Bollard pull
Bollard pull ( English pull bollard is) shortening both the Pfahlzugmessung , a method for determining the tensile force of ship propulsion systems (particularly for tractors ), as well as for the thus determined maximum bollard pull . This comparative value denotes the mass that could theoretically be lifted by the tensile force of the ship against the acceleration of gravity . In comparison, the mass of a ship that can be pulled horizontally with this force is much higher. The thrust when maneuvering is usually equated with the bollard pull.
When measuring the bollard pull, a pull balance is attached between the tow rope of the ship and a bollard or pole. The tensile force generated can be read off the tensile scales for different performance levels of the ship's propulsion system. For hydrodynamic reasons, their maximum does not necessarily have to be at the maximum machine power or speed.
When measuring the bollard pull, the propeller always has 100 percent slip due to the lack of movement of the ship through the water , which is a hydrodynamically unfavorable operating condition, somewhat comparable to the spinning drive wheels of a land vehicle. However, tug propellers are optimized for this. They are a large area to produce water flow with a large cross-section, have little inclination, so that flow is slow at a standstill ship and turn very slowly to performance-reducing stalls to avoid.
The maximum bollard pull determined in this way is usually specified in kilograms or in metric tons , although a force is determined during the measurement (see weight force ). If only “ tons ” is mentioned, weight units from the Anglo-American system of measurement can also be meant.
The actually usable thrust and pulling force can sometimes differ considerably from the maximum bollard pull and is dependent on wind, current , water depth , sea state and, in particular, the speed of travel, which determines the actual slip and the propeller flow .
The bollard pull of ocean tugs can be over 250 tons. Modern harbor tugs develop a bollard pull of 20 to 60 tons. Well-designed tractor drives achieve a bollard pull of over 15 kilograms per kilowatt of installed drive power. Fishing trawlers and trawlers for trawling achieve similar values.
The drives designed for profitability for passenger or cargo ships and slow sailing and motor yachts reach around 10 kg / kW, speed boats and racing boats can be well below 5 kg / kW. The drives of the last-mentioned types of ships only achieve their maximum efficiency when they are flown against at their design speed. The force developed is not directly related to the bollard pull, because these drives are designed for operation with as little slip as possible. As a rule, no bollard pull measurement is carried out for such ships, but rather the time or distance required to accelerate to a specified speed or to brake.