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5. RESULTS
5.1 Different ways to pass a river bend
For three of these simulation runs with different settings of the control devices the path of the ship is given in the following figures. In Fig. 11 the standard case using only the main rudder is plotted. The speed for these manoeuvres was fixed at half engine power in order not to affect the effectivity of the bow thruster too much. For visibility purposes some of the ship contours were erased and replaced by a single contour drawn in black.
Fig. 11 Manoeuvre only using the main rudder (a)
The application of only the bow thruster as control device is shown in Fig. 12. The ships outline shows that in case of main rudder usage the stern is swilling out, while steering with the bow thruster causes the stern of the vessel to be dragged along like it is known from cars.
Fig. 12 Manoeuvre only using the bow thruster (b)
Case (a) has drift angle directing towards the turning direction, while in case (b) it points outwards. For both an increase of the traffic lane width is to be observed due to the drift motion. This can be improved by using the main rudder and the bow thruster simultaneously. If the forces of both are balanced perfectly the drift angle will decrease to zero and the traffic lane width will reach its theoretical minimum b, which can be calculated by (6) using the geometrical relationship shown in Fig. 13.
Fig. 13 Theoretical minimum of traffic lane width
The simulated manoeuvre performed with this setup is shown in Fig. 14 where the drift angle is zero and the pivot of the ship is exactly in the middle of the vessel.
Fig. 14 |
Manoeuvre using main rudder and bow thruster in balance |
5.2 Forces affecting the vessel
Normally the force of a stern rudder is used to turn the vessel to a drift angle and the drift force laterally affecting the hull is acting as zentripetal component to generate a curve with a certain radius.
If there is only a bow steering system the main force affecting the hull and pointing inwards has to be generated by this system. It has to be many times stronger than the force of the main rudder, because it also has to compensate the hull force due to the sign reversed drift angle pointing outwards.
In combining bow and stern control devices the drift force can be brought to zero. The force of the forward system however has not only to generate the zentripetal force for the curve but also to compensate the force of the main rudder at the stern pointing outwards.
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