Bollard pull is a conventional measure of the pulling (or towing) power of a watercraft. 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. The bollard pull of a vessel may be reported as two numbers, the static or maximum bollard pull – the highest force measured – and the steady or continuous bollard pull, the average of measurements over an interval of, for example, 10 minutes. 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.
Bollard pull is primarily (but not only) used for measuring the strength of tugboats, with the largest commercial harbour tugboats in the 2000-2010s having around 60 to 65 short tons-force (530–580 kN; 54–59 tf) of bollard pull, which is described as 15 short tons-force (130 kN; 14 tf) above "normal" tugboats. [2] [3] The worlds strongest tug since its delivery in 2020 is Island Victory (Vard Brevik 831) of Island Offshore, with a bollard pull of 477 tonnes-force (526 short tons-force; 4,680 kN). [4] Island Victory is not a typical tug, rather it is a special class of ship used in the petroleum industry called an Anchor Handling Tug Supply vessel.
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]
Background
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. [6]
Total resistance is the sum of frictional resistance, 
, residual resistance, 
, and air resistance, 
. [6]
Where: [7]
is the density of water
is the density of air
is the velocity of (relative to) water
is the velocity of (relative to) air
is resistance coefficient of frictional resistance
is resistance coefficient of residual resistance
is resistance coefficient of air resistance (usually quite high, >0.9, as ships are not designed to be aerodynamic)
is the wetted area of the ship
is the cross-sectional area of the ship above the waterline
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