Beyond Bollard Pull: Hydrodynamics in the Spotlight at ITS 2026

The maritime industry is increasingly recognising the value of active-escort towage as a critical risk mitigation tool when a ship suffers engine or rudder failure in confined or environmentally sensitive waters. However, despite this growing awareness, the operational understanding of escort towage often remains
overly simplistic.

At ITS 2026 in Gothenburg, Sweden, SeaWays Managing Director Captain Arie Nygh presented his paper “Beyond Bollard Pull – A Whole‑System View of Escort Tug Capability”. In this article, we summarise one of the key ideas behind that paper and why the industry needs to move past bollard pull as the primary measure of escort tug performance.

For decades, the first question about an escort tug has been, “How many tonnes of bollard pull?” From SeaWays’ perspective, this is no longer an adequate way to judge escort capability.

In real escort operations, the tug rarely pulls straight astern. Instead, it often works at significant angles to the ship’s heading. At higher towline angles, the effective force on the ship is multiplied. Under certain conditions, a tug with a static bollard pull of around 80 tonnes can impose well over 200 tonnes of load into the towline and onto the ship’s fittings. Many vessels were never designed with these levels of dynamic load in mind.

Arie emphasises that high‑bollard‑pull tugs are not inherently inappropriate. The key is understanding the actual forces being generated in escort modes and ensuring that every part of the system – especially the ship’s fairleads and bitts – can safely withstand them.

SeaWays differentiates strongly between passive and active escort towage.

In passive escort, a tug runs in company with the ship but is not tethered. It can respond to an emergency, but it does not continuously apply steering or braking forces.

In active escort, the tug is connected – most commonly at the centre lead aft – and intentionally generates forces into the ship’s hull through the towline. Depending on the ship’s speed through the water, the tug may be:

• Providing indirect steering assistance at higher speeds
• Using powered indirect steering at intermediate speeds
• Offering direct steering or direct braking at lower speeds
• Performing transverse arrest where substantial lateral stopping forces are required

Each mode produces a different pattern of forces on the tug and the ship. SeaWays stresses that escort plans and procedures must recognise these differences and respect the safe operating envelope of both tug and ship.

A central theme of the paper is the role of hydrodynamic lift in modern escort tug performance. Escort‑grade tugs are typically fitted with keels, skegs or fins that act as vertical foils in the water. Once ship speed through the water climbs above about 6 knots, these foils begin to generate lift as the tug adopts a controlled drift angle. That lift is transmitted into the towline as steering or braking force.

In practice, this means a well‑designed escort tug can generate towline forces that equal or exceed its static bollard pull while running at only moderate engine power. For example, a tug with a bollard pull of around 85 tonnes may be able to develop approximately 90–120 tonnes of steering force at escort speeds purely through hydrodynamic lift.

This is essential because the ship’s rudder, also a foil, becomes very powerful at speed. If a rudder locks or goes hard‑over in a confined channel, the tug must be able to produce forces greater than the rudder’s to bring the ship back on track. SeaWays’ position is that escort capability must always be assessed against this requirement, not just against static bollard pull.

By contrast, tugs without escort‑grade appendages rely mainly on propulsion to generate towline force. As speeds rise, they quickly encounter diminishing returns, which is why we say that for such tugs, speed through the water “becomes the enemy” in escort operations.

Hydrodynamics cannot be considered in isolation. The same forces that provide powerful steering and braking also generate capsizing moments on the tug.

Under indirect steering, the tug heels substantially, driven by towline tension and lateral hydrodynamic forces. Classification rules limit acceptable heel based on hydrostatic righting energy, commonly around the point where the main deck immerses. Beyond that, the tug’s safety margins are rapidly eroded.

For SeaWays, effective escort design must therefore include:

• Increased beam to improve initial stability
• Sufficient freeboard to keep the main deck well clear of immersion
• Sponsons to add hydrostatic lift at large angles of heel
• Carefully optimised underwater hull forms to balance lift and stability

Towing staple design also plays a vital role. Traditional staples can act as tripping levers, increasing the capsizing moment. Modern A‑frame style staples, as discussed in the paper, are designed to reduce this effect, allowing higher towline forces before the tug approaches its safe heel limit. These kinds of details make a substantial difference to real‑world escort capability.

SeaWays’ message from ITS 2026 is that bollard pull remains useful, but it is only one piece of a much larger puzzle.

True escort capability depends on:

• Hydrodynamic performance at speed (including keels, skegs and fins)
• Hull stability and righting energy under indirect loads
• Towing geometry and towline angles
• The strength and arrangement of shipboard fittings
• Winch performance and towline characteristics
• Competent, well‑trained crews and pilots, supported by robust procedures

When ports, tug operators and pilots focus solely on bollard pull, they risk overlooking factors that are critical when something goes wrong at speed in a narrow channel. SeaWays advocates a whole‑system approach that assesses how all of these elements work together, particularly in emergency scenarios.

By reframing the discussion around hydrodynamics and system integrity, our aim is to help the industry make better‑informed decisions about tug selection, escort policies and training priorities – and ultimately to reduce risk for people, ships and the environment.

A central theme of our paper is the role of hydrodynamic lift in modern escort tug performance. Escort‑grade tugs are typically fitted with keels, skegs or fins that act as vertical foils in the water. Once ship speed through the water climbs above about 6 knots, these foils begin to generate lift as the tug adopts a controlled drift angle. That lift is transmitted into the towline as steering or braking force.

In practice, this means a well‑designed escort tug can generate towline forces that equal or exceed its static bollard pull while running at only moderate engine power. For example, a tug with a bollard pull of around 85 tonnes may be able to develop approximately 90–120 tonnes of steering force at escort speeds purely through hydrodynamic lift.

This is essential because the ship’s rudder, also a foil, becomes very powerful at speed. If a rudder locks or goes hard‑over in a confined channel, the tug must be able to produce forces greater than the rudder’s to bring the ship back on track. SeaWays’ position is that escort capability must always be assessed against this requirement, not just against static bollard pull.

By contrast, tugs without escort‑grade appendages rely mainly on propulsion to generate towline force. As speeds rise, they quickly encounter diminishing returns, which is why we say that for such tugs, speed through the water “becomes the enemy” in escort operations.

To explore the full analysis, diagrams and case studies behind these concepts, view the complete ITS 2026 paper “Beyond Bollard Pull – A Whole‑System View of Escort Tug Capability”