A tow force is a test or rather the results returned by tests that are used to establish the maximum traction that working waterjet can exert. There are two ways to establish these values: practical experiments and simulations. Practical testing involves physically connecting a unit of measurement to a boat and a fixed object and reading the resulting force when the boat is placed under full thrust. The simulation results are based solely on calculations performed on advanced software. Practical studies are not always accurate due to the large number of peripheral conditions that affect the results while simulations are expensive and are usually only used by large shipping companies.
Establishing the trait potential of ships, especially tugs, is considerably more complicated than calculating the horsepower of a land vehicle. There are many factors such as different propulsion systems, transmission types, and associated efficiency losses that make them use power plant output values inaccurately. Tow power tests return more realistic and representative results and are widely used to establish working watercraft pull values. Towing force values can be achieved in two ways – by simulation or by practical testing.
Practical test tests involve a tension flap on a marine cable attached at one end to a fixed object and at the other to test vessels. The mooring bollards are often used as anchor points where the tests get their name. When maximum thrust is applied to the boat’s engines, the amount of traction exerted on the cable is read by the meter. These are cheaper of the two methods and are most often adopted by smaller ship manufacturers to test a one-off ship. However, this type of test bollard is difficult to perform precisely because there are a number of critical boundary conditions that must be met before the results can be considered final.
These factors include the use of a deep, undisturbed water exposed to strong wind and current conditions for the test. Thrust also has to be generated by ship propeller alone and not helped by rebound forces produced by adjacent obstacles. Propeller stroll or the propeller tendency has to push the boat out to one side also has to be factored into the equation. The relationship between the heights of the bollard and the ship clamp as well as the towline geometry are also important factors. Even the salinity or salt content of the water plays a role in the accurate measurement of drag force values.
Simulated bollard pull tests are easier to perform, but far more expensive than practical tests. These are pure mathematical calculations performed by highly advanced and accurate marine simulation programs. The high cost of simulated towing tests makes this a more suitable option for larger shipyards producing ship lines. However, as accurate as they are, simulated pull tests are often backed by practical test results.