A bollard pull is a test or rather the results returned by tests used to establish the maximum pulling force working watercraft can exert. There are two ways of establishing these values: practical trials and simulations. Practical trials involve physically connecting a measuring device to the boat and an immovable object and reading the resulting force when the boat is placed under full thrust. Simulation results are based purely on calculations carried out on sophisticated software. Practical trials are not always accurate due to the large number of peripheral conditions which affect the results while simulations are costly and generally only used by large shipping companies.
Establishing the pulling potential of ships, particularly tugs, is considerably more complex than calculating the horsepower of a land vehicle. There are many factors such as different propulsion systems, transmission types, and associated efficiency losses which make the use power plant output values inaccurate. Bollard pull tests return more realistic and representative results and are used extensively to establish working watercraft pulling values. Bollard pull values may be achieved in one of two ways — by simulation or by practical trial.
Practical trial tests involve suspending a strain gauge on a marine cable that is attached on one end to an immovable object and on the other to the test craft. Mooring bollards are often used as anchor points which is where the tests get their name. When maximum thrust is applied to the boat's engines, the amount of pulling power exerted on the cable is read off the gauge. This is the cheaper of the two methods and is most often adopted by smaller ship manufacturers to test once-off vessels. This type of bollard test is, however, difficult to carry out accurately because there are a number of critical boundary conditions which need to be met before the results can be considered definitive.
These factors include the use of a deep, undisturbed body of water devoid of strong winds and currents for the test. Thrust also has to be generated by the ships propeller alone and not aided by rebound forces produced by adjacent obstructions. Propeller walk or the tendency propellers have to yaw the boat to one side also has to be figured into the equation. The relationship between the heights of the bollard and ships cleat as well as the tow line geometry are also important factors. Even the salinity or salt content of the water plays a role in the accurate measurement of bollard pull values.
Simulated bollard pull tests are simpler to execute but far more costly than practical trials. These are pure mathematical calculations executed by highly sophisticated and accurate marine simulation software. The high costs of simulated bollard pull tests make this a more suitable option for larger shipyards producing ship lines. As accurate as they are, simulated pull tests are, however, often backed up by practical trial test results.
