Understanding Bollard Pull: A Key Factor in Ship Maneuvering

Name: ESC Group
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ESC Group
2 days ago
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Bollard pull is our marine industry's standard measurement for a vessel's pulling capability. Simply put, it measures the maximum force a ship can exert when stationary in the water - essentially our equivalent to horsepower or torque in land vehicles. To measure bollard pull, we secure the vessel to a fixed pier bollard (those strong metal posts you see on docks) and use specialized load cells to calculate the tension generated in the line when the engines run at full power. This testing approach closely resembles the drawbar pull concept used with land-based towing vehicles like trucks and tractors.

The technical specifications for bollard pull are typically expressed in kilonewtons (kN), short tons (stf), or tonnes-force (tf), providing a standardized way to quantify pulling strength. This measurement is absolutely critical for tugs and other towing vessels since it reveals their true pulling capability under load conditions. Every commercial vessel intended for towing operations must obtain proper bollard pull certification from internationally recognized classification societies such as the American Bureau of Shipping (ABS), Bureau Veritas (BV), or Indian Register of Shipping (IRS). This certification ensures the vessel can safely perform its intended towing functions and meets industry standards for pulling capacity.

Here at ESC, we offer international qualified mooring bollards that ensures the safety of your vessels from light to heavy-weight vessels and maintain the integrity of your marine and docking facilities.

Why Bollard Pull Matters

Bollard pull measures a vessel's pulling force at zero forward speed - similar to how horsepower rates land vehicles. Provides accurate measurement of a tug's true power capabilities, unlike easily manipulated horsepower ratings.

Critical applications:
- Holding large ships steady when they exert reverse thrust
- Emergency stopping/course correction of large vessels in ports
- Towing heavy marine structures like semi-submersible oil rigs
Operational significance:
- Ensures tugs can generate maximum thrust when needed
- Gives operators reliable data about vessel capabilities
- Certified measurement recognized by international standards
Logistics optimization:
- Determines realistic load-pulling capacities
- Helps calculate accurate towing speeds and transit times
- Critical for port operations planning and scheduling
Special case - AHTS vessels:
- Required for moving massive semi-submersible oil rigs
- Needs precise measurement due to extreme weight of structures
- Essential for planning complex offshore operations
Regulatory importance:
- Now required certification for most tugs and support vessels
- Prevents misrepresentation of vessel capabilities
- Standardizes power measurement across the marine industry

How to Calculate Bollard Pull

calculating bollard pull — Photo credit: Marine Insight

Where:

Engine Power is the total power in kilowatts.
Propeller Efficiency accounts for losses in power transmission.
0.101972 is a conversion factor.
9.81 adjusts for gravitational acceleration.

Bollard pulls testing measures a vessel's maximum towing capacity by securing it to a fixed point and measuring the force generated at full thrust while maintaining zero forward speed. The test involves anchoring the vessel to a pier bollard and recording the tension developed in the connecting line when maximum propulsion is applied.

Ideal testing conditions require calm, tide-free waters with sufficient clearance in all directions, uniform vessel draft, and unobstructed propeller wake. In reality, perfect conditions are rarely achievable, so engineers must account for a margin of error in the measurements. Environmental factors like water salinity significantly impact results since propeller thrust depends on water density and mass displacement characteristics.

conducting bollard pull tests — Photo credit: Marine Insight

For accurate data collection, engineers typically conduct tests over ten-minute intervals or until oscillations stabilize. Multiple measurements of thrust, tension, and other parameters are averaged to determine the official bollard pull rating. Between test runs, sufficient settling time must be allowed for the surrounding water to return to its undisturbed state.

For vessels equipped with supplementary propulsion systems like electric thrusters, additional measurements are recorded as Hybrid Bollard Pull (HBP). This separate classification accounts for the additional thrust capability provided by these alternative power sources, particularly relevant with modern green propulsion technologies.

While computational fluid dynamics simulations can provide theoretical bollard pull values, physical testing remains the industry standard despite its inherent variability. For fleets requiring multiple vessels with similar towing characteristics, computational methods become more cost-effective as the initial investment can be amortized across multiple units.

Waves-induced oscillations or slamming effects at the bow are the cause of wave resistance. Any vessel's ability to travel is greatly impacted because this resistance needs to be overcome.

When the wind is blowing against the ship's sail area, wind forces enter the picture and might impede forward motion. In addition to these external forces, the hull's inherent resistance also plays a role. The following formula is used to determine this hull resistance:

Resistive force formula:

R = 0.5 x water density x wetted surface area x velocity2

Where at zero forward speed, the tug’s own resistance is zero, so the full bollard pull is available for towing. At higher speeds, some bollard pull is used to overcome the tug’s resistance, reducing towing capacity.

Enhancements to Increase Bollard Pull

To increase the bollard pull of the vessels, a common and effective way of doing this is by adding a nozzle casing (duct) around the propeller blades, where it improves thrust at low speeds under heavy loads. The nozzle acts like a hydrofoil, increasing by up to 30% the flow efficiency and thrust of the vessels.

Installing multiple propellers with shrouds and balance thrust and improve stability. Where most of the vessels that require higher power or speed often have multiple propellers, where each propeller is enclosed properly with shroud or nozzle to maintain hydrodynamic balance. Resulting in an evenly thrust distribution across the vessels’ centerline, improving stability and maximizing total bollard pull.

Some advanced versions incorporate the ducted propeller system directly with rudders and steering devices. Through the optimization of water flow and thrust direction, this integration improves mobility and can further improve bollard pull. Better control during towing or station-keeping activities is made possible by such systems.

Although this installation gives advantage and improvements, there is still persist limitations. At low speeds, ducted propellers and nozzle casings increase bollard pull; at higher speeds, however, they decrease propulsion efficiency. There is a greater chance of cavitation, a condition in which vapor bubbles form around quickly rotating propeller blades due to low pressure. In addition to wasting energy and decreasing thrust, cavitation over time can harm propeller blades.

Despite these limitations, the advantages of tugs exceed the disadvantages because they are primarily used at moderate speeds for maneuvering and towing.

Considering Different Ship Sizes

When we look at different vessels, their size directly affects how much pulling power they need:

For small vessels like fishing boats and harbor tugs, we're working with less engine power and simpler propellers. They don't need massive pulling strength, but they must be quick to respond when maneuvering. We use these mainly to help ships dock and for moving small loads around harbors.

With medium-sized vessels such as cargo ships and ferries, we need moderate pulling capability for port operations. In tight spaces, these ships often need help from specialized tugs. Their weight distribution creates more water resistance, which affects how they handle.

The big ships - tankers, cruise liners, and container vessels - require substantial pulling power to control their enormous weight and overcome water resistance. These giants typically need several tugboats working together just to dock and depart safely. Getting the math right is crucial for handling these vessels safely in deep water.

Different ship designs also change our bollard pull calculations. Tugboats are built specifically to maximize pulling strength compared to their size. When working with barges, we have to factor in both drag forces and how fast we're towing. For massive vessels like cruise ships and tankers, we need extremely powerful tugs because these ships have tremendous momentum once they're moving - they simply can't stop quickly on their own.

According to design rules, bollards should usually be positioned 15–30 meters apart, generally at the same distance as fenders (either halfway or at the same point). 15% of the shortest ship's length is another rough guideline.

Ship Displacement - Bollard Size

Up to 2000 tons - 10 tons

2000 - 10000 tons - 30 tons

10,000 - 20,000 tons - 60 tons

20,000 - 50,000 tons - 80 tons

50,000 - 100,000 tons - 100 tons

100,000 - 200,000 tons - 150 tons

Over 200,000 tons - 200 tons

ESC offers a broad selection of mooring bollards categorized by their displacement with ratings from 10 to 300Tons and many different types - the T-Head, T-Horn, Kidney, Cleat, Double Bitt, Single Bitt, and Pillar. In order to provide our clients with the highest caliber of mooring bollards, we took into account the design and standard codes.

Take away

To conclude, bollard pull remains the cornerstone measurement for evaluating a vessel's true towing capabilities in marine operations. While technological advancements continue to improve how we measure and simulate these forces, the physical bollard pull test persists as our industry standard for good reason - it provides real-world performance data that directly reflects operational conditions. For maritime professionals, understanding these measurements is crucial whether managing harbor tugboats or coordinating massive offshore operations with AHTS vessels. The certification process, though imperfect due to environmental variables, gives us the reliable benchmarks necessary for safe and efficient marine operations across vessel sizes and types.

As we continue to develop hybrid propulsion systems and more efficient hull designs, accurate bollard pull measurements will remain fundamental to matching vessels with their intended tasks and ensuring the seamless movement of ships through our increasingly busy waterways.