Bollard pull
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Bollard pull is a conventional measure of the pulling (or towing) power of a
), which is utilized to move a load.Bollard pull is primarily (but not only) used for measuring the strength of
Background
Unlike in ground vehicles, the statement of installed
Effective towing power is equal to total resistance times velocity of the ship.[5]
Total resistance is the sum of frictional resistance, , residual resistance, , and air resistance, .[5]
Where:[6]
- 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
Measurement
Values for bollard pull can be determined in two ways.
Practical trial
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 propeller discharge. If the ship were too close to a wall, water could rebound back, creating a propulsive wave. This would falsify the measurement.
- 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.
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
See also
- Azipod
- Kort nozzle
- Tractive force
Notes
- ^ a b "Bollard Pull - an overview". ScienceDirect Topics. Retrieved 1 April 2021.
- ^ "Rotor Tug "RT Zoe"". Marineline.com. 13 September 2006. Retrieved 19 August 2013.
- ^ "Western Marine to build tugboat, vessel for Ctg port". The Independent. 4 June 2012. Archived from the original on 15 April 2016. Retrieved 19 August 2013.
- ^ "MV ISLAND VICTORY". Island Offshore. 11 December 2019. Retrieved 2022-12-31.
- ^ a b https://www.man-es.com/docs/default-source/marine/5510-0004-04_18-1021-basic-principles-of-ship-propulsion_web.pdf
- ^ Please note that the velocity of air or water is not necessarily equal to the velocity of the ship as the velocity of wind and water currents must be added vectorially
Further reading
- "Bollard Pull Trial Code for tugs with Steerprop propulsion". Retrieved 2011-08-24.