How bollard pull determines the right tugboat for towing work

A towing plan can look adequate on paper until the vessel reaches open water, the tow begins to yaw, wind rises across a broad deck area, and the towing speed falls below expectation. At that point, the question is no longer whether a tugboat is available. The real question is whether it can maintain enough controlled pulling force to keep the operation safe.

This problem often appears during early equipment selection. A proposed tug may have suitable dimensions, engines that appear powerful, and a familiar nameplate rating. Yet those details alone do not show whether it can start a loaded barge moving, hold a disabled vessel against current, or control a tow through restricted water. Bollard pull provides a practical starting point because it measures the pulling force a tug can deliver under defined static conditions. Used correctly, it helps turn a vague capacity discussion into a more defensible towing decision.

The capacity figure that can be misunderstood

Bollard pull is the steady force produced by a tug when it is secured to a fixed bollard or equivalent restraint, usually at full engine output under specified test conditions. It is commonly expressed in tonnes or kilonewtons. The value indicates the vessel’s maximum static pulling capability at the point of measurement.

That definition matters because towing work is not static. A tug working on a towline is affected by propeller loading, water depth, vessel speed, sea state, towline angle, wind, current, hull resistance, and the behavior of the object being towed. The bollard-pull figure is therefore not a promise that the same force will be available throughout every operation. It is a baseline for assessing whether the tug has sufficient reserve before the real variables begin to reduce effective performance.

A common procurement mistake is to treat the highest stated bollard pull as the sole selection criterion. Another is to choose a tug based only on engine power. Two vessels with similar installed power may perform differently because of propulsion type, propeller diameter, hull form, draft, gearbox arrangement, and the condition in which the power can be continuously delivered. A higher rating can also be irrelevant if the towing gear, deck layout, or operating limits do not support the planned job.

Start with the tow, not the tug

Before comparing vessel specifications, define the work that the tug must actually perform. “Towing a barge” is not enough detail for a reliable selection. A loaded construction barge, a dredger under relocation, a floating pipeline section, and a disabled vessel may all require very different pulling force even when their displacement appears similar.

Begin by recording the tow’s physical and operational characteristics:

• Dimensions, draft, displacement, and hull condition of the towed unit.
• Whether the tow is loaded, partly loaded, light, submerged, or likely to change condition during the voyage.
• Expected towing speed and whether the job involves starting from rest, continuous transit, holding position, or maneuvering in confined water.
• Route conditions, including exposed passages, bends, bridges, shallow areas, currents, traffic, and available shelter.
• Projected windage area, especially for deck cargo, cranes, accommodation blocks, or tall equipment.
• Connection method, towline length, chafing protection, emergency release arrangements, and required deck machinery.

This information reveals the resistance the tug must overcome. Hydrodynamic resistance generally rises as towing speed increases. Wind resistance can become dominant for a light barge with a large exposed profile. Current may either assist or oppose the tow, but it can also create lateral loads that are more difficult to control than a simple head-on force. In shallow water, reduced under-keel clearance can change resistance and maneuvering behavior substantially.

The purpose is not to force every project into a single formula. It is to prevent an apparent match between a tug’s advertised bollard pull and a towing job whose real resistance has never been properly described.

Separate the forces that occur during the job

A useful assessment distinguishes between several operating moments rather than looking for one universal force requirement.

Breaking the tow away

Starting a grounded-feeling, heavily laden, or long-idle floating unit can require a short period of high pull. Mooring friction, mud suction, shallow-water effects, or resistance from multiple connected pontoons may make the initial movement more demanding than the later transit. If the operation depends on breaking a tow free from a berth, beaching area, or dredging location, this condition should be discussed separately from normal towing speed.

Maintaining passage speed

Once moving, the required force changes. The tug must overcome hull drag and environmental resistance while retaining enough control authority for safe navigation. A tug that can start the tow may still struggle to maintain the required speed in adverse current or wind. Conversely, specifying a very high bollard-pull tug for a long, low-resistance transit can add unnecessary fuel use, crewing complexity, and charter cost if the route is protected and the speed requirement is modest.

Controlling the tow rather than merely pulling it

Control is often the deciding factor in narrow channels, harbor approaches, or areas near fixed structures. The tug may need to alter the tow’s heading, check its forward movement, or keep it clear of channel edges. In these conditions, thrust direction, maneuverability, towing point location, visibility from the wheelhouse, and deck crew access can matter as much as the maximum static pull.

An azimuthing propulsion arrangement can provide strong directional control, while conventional propulsion may suit other operating patterns. Neither configuration is automatically correct. The practical question is whether the propulsion and towing arrangement match the route, tow geometry, and level of close-quarters handling required.

Use bollard pull as a margin, not as a target

After estimating towing resistance, compare it with the tug’s verified bollard pull and then allow for operational margin. The margin is not a decorative percentage added at the end of a spreadsheet. It represents uncertainty and changing conditions: fouling on the towed hull, stronger-than-forecast wind, variable current, reduced engine output in high ambient temperatures, line-angle losses, or a need to maneuver rather than pull directly ahead.

The appropriate reserve depends on the consequences of losing control and on how predictable the operating environment is. A short movement inside sheltered water may permit a more tightly matched selection than an exposed coastal tow with changing weather and limited refuge. Projects involving constrained clearances or high-value floating equipment usually justify a more conservative approach, because a loss of towing control can quickly become an operational interruption, damage event, or recovery problem.

It is also important to ask whether the published bollard pull is continuous or related to a limited power condition. Some specifications distinguish between maximum and sustained output. For a long tow, continuous performance is usually more relevant than a peak figure that cannot be maintained through the intended duty cycle. Request the test basis and clarify the engine rating used during the measurement.

Questions that expose weak comparisons

When several tugboats appear similar, the quality of the supporting information often separates a dependable choice from a risky one. Rather than comparing only a headline force value, ask the supplier or operator to explain how that value was established and how the vessel is equipped for the proposed work.

These questions are especially important when a tug will work with dredging equipment or other heavy floating plant. The tow may include components with unusual shapes, variable draft, exposed pipework, or equipment that creates uneven wind loading. In such cases, a general-purpose towing estimate should be reviewed against the actual configuration before a vessel is committed.

Do not overlook towing gear and line behavior

A tugboat with adequate propulsion can still be unsuitable if its towing equipment is mismatched to the task. Towline strength, elasticity, length, fairlead arrangement, winch response, and emergency release capability all affect safety. A line that is too short for the sea condition can transmit shock loads more directly. A poor lead angle can create side loading or complicate recovery. Insufficient brake capacity can prevent the crew from controlling a surge load when the tow starts to overrun.

For this reason, bollard pull should be considered alongside the safe working limits of the towing system. The tug’s pull is only useful when it can be transferred through the towing arrangement in a controlled manner. Review the intended towline configuration early, including any bridles, pennants, shackles, and connection points on the tow. Compatibility at the interface is often where an otherwise sound plan develops avoidable risk.

A practical way to make the selection

First, write a short operating description that states the tow condition, route, intended speed, expected weather window, and critical maneuvering points. This should be specific enough that a marine operator can identify where the highest resistance or control demand will occur.

Next, estimate the required towing force for normal transit and identify any separate peak-demand events, such as initial breakaway, cross-current maneuvering, or holding the tow while another vessel passes. Where the job is complex or the consequences of failure are significant, involve qualified naval architectural or marine operational support rather than relying on a generic rule of thumb.

Then compare candidate tugs against the required force with an appropriate operating reserve. Eliminate vessels whose bollard pull is only marginal under ideal static conditions. For the remaining options, examine propulsion, winch capacity, deck arrangement, endurance, crew familiarity, and maintenance readiness. A slightly lower-rated vessel with a well-suited towing system and better maneuverability may be the more practical choice for a controlled harbor movement, while an exposed long-distance tow may justify greater static pull and endurance.

Finally, confirm the plan before mobilization. Review towing certificates and vessel records where applicable, inspect the available towing gear, verify communication arrangements, and agree on weather limits and contingency actions. If the tow condition changes after planning—for example, additional deck cargo is added or the route changes—revisit the force assessment rather than assuming the original tug selection remains valid.

When a higher bollard pull is not automatically better

More pull provides reserve, but it can also mean a larger vessel, higher operating cost, deeper draft, or less convenient access to shallow sites. A high-capacity tug may not be the best fit for a narrow inland route, a small work basin, or a project where frequent close handling is more important than sustained straight-line pull.

The goal is not to procure the largest available tugboat. It is to select a vessel whose verified pulling force, control characteristics, equipment condition, and operating profile align with the actual towing duty. Bollard pull gives the assessment a firm technical anchor, but the final decision should reflect the whole towing system: tug, tow, route, crew, connection gear, and environmental exposure.

When those elements are reviewed together, the selection process becomes less dependent on headline specifications and more focused on whether the vessel can perform the work with manageable operational risk.