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Question:-

Improvement in rudder efficiency can considerably contribute in improving propulsion effectiveness. Discuss common rudder-efficiency improvement devices/techniques currently in use, referring to the following:-

1. Rudder Thrust Fins
2. Asymmetric Rudder
3. Rudder Bulbs
4. Grim Vane Wheels.

Answer:-

Rudder provides following functions in a ship:-

1. Course keeping.
2. Manoeuvring.
3. In exceptional conditions, can be used for emergency stopping and roll stabilizing.

Effect of rudder on ship’s propulsion efficiency:-

1. The propellor accelerates and rotates the wake of the hull towards the rudder. The rudder works in propellor slipstream including manoeuvring forces and movements and affects propellor generated thrust and required torque.
2. When the rudder is angled, it creates a lift force that turns the ship by deflecting the flow of water.
3. When the rudder is angled away from centreline, it creates drag which needs to be overcome by engine by producing more thrust to maintain the speed of the vessel.
4. This increase in thrust lead to more fuel consumption.

An optimal rudder design by streamlined rudder profile reduces drag when the ship is at zero or small angles while a poor design can create turbulance that reduces propellor efficiency. Following things are kept in mind while designing rudders for better propulsion efficiency :-

1. Increasing the rudder area in the propellor slipstream.
2. Enlarging the total rudder area
3. Improving the rudder hydrodynamic characteristics by changing the profile.
4. the type of rudder.

Numerially, Rudder Force, F(x) = Drag Force BY VISCOSITY, F(d)+ Drag by Pressure Gradient F(p)+ Rudder Thrust, F(t)

1. Drag by pressure gradient– A velocity gradient is produced by the accelerated flow due to the working of propellor. Now the pressure field in the trailing end is lower than the pressure field in the leading end due to the negative gradient behind the propellor. This lead to drag and effect the overall rudder force.
2. Rudder Thrust– is generated due to the circumferential flow in the propellor slipstream. But an inflow velocity vector is formed due to the axial and circumferential flow component and the lift produced by this inflow with an attach angle becomes thrust.

The rudder thrust reduces the rudder drag and hence contribute to propulsive performance.

Following rudder-efficiency improvement devices/techniques currently in use:-

Rudder Thrust Fins :- The rudder thrust fins which is a horizontal wing has the same effect as rudder which is a vertical wing, and generates fin thrust and helps in increasing rudder performance by reducing drag.

Asymmetric Rudder :- is a rudder that has a cross sectional shape which not the same on both sides of the centre-line. one side is more curved than the other. This helps to match the swirl of the water from the propellor which lead the water to hit the rudder at a better angle. This improves the lift for the same rudder angle. hence the rudder doesnot need to be deflected as much, hence less drag and more efficient turning leading to better propulsion efficiency. This also reduces the turbulance and hence increases the propulsion efficiency.

Rudder Bulbs:- is a streamlined bulbs fitted at the leading edge of the rudder. It improves the water flow in front of the rudder and fills the vacuum behind the centre of the propellor. Its main function is to reduce the hub vortex by decreasing the distance between the propellor and the rudder leading to a more streamline flow of water which lead to lesser difference in pressure in leading and the trailing end of the rudder, hence F(p) which causes the reduction in energy flow reduces and hence the Rudder Force F(x) increases. The streamline flow also reduces cavitation and hence increase the life of rudder as well.

Grim Vane Wheels:- is an energy saving device fitted behind the propellor with diameter bigger than the propellor diameter so that only the inner part is located in the propellor stream. The vane wheel blades have turbine profile in this inner section while having propellor profile at the outer section. In the turbine section the kinetic energy is taken from the propellor slipstream, which is directly transformed again in the propellor section into additional thrust, hence increasing Rudder Thrust F(t). This lead to increase in Rudder Force F(x).