If the rudder has its entire area aft of the rudder stock then it is unbalanced .A rudder with between 20 and 40% of its area forward of the stock is balanced since there will be some angle at which the resultant moment on the stock due to the water force will be zero. Most modern rudders are of the semi-balanced design. This means that that a certain proportion of the water force acting on the after part of the rudder is counter acted by the force acting on the for'd half of the rudder; hence, the steering gear can be lighter and smaller. A rudder may lift due to the buoyancy effect, the amount of lift is limited by the jumper bar fitted to the stern frame. The jumper/rudder clearance must be less than the steering gear cross head clearance to prevent damage. A rudder is supported by means of a bearing pintle or a lower bearing depending upon the design. Where a lower bearing is employed the rudder is actually supported on split bearing rings fitted on the lower face of the rudder and the upper face of the sole piece ( the extended lower section of the stern frame upon which the rudder sits)
SEMI BALANCED RUDDERSemi balanced rudder with rudder horn
Fully balanced rudderTo reduce the amount of torque required to turn a rudder the pivot point is moved back from the leading edge. The amount of torque then varies depending on the angle of attack. Zero torque leads to instability with rudder moving within its clearances.
It is possible to have the blades angled in or out when the wheel is amid ships to increase propulsive efficiency.
SPECIAL STEERING DEVICES
THE KORT NOZZLEAdequate clearance is essential between propeller blade tips and sternframe in order to minimise the risk of vibration. As blades rotate water immediately ahead of the blades is compressed and at the blade tips this compression can be transmitted to the hull in the form of a series of pulses which set up vibration. Adequate clearance is necessary or alternatively constant clearance, this being provided with ducted propellers such as the Kort nozzle Originally designed to reduce erosion on river banks the nozzle has proved itself also able to increase thrust without increase of applied power.
The nozzle consists of a ring of aerofoil section which forms a nozzle surrounding the propeller. The suction of the propeller causes an acceleration of flow in the mouth of the nozzle and hence a drop of pressure in this region. Since the pressure on the outer part of the nozzle remains relatively unchanged, there is a resulting differential in pressure, which acting on the projected annulus of the nozzle, gives the additional forward thrust. This additional thrust is transmitted direct from the kort nozzle to the hull via the nozzle supports ,so that no additional force acts on the propeller and shaft thrust block.
There are two types of Kort nozzles. The fixed type has a conventional rudder behind it, whereas with the swivelling rudder type , the whole assembly is supported by a carrier attached to the rudder stock and actuated by the steering gear.
In the case of nozzle rudders ,when helm is applied , the increased thrust has an athwartship component which has powerful steering effect, so that hard over angles of 25' ( or 30' in special cases ) are sufficient to provide effective steering ahead during a crash stop and ,provided the hull is a reasonable design , astern.
This device is especially valuable for tugs, trawlers, special vessels and more recently ,VLCC, which are required to manoeuvre well , particularly at slow speed , and have the best propulsive efficiency. Bollard pull gains between 30 and 50% , equivalent to re-engining up to 1 3/4 times the original power , have been obtained in tugs and trawlers and in VLCC gains in propulsive efficiency between 6 to 13% can be expected. The normal method of calculating rudder torque's can be applied to nozzle rudders . The maximum steering effort is required to return the rudder towards midships and not to move the rudder over from amidships. Thus , the steering gear must be designed to keep control of the rudder under these conditions. For diagram and additional notes see 'method of reducing vibration' and 'increasing propulsive efficiency'
PLEUGER RUDDERA normal rudder can only be effective when the ship is moving, and the torque it exerts varies with the square of the speed ,so that at very low speeds it can be very ineffective. A pleuger rudder incorporates a submersible electrically driven propeller which can be run when the main propulsion is at rest . In order to attain maximum effect and manoeuvre the ship at rest the rudder is able to turn to 90',owing to this normal floating linkage hunting gear cannot be used, and a special cam hunting gear used. For normal course keeping the angle is limited to 35',and a warning signal initiated if exceeded.
THE VOITH SCHNEIDER PROPELLORThis propeller consists of a series of blades of aerofoil section which project vertically downwards from the ship's hull and rotate about a vertical axis. The blades are mounted on axes on a circle around the central axis and are linked together with a mechanism which can cause them to oscillate so as to provide thrust in any direction. The amount of thrust can be varied by varying the degree of oscillation, thus with the blade assembly rotating in the same direction, manipulation of the blades can give ahead or astern thrust, or port and starboard thrust without an ahead or astern thrust component, or any angle of port or starboard thrust with ahead or astern thrust.
The cycloid motion of the blades can be made to produce thrust in any direction round the circle of rotation. This means that the propeller in addition to providing the main drive for a ship provides full manoeuvrability without the need to provide a rudder and steering gear. This unusual degree of control is of particular value for special craft or floating equipment such as floating cranes or drilling ships which must be kept in position. The location of the propeller depends upon the particular application and it can be placed where the maximum desired effect can be achieved.
JET FLAP RUDDERAnother device which is being investigated at the N.P.L. is the jet flap rudder. In the trailing edge of an otherwise conventional rudder, a fluidic switch is fitted, which can direct a jet of water to port or starboard. The water is pumped into the hollow rudder through a hollow rudder stock. Considerable increase in manoeuvrability is claimed, especially at low speeds.
ROTATING CYLINDER RUDDERSThis is a device to make a ship equally manoeuvrable at all speeds and was developed in the U.K by the Ship Division of the National Physical Laboratory (N.P.L.).
A normal rudder is effective up to angles of about 35', after which the flow over the rudder stalls in a manner similar to that over an aeroplane wing at high angles of incidence. There are various methods of preventing this from occurring and they all involve feeding energy into the stream of fluid adjacent to the rudder or aerofoil surface. This is called boundary layer control. One such method is to rotate a cylinder at the leading edge of the section at such a speed that the rudder can be put over to 90' without stall, and this is the basic principle of operation of the rotating cylinder rudder. It is, of course, necessary to reverse the direction of rotation of the cylinder depending on whether the rudder is put to port or starboard, and such a system can be fitted to almost any type of rudder, balanced or unbalanced. The major advantage of putting a rudder over to such a high angle is that the flow from the main engines may be deflected through a much larger angle than with a conventional rudder, and static side thrusts of over 50 per cent of the bollard pull have been measured. Another main advantage is that its effect is independent of forward speed and it works as effectively at zero as at full speed.
Becker flapThe flap is attached to the hull. As the rudder rotates the flap is turned in an same direction increasing the aerofoil shape and thereby increasing lift. This system can be used at very large rudder angles giving side thrust capabilities
This design allows for larger turning forces for the same sized rudder or reduced size requirements compared to other sizes
Anouther format allows the flap to be steered independently. This again allows increases in thrust as well as improving steering with the vessel underway as only the flap has to be turned to cause small corrections to be carried out