Bollard pull

It is defined as the force (usually in tonnes-force or kilonewtons (kN)) exerted by a vessel under full power, on a shore-mounted bollard through a tow-line, commonly measured in a practical test (but sometimes simulated) under test conditions that include calm water, no tide, level trim, and sufficient depth and side clearance for a free propeller stream.

[1] Like the horsepower or mileage rating of a car, it is a convenient but idealized number that must be adjusted for operating conditions that differ from the test.

An equivalent measurement on land is known as drawbar pull, or tractive force, which is used to measure the total horizontal force generated by a locomotive, a piece of heavy machinery such as a tractor, or a truck, (specifically a ballast tractor), which is utilized to move a load.

For vessels that hold station by thrusting under power against a fixed object, such as crew transfer ships used in offshore wind turbine maintenance, an equivalent measure "bollard push" may be given.

[5] Unlike in ground vehicles, the statement of installed horsepower is not sufficient to understand how strong a tug is – this is because the tug operates mainly in very low or zero speeds, thus may not be delivering power (power = force × velocity; so, for zero speeds, the power is also zero), yet still absorbing torque and delivering thrust.

Bollard pull values are stated in tonnes-force (written as t or tonne) or kilonewtons (kN).

[1] Effective towing power is equal to total resistance times velocity of the ship.

It is limited in precision - a number of boundary conditions need to be observed to obtain reliable results.

Summarizing the below requirements, practical bollard pull trials need to be conducted in a deep water seaport, ideally not at the mouth of a river, on a calm day with hardly any traffic.

See Figure 2 for an illustration of error influences in a practical bollard pull trial.

Note the difference in elevation of the ends of the line (the port bollard is higher than the ship's towing hook).

Furthermore, simulation tools and computer systems capable of determining bollard pull for a ship design are costly.

Hence, this method makes sense for larger shipyards and for the design of a series of ships.

Practical bollard pull tests under simplified conditions are conducted for human powered vehicles.

There, bollard pull is often a category in competitions and gives an indication of the power train efficiency.