework, accessed in 2026, groups bollard pull as follows:
| Singapore category | Bollard pull |
|---|---|
| Small | 10–16 tonnes |
| Medium | 17–24 tonnes |
| Big | 25–45 tonnes |
| Extra-big | Above 45 tonnes |
These bands are a local assignment framework, not a universal engineering standard. A practical selection must convert the ship’s windage, displacement, draft, speed, turning behavior and environmental limits into required line, steering and braking forces. The calculation should also include interaction effects between tug and ship, available water depth, current, under-keel clearance, berth shape and the number and location of assisting tugs.
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Environmental and operational factors to check
Berth and approach geometry
Map the turning basin, channel width, obstructions, fender faces, dolphins, leads and safe escape routes. An ASD’s lateral thrust is most valuable when the geometry leaves little room to gather speed. A conventional tug may be entirely adequate where the approach is long, straight and unobstructed.
Windage, tide and current
Container ships, car carriers and high-sided tankers can experience large wind forces even when their displacement is modest relative to a tug’s pull. Record wind direction and gust limits, tidal stream, river current and the interaction between the moving ship and the bank or berth. The tug’s required force is a vector that changes as the ship’s heading and speed change.
Depth and underwater clearance
Draft, squat, siltation and charted depth affect propeller and thruster performance. Confirm the tug’s operating draft and the depth limits for the intended berth, channel and escort route. A design that performs well in deep water may lose efficiency or maneuvering margin in a shallow approach.
Speed envelope and sea state
Low-speed berthing, coastal towing and high-speed escort impose different loads. Establish the maximum assist speed, significant wave conditions, gusts, visibility limits and allowable downtime. For escort work, check roll, yaw, dynamic towline loads and winch response in the expected waves rather than extrapolating from calm-water bollard pull.
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Fendering and contact loads
Verify the point-load limits of the tug’s fenders and the ship’s shell plating, beltings and designated contact areas. The line arrangement must keep loads within the rated capacity of hooks, bitts, fairleads, winches and shackles. A highly maneuverable tug can apply force quickly; the ship-assist plan must define how that force is limited and communicated.
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Safety controls that apply to either configuration
The UK regulator’s ship-assist syllabus treats the following as core competencies rather than optional features:
- Recognizing tug-ship interaction forces, including suction, cushion and pressure changes near a moving hull or bank.
- Respecting point-load and fendering limits at every intended contact location.
- Planning operations in restricted visibility and defining radar, communications and abort procedures.
- Preparing for loss of propulsion, steering, winch, communications or other critical systems.
- Using bow-to-bow ASD techniques only with suitable equipment, line plans and trained personnel.
- Understanding direct and indirect towing, escort steering and escort braking forces.
- Managing towing-point hazards, snap-back zones and line-of-fire exposure.
- Avoiding girting, in which towline forces can rotate or capsize a tug when the line leads abeam or outside a safe sector.
Briefings should identify who controls speed, heading, line tension and emergency release. The pilot, tug master and deck crew need a shared vocabulary for “stop,” “slack,” “hold,” “let go” and abort actions. Procedures should be practiced for a failed azimuth unit, rudder or engine, a fouled line, a sudden wind shift and a loss of communication.
How to decide what bollard pull and configuration you need
- Define the mission. Separate berthing, unberthing, turning, emergency braking, barge towing and escort. Record how often each occurs and whether one tug must cover all of them.
- Describe the assisted ships. List displacement, length, beam, draft, windage, propulsion arrangement, thruster availability, minimum maneuvering speed and known handling limitations.
- Describe the waterway. Document berth and approach geometry, depth, current, tide, wind statistics, wave climate, traffic and restricted-visibility requirements.
- Set force requirements. Obtain naval-architect or towage-engineering calculations for longitudinal pull, lateral force, braking force and steering moment. Include interaction, line angles, fender loads and environmental margins.
- Choose the operating mode. Decide whether the tug will work bow-to-ship, stern-to-ship, on a line, in direct escort or in indirect escort. Confirm that the hull, skeg, winch and fairleads are intended for those modes.
- Validate with trials and procedures. Require documented bollard-pull results, maneuvering data, escort-force evidence where applicable, crew qualifications, maintenance support and emergency drills before assigning the tug to routine work.
Procurement checklist for a port or operator
- Specify the ship classes, displacement range, windage and thruster arrangements to be assisted.
- State the berth, channel, depth, current, wind, wave and visibility limits.
- Set minimum ahead, astern, lateral, steering and braking forces, not just a single bollard-pull number.
- Identify required operating modes: alongside push, line towing, bow-to-bow ASD work, direct escort and indirect escort.
- Check skeg dimensions, stability criteria, tow-point geometry, winch type, release speed and dynamic line-load limits.
- Verify fender type, energy absorption, allowable point loads and compatibility with the ships’ contact areas.
- Review propulsion redundancy, steering controls, emergency stops, fire protection, communications and blackout recovery.
- Require crew training in interaction, girting avoidance, restricted visibility, escort forces and loss-of-system procedures.
- Confirm planned maintenance, spare-parts access, dry-docking capability and local technical support.
- Use sea trials or simulator validation to confirm the tug’s behavior in the actual operating envelope.
Practical recommendation
Choose an ASD as the default starting point when the core work is confined-water ship assist: frequent berthing and unberthing, rapid push-pull changes, lateral positioning, pivoting or tight approaches. Its independent azimuth units give the bridge team more ways to apply force precisely.
Choose a conventional twin-screw tug when the work is dominated by straight-line barge or utility towing, when higher-sea towing robustness is the priority, or when the port’s waterway gives the tug ample room to maneuver. Its simpler force pattern can be an advantage when the mission does not require constant vector changes.
For escort, make neither choice from bollard pull or propulsion type alone. Specify steering and braking forces, speed, stability, skeg and tow-point geometry, winch response, dynamic towline loads and crew procedures for the exact ship and route. The best ship-assist tug is the one whose complete design and operating system match the port’s real missions.
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