Bollard pull
Bollard pull ( en.: bollard pull) is an abbreviation for both the Pfahlzugmessung, a method for determining the tensile force of marine propulsion (especially with tractors ), and for the thus determined maximum bollard pull. This comparison value refers to the mass, which could theoretically be raised by the tensile force of the vessel in relation to the gravitational acceleration. The mass of a vessel that can be dragged horizontally with this force, by contrast, is much higher. The thrust of the maneuver is usually equated with the bollard pull.
When Pfahlzugmessung a Zugwaage between the towline of the ship and a bollard or pile is attached. For different levels of ship propulsion, the tensile force generated can be read on the Zugwaage. Their maximum must hydrodynamically conditioned, not necessarily lie at the maximum engine power or speed.
When Pfahlzugmessung the propeller lack of ship movement through the water is always 100 per cent slip, which is a hydrodynamically unfavorable operating condition, a little similar to the spinning drive wheels of a land vehicle. Tractor propeller, however, are optimized for it. They are a large scale to produce water flow with a large cross-section, have little slope, since this flow is slow at a standstill ship and turn very slowly to prevent power-robbing stalls.
The determined maximum bollard pull is usually given in kilograms or metric tons, although in the measurement of a force is determined (see gravitational force). Is only of "tons " is mentioned can be meant from the Imperial measurement system may also weight units.
Actual usable thrust and pulling force can significantly differ in part from the maximum bollard pull and is dependent on wind, current, water depth, sea state, and especially the driving speed, which decides on the actual slip and the flow into a propeller.
The bollard pull of tugs can be over 250 tons. Modern Tug Boat develop a bollard pull of 20 to 60 tons. Well-designed tractor drives to achieve a bollard pull of over 15 kilograms per kilowatt of installed drive power. Trawler Trawler and for trawling reach similar values .
The systems designed to profitability drives for passenger or cargo vessels and slow sailing and motor yachts reach about 10 kg / kW, fast and speedboats can be well below 5 kW kg /. The drives of the latter types of ships reach their maximum efficiency only when they are flowing at their design speed. The force developed here is not directly related to the bollard pull, because these drives are designed for a low-slip operation as possible. For such vessels, no Pfahlzugmessung is performed normally, but it can be measured the time or the distance required to accelerate to a given speed or braking.