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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. In another words Bollard pull is the maximum pulling capacity of a tug that can exert.

Why in  Bollard Pull not BHP?
The BHP of the tug is not sufficient to show the efficiency because of loss in transmission, propulsion efficiency and propulsion type Whereas Bollard pull shows the efficiency of the maximum pulling capacity of a tug that can exert.

TEST
Bollard pull test is carried out by dynamometer comprised of load cell & load indicator.
Loadcell is the force measuring link and it is made as part of the rope, connected to the towing hook. 



Sustained Bollard Pull is the mean value of the pull during a specific time period (5 to 10 min.). If the measurements are taken every 30 sec., the sustained Bp is the mean value of the readings.

Maximum Static Bollard Pull is the highest 30-second value read during the test. If the measurements are taken every 30 sec., the maximum static Bp is the highest mean value of two consecutive readings.

Maximum Bollard Pull is the highest single value measured.


Requirements for  the Bollard Pull Test:
  • The test is carried out at slack water
  • Trial site: should allow the use of a long enough towing line and be free of wind, waves, and be located in very deep water with no tides or currents.
  • The towline length should be enough to allow free water astern of the vessel. (200 meters minimum recommended).
  • 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 tug has to be loaded to its designed waterline, in order to ensure the adequate immersion of the propellers.
  • Wind speed during the test should not exceed 5m/s.
  • The trial should be done with no currents if possible; if there’s a current, it should not exceed 1 knot.
  • 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.
  • Measurement should be taken using a calibrated dynamometer or a mechanical load gauge.
  • Steering during the trials should be avoided as much as possible.
  • The readings on the dynamometer should be continuously recorded by a computer, or at 30-sec intervals if a mechanical load gauge is used.
  • The maximum acceptable deviation of the dynamometer is +-2%.
  • Communication between shore and tug should be excellent, for an accurate recording of data relating the bollard pull to the rpm.
  • Bollard and the towing hook should be at the same height

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