Experimental investigation of the efficiency of coupled – and uncoupled rudder work on twin-screw twin-rudder ferry model ship in proximity of a pier in shallow water: Part 1
Twin-screw and twin-rudder are the dominant design solutions for highly-manoeuvrable ships such as ferries. Such a configuration is chosen by designers because the combination of the propellers’ thrusts and the rudders’ side force is capable of providing a lateral force and a yawing moment at zero ship speed, which is very advantageous for harbour manoeuvres in proximity of quays.
Setting the equal thrusts of propellers in opposite directions provides a side force and a yaw moment without a net surge force – in such a case the rudders’ deflection angles may be zero, so there will be almost no side force produced by the rudders.
The way to control the magnitude of the side force and the yaw moment is to change the propellers’ differential thrusts (thrusts acting in opposite directions). By deflecting the rudders the ship pilot may also control the magnitude of the side force and the yaw moment – the rudders are usually used to increase these two things.
When the propellers produce thrust in opposite directions only one of the rudders receives the propeller’s slipstream and is capable of producing significant side force – it is the rudder located behind the propeller producing the ahead thrust. The other propeller sucks the water from the aft of the ship and this stream somehow flows around the other rudder. In such a situation the question from pilots arises – what should be the deflection angle of such a rudder located behind a propeller producing an astern thrust?
There are two designs of rudder control systems: the rudders working synchronously (coupled), i.e., being deflected in the same manner or the rudders working in a decoupled manner, each of them being deflected independently. In the first case the former question is irrelevant. In the second case the ship pilot asks whether it is more efficient to work both the rudders synchronously or to deflect only the rudder exposed to the propeller slipstream while keeping the other rudder – the one behind the propeller working astern – at some fixed deflection angle.
The information for the ship pilots with regard to the effectiveness of twin-rudder usage is important, since it provides the pilots with objective information affecting navigational safety. If any of the methods of rudder operation has an advantage then this method should be advised to be employed. Also, knowledge of the manoeuvring efficiency of each of the rudder usage methods is useful when other operational factors are considered, for example, the resulting load on the rudder steering gear (which may discourage the deflection of the rudder placed behind the propeller producing the astern thrust).
The estimation of rudder effectiveness in the described situation requires some mathematical modelling of rudder forces and hull forces. Unfortunately, the four quadrant models of hullpropeller-rudder interactions are far from perfect.
The situation is even worse for shallow water manoeuvring, when the presence of flow boundaries (bottom and quay) has significant effect on hydrodynamic forces and, precisely, the shallow waters are areas where the manoeuvring at zero speed is performed.
Therefore, we have designed a model ship experiment that would provide the data about forces and moments acting on a twin-screw, twin-rudder model ship of a ferry which manoeuvres in shallow water using propellers working with differential thrust and with synchronous or decoupled way of rudder operation.
The data allows us to answer the question which of the rudder operation methods results in larger total yaw moment and side force.