Experimental investigation of the efficiency of coupled- and uncoupled rudder work on twin-screw, twinrudder ferry model ship in proximity of a pier in shallow water: Part 2
Part 1 of this article was originally published in edition 33 of Port Technology International. It is available for download at www. porttechnology.org under journal archives.
The authors have designed a model ship experiment that would provide the data for forces and moments acting on a twin-screw, twin-rudder model ship of a ferry manoeuvring in shallow water while using propellers working with differential thrust and with synchronously or decoupled way of rudders operation.
The data collected will answer the question: Which of the rudder operation methods results in larger total yaw moment and side force? The research indicates that the rudders may be operated in any way, since the mode of their operation barely influences the total side force and the total yaw moment.
The research also indicates that in shallow water (H/d = 1,25) the vertical wall influences the side force and the yaw moment at least to the distance of one ship beam (B) between the wall and the ship’s side. Part 1 of this article set out the test objectives and explained the experiment design including set-up and conditions.
During experiments longitudinal (FX1,2) and lateral (FY1,2) force components were measured on two dynamometers located at the bow and at the stern of the model ship. The force components were summed to provide a total force components (FX and FY). Using the lateral components (FY1,2) the yawing moment Mz around the centre of gravity was computed.
The dimensional values of force components and yaw moment were non-dimensionalised using the model ship length Lpp and the half of the average total FX force for the case of zero rudders deflection angle and ahead-oriented propeller thrust (T0). Nondimensional equivalent location of the side force (assuming that only the side force causes the yaw moment) X’FY is given by the ratio of yaw moment and side force.
Forces due to propellers’ action alone
The total force and moment due to the propellers action alone (rudders set to zero deflection angle) show the amount of interaction between the ship and the flow boundaries (the bottom and the vertical wall). Tables 1 and 2 summarise the forces and moments for various combination of propeller settings.
The data show that the presence of the vertical wall induces side force oriented toward the wall for every mode of propeller operations. The shallow water increases the magnitude of the side force. More force is produced for higher propeller loads – as it was expected, since the propeller load causes the flow of the water around a model ship dead in water.