In the maritime industry, one of the most important measures of a tugboat’s strength and capability is its bollard pull. Whether in ship-handling operations, offshore towing, or harbor assistance, the term bollard pull ahead and astern plays a crucial role in understanding how effectively a tug can push or pull a vessel. While the concept may sound technical, it is actually quite straightforward when explained in the context of a tug’s performance in both forward (ahead) and reverse (astern) directions.
Understanding Bollard Pull
Bollard pull is the measure of the pulling power of a tugboat or vessel, expressed in tons or kilonewtons (kN). It represents the maximum force a tug can exert when it is tied to a fixed point typically a bollard on a dock or quay and pulling at full engine power. This test is conducted under controlled conditions to determine the vessel’s true towing strength. The bollard pull value helps ship operators, harbor authorities, and engineers assess a tug’s suitability for different types of operations.
The term bollard pull ahead and astern refers to two separate measurements one taken when the tug is moving forward (ahead) and one when it is moving backward (astern). Both values are important, as tugboats often need to perform maneuvers in both directions while assisting ships in tight harbor environments or during offshore operations.
The Difference Between Bollard Pull Ahead and Astern
While both measurements serve the same purpose to indicate the tug’s pulling power the results are usually not the same. In most cases, the bollard pull ahead is higher than the bollard pull astern. This difference occurs due to the design of the tug’s propulsion system, hull shape, and propeller configuration.
Bollard Pull Ahead
The bollard pull ahead is measured when the tugboat is pulling in the forward direction. During this test, the engines are operated at full power, and the thrust generated by the propellers is directed aft, pulling the tug forward against the fixed bollard. This measurement represents the tug’s maximum towing capability under optimal conditions.
For most modern tugs especially those equipped with azimuth thrusters, Voith Schneider propellers, or conventional screw propellers the bollard pull ahead figure is typically used as the main performance indicator. Since most towing and ship-assist operations are performed in a forward motion, this value is critical for determining the tug’s overall strength and suitability for tasks like escorting tankers, towing barges, or assisting ships at sea.
Bollard Pull Astern
The bollard pull astern is measured when the tugboat is pulling in reverse. This measurement is often lower than the ahead value because the tug’s hull and propellers are optimized for forward motion. When moving astern, hydrodynamic efficiency decreases, and the propellers may experience turbulent water flow or reduced thrust efficiency.
However, the bollard pull astern value is still very important especially in harbor operations, where tugs must sometimes pull or push vessels while maneuvering backward. For example, during docking or undocking operations, a tug may need to operate in reverse to control the movement of a large ship safely.
Typical Bollard Pull Values
The actual bollard pull value depends on the tug’s size, power, and propulsion system. Modern harbor tugs can have bollard pull ratings ranging from 30 to 100 tons or more. Offshore tugs and anchor-handling vessels (AHTS) can exceed 200 tons of bollard pull. As a general rule, the astern pull is about 80 90% of the ahead pull, though this varies depending on the vessel’s design.
- Small harbor tugs 25 40 tons ahead, 20 35 tons astern
- Escort tugs 60 90 tons ahead, 50 75 tons astern
- Offshore anchor-handling tugs 150 250 tons ahead, 130 220 tons astern
These figures help determine what kind of operations a tug is capable of handling and how efficiently it can perform in both directions.
How Bollard Pull Is Tested
The bollard pull test is carried out under specific conditions, often supervised by a classification society or maritime authority. The tug is secured to a strong bollard or structure using a towline. The engines are then run at full power, and the force exerted is measured using a calibrated dynamometer or load cell attached to the towline. The test is typically conducted for a few minutes to ensure stable readings.
Two tests are usually performed one for bollard pull ahead and one for bollard pull astern. These tests help ensure that the tug meets the required specifications and that its propulsion system functions correctly in both directions. It’s also a valuable diagnostic process for verifying engine performance, propeller condition, and hull efficiency.
Factors Affecting Bollard Pull Ahead and Astern
Several factors can influence the results of a bollard pull test. These include
- Propulsion system typeAzimuth thrusters and Voith Schneider propellers provide better astern performance compared to traditional fixed-pitch propellers.
- Hull designTugs with symmetric hull designs or optimized stern shapes perform better in reverse.
- Water depthShallow water can reduce thrust efficiency due to increased turbulence.
- Sea conditionsWaves, current, and wind can all affect the accuracy of bollard pull measurements.
- Engine power and maintenanceA well-maintained propulsion system ensures consistent performance in both directions.
Because of these variables, bollard pull tests are usually performed in calm, deep waters to minimize external influences and ensure accurate readings.
The Importance of Bollard Pull Ratings
Knowing the bollard pull ahead and astern values of a tugboat is essential for safe and efficient maritime operations. These ratings determine whether a tug can handle specific tasks, such as towing large vessels through narrow waterways or controlling a ship’s movement during docking. In ports with heavy traffic, selecting the right tug with adequate bollard pull ensures that ships are maneuvered safely without delays or accidents.
In offshore applications, high bollard pull is necessary for anchor handling, rig towing, and positioning large floating structures. Understanding both ahead and astern pull capabilities allows operators to plan operations more effectively and avoid overloading a tug beyond its designed capacity.
Operational Scenarios
Let’s consider a few examples of how bollard pull ahead and astern values are applied in real-world operations
- Harbor assistanceA tug with strong astern pull can maneuver large ships backward into berths or guide them safely away from piers.
- Escort operationsHigh ahead pull allows a tug to stabilize or slow a large tanker in case of engine failure.
- Offshore towingWhen towing oil rigs or barges over long distances, maximum ahead pull ensures steady progress against ocean currents.
In all these situations, both ahead and astern bollard pull values contribute to the tug’s overall effectiveness and safety profile.
Modern Tugboat Designs and Balanced Performance
Modern tugboats are increasingly designed to provide balanced bollard pull performance in both directions. This is especially true for advanced azimuth stern drive (ASD) tugs and rotor tugs. Their propulsion systems can rotate 360 degrees, allowing them to generate nearly equal thrust ahead and astern. Such designs enhance maneuverability, efficiency, and safety during complex ship-handling tasks.
By achieving closer parity between ahead and astern performance, these modern tugs reduce operational limitations and provide greater flexibility for harbor and offshore operations. This technological advancement reflects the maritime industry’s shift toward higher precision and control in vessel movement.
Understanding bollard pull ahead and astern is key to appreciating how tugboats perform their essential duties in maritime operations. The bollard pull ahead represents a tug’s maximum pulling power when moving forward, while the bollard pull astern measures its strength in reverse. Although the ahead value is typically higher, both measurements are critical for ensuring safe and efficient vessel handling. Factors such as propulsion type, hull design, and sea conditions all influence these results. As tugboat technology continues to evolve, newer designs are achieving near-equal bollard pull in both directions, enhancing maneuverability and reliability across diverse marine environments.