Bollard pull

Bollard pull is a conventional measure of the pulling (or towing) power of a watercraft. It is defined as the force (in tons 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. 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 60-65 tons of bollard pull, which is described as 15 tons above "normal" tugboats.[1][2]

Background

Unlike in ground vehicles, the statement of installed horsepower is not sufficient to understand how strong a tug is - this is because other factors, like transmission losses, propulsion type, and propulsion system efficiency have an influence as well.

Bollard pull values are stated in tons (written as TBP) or kiloNewtons (kN).[3]

Measurement

Values for bollard pull can be determined in two ways.

Practical trial

Figure 1: bollard pull trial under ideal (imaginary) conditions

This method is useful for one-off ship designs and smaller shipyards. 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.

• The ship needs to be in undisturbed water. Currents or strong winds would falsify the measurement.
• The static force that intends to move the ship forward must only be generated by the friction between the propeller discharge race and the surrounding water. If the ship were too close to a wall, the measurement would be falsified.
• The ship must be in deep water. If there were any ground effect, the measurement would be falsified. The same holds true for propeller walk.
• Water salinity must have a well-defined value, as it influences the specific weight of the water and thereby the mass moved by the propeller per unit of time.
• The geometry of the towing line must have a well-defined value. Ideally, one would expect it to be exactly horizontal and straight. This is impossible in reality, because
• the line falls into a catenary due to its weight;
• the two fixed points of the line, being the bollard on shore and the ship's towing hook or cleat, may not have the same height above water.
• Conditions must be static. The engine power, the heading of the ship, the conditions of the propeller discharge race and the tension in the towing line must have settled to a constant or near-constant value for a reliable measurement.
• One condition to watch out for is the formation of a short circuit in propeller discharge race. If part of the discharge race is sucked back into the propeller, efficiency decreases sharply. This could occur due to a trial that is performed in too shallow water or too close to a wall.

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, there is the partial short circuit in propeller discharge current, the uneven trim of the ship and the short length of the tow line. All of these factors contribute to measurement error.

Figure 2: bollard pull trial under real conditions

Simulation

This method eliminates much of the uncertainties of the practical trial. However, any numerical simulation also has an error margin. 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.

Both methods can be combined. Practical trials can be used to validate the result of numerical simulation.

Human-powered vehicles

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. Although conditions for such measurements are inaccurate in absolute terms, they are the same for all competitors. Hence, they can still be valid for comparing several craft.

Bollard pull types

Sustained bollard pull

Mean value of the pull during a specific period of time, often 5 to 10 minutes. If the readings of the test are measured at intervals of 30 seconds, sustained bollard pull is the mean value of the readings.

Maximum static bollard pull

Maximum static bollard pull is always higher than sustained bollard pull as it is the measure of the highest 30 seconds measured during the test. If it is measured at intervals of 30 seconds, the highest mean value of 2 subsequent readings are taken.

Maximum bollard pull

The single highest measured value. This value is substantially higher than sustained bollard pull and should not be referred to as "bollard pull".