House D.J. Ship Handling Theory and Practice

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Rudder types and stern arrangements

Rudders

With so many rudder types available to the ship owner today, it would be totally

inappropriate to discuss one kind of rudder with possibly a ‘Rudder Post’ and ‘Bearing

Pintle’. The fact that the majority of rudders have many variations of rudder bearings

and rudder securings, together with different transmission methods from the steering

consul, tends to make the operational needs different for each particular rudder.

Rudders are now active in nature, having hydrodynamic flaps, or power motors

which provide improved positive response to steering orders. Many ships are fitted

with the modern ‘Schilling rudders’ or the popular hanging ‘Becker rudder’ or the

‘Becker King support’ variety.

196 SHIP HANDLING

Improved Flap

Rotor Cylinder Rudder

Combined Rotor

and Flap provides

reduced turning

circle

Stern rudder/propeller arrangements

Appendix C-H8530 4/9/07 10:07 AM Page 196

Arguments for ducted propellers

If the ducted propeller is compared with the open propeller, several distinctive aspects

are noticed, the most influential probably being the enhanced speeds gained of up to

about 0.6 knots. This can clearly be translated to a fuel burn/power saving over an

example voyage. Such a saving could justify the initial expense of fitting the ducting

arrangement at the building stage.

APPENDIX C 197

Stern post trunking

Reaction fin

Ducting

Turning

axis of

rudder

Bearing pintle

Sole piece

Propeller shaft

Ducting

Rudder

Stern arrangement with propeller ducting and reaction fins. Propeller not included for clarity.

Appendix C-H8530 4/9/07 10:07 AM Page 197

Experience has also shown that ducting may reduce vibration effects, particularly

where cavitation is a feature. In comparison with an open propeller, vessel vibration

about the stern and the ‘thrust block’ is noted as being suppressed by the installation of

ducting. The reduced vibration effects may be as a result of the reduced overall size of

the propeller where ducting is employed, as compared to a larger propeller in open

design where ducting is not present. An exception to the norm would be where a vessel

is a lightship or in ballast when excessive vibration (nearly twice the normal levels) can

expect to be experienced. When the vessel is deep laden and fitted with ducting, such a

vessel benefits from reduced vibration effects. Assuming the ship is gainfully employed

with continuous cargoes, ducted propellers would seem to be a favourable addition.

Obviously with smaller propellers being used in construction (including the spare)

production costs are reduced. However, this reduction would be offset by the cost of

installing the duct itself. In practice, if a vessel suffers damage in the propeller region,

the duct itself may suffer damage. This may be seen to afford some protection to the

propeller and the duct could be repaired by regular shipyard methods; whereas a

damaged propeller would need to be drawn and possibly recast.

Ducted propellers would seem to be economically viable to the ship owner. How-

ever, corrosion on such an additional fitment below the waterline must also be con-

sidered in the economic equation. It would also seem to be better to fit at the new

build stage than to retro-fit ducting to an existing vessel. Retro-fitting is more labour-

intensive and therefore some thought should be given at the design stage as to the

needs of the trade and the voyage plans.

198 SHIP HANDLING

Stern arrangements with ducted propellers. Flap rudder aft of the propeller duct on a new

build twin screw anchor handling vessel. Twin stern thrusters are seen built into an enhanced

skeg. The whole area is fitted with sacrificial anodes because of the use of dissimilar metals

employed in the manufacture of the hull and propeller units.

Appendix C-H8530 4/9/07 10:07 AM Page 198

APPENDIX C 199

Rudder Stoc

Rudder

Trunk

Rudder Horn

Pintle

Rudder

Blade

Cone

Block

Stiffening

Stern

Rotary Vane

Actuator

The Mariner rudder.

Appendix C-H8530 4/9/07 10:07 AM Page 199

200 SHIP HANDLING

Turning Axis of Rudder

Rudder Stock

Rudder Carrier Bearing

Rudder Trunk

Outer Trunk

Neck Bearing

Webs

Main Rudder Blade

Future Burn Out, for Shaft

Removal

Heel (Bearing) Pintle

Sole Piece

of Sternframe

Turning Axis

of the Flap

Hinge

Flap

System

Mechanical Link

Turning Axis of the

Link System

Flap rudder arrangement. General arrangement of ‘Becker’ standard type flap rudder.

Rudder Trunking

Rudder

Carrier

Hanging Bearing

Inspection

Plate

Propeller

Hub

Propeller Shaft

Watertight Gland

Watertight seals on the

Sterntube and the Rudder

post. Glands and stuffing

boxes would be inspected

and re-packed if required

Suspended rudder – stern arrangement. Hanging/suspended rudders.

Appendix C-H8530 4/9/07 10:07 AM Page 200

APPENDIX C 201

CR Seal

Radial Bearing

CR Radial

Bearing

CR Thrust

Bearing

Inner Propeller

Shaft

Geislinger

Coupling

Steady

Bearing

Machinery

Thrust

Block

Intermediate

Shaft

Outer Propeller

Shaft

CR Gearing

Seals

Propeller shaft arrangement (contra-rotating propellers).

Appendix C-H8530 4/9/07 10:07 AM Page 201

202 SHIP HANDLING

Becker King support rudder

Becker-Ruder support rudder.

These rudders are connected with the ship’s hull by means of a rudder trunk which

is a cantilever design with an inside bore for location of the rudder stock. The end

part of the rudder trunk is provided with an inside bearing for mounting the rudder

blade. The exposed, lower part of the rudder stock, is fixed to the rudder body for

input of the torque.

Appendix C-H8530 4/9/07 10:07 AM Page 202

Schilling rudders

With the advance of rotors and flaps the turning circle of vessels was able to be con-

siderably reduced. The introduction of the schilling rudder meant that a further

major reduction of the turning circle was achieved.

This particular design is a single-piece construction with no moving parts. It has a

hydrodynamic shape fitted with end plates, which help to extract slipstream rota-

tional energy. The trailing wedge reduces yaw to provide excellent course stability

according to the manufacturers.

When operational, the 70 to 75° helm position takes the full slipstream from the

propeller and diverts it at right angles to the hull, eliminating the need for stern

thrusters; an ideal property for berthing operations, giving a sideways movement to

the vessel.

The rudder design may be a fully hung spade, or a simplex type, with a balance of

about 40 per cent. Sizes of rudders vary, but the positioning should take account of

the recommended distance between the trailing edge of the propeller and the stern

of the vessel, to equal 1

times the propeller diameter.

Rudder movement is achieved by steering motors supplied by reputable manufac-

turers, although several fittings have employed rotary vane steering gear with

extremely responsive results; torque affecting the stock being similar to conventional

rudder designs.

Schilling VecTwin rudders

This system of twin Schilling rudders operated by rotary vane steering is expected

to provide improved course stability over and above a conventional rudder. From

the point of view of the ship handler, a single joystick control provides compara-

tively easy manoeuvring, the ship moving in the direction of the joystick movement;

the propulsive thrust being proportional to how far the stick is pushed from the

‘hover’ position.

Again there are no moving parts underwater, so providing reduced maintenance

and less wear and tear compared to conventional rudders. A further advantage is

that with the rudders in the clam shell position, the stopping distance of the vessel is

greatly reduced and the heading is still retained. The arrangement tends to work

well with bow thrust in opposition at the fore end, giving the vessel an extremely

tight turning circle practically within its own length.

If the turning circle of the vessel with a VecTwin rudder arrangement is considered,

the rapid speed reduction caused by the large rudder angles (65/70°) would expect

to result in a reduced angle of heel. Also, because the speed is reduced so are the

‘advance’ and ‘transfer’ values of the vessel in the turn. This subsequently provides

a tighter turning ability than, say, a conventional vessel.

APPENDIX C 203

Appendix C-H8530 4/9/07 10:07 AM Page 203

Bow rudders

The bow rudder is a specialist feature and generally not a common fitting other than

to ferries and similar vessels that have a need to navigate stern first on a regular basis.

Its effectiveness comes from the main propeller’s right aft, which sends the wash of

water forward passing under the keel of the ship.

The bow form is shaped to accommodate the structure of the bow rudder and frame

spacing is reduced in the structural area to provide added strength to accept the rud-

der activity. The pivot point of the vessel will have a tendency to move towards the

204 SHIP HANDLING

The Schilling, high lift, high performance rudder.

Appendix C-H8530 4/9/07 10:07 AM Page 204

bow region to a position of about

L, measured from the forward perpendicular,

when navigating with astern propulsion. The position of the rudder, in conjunction

with the new position of the pivot point of the vessel, will usually achieve max-

imum effect from the arrangement.

APPENDIX C 205

Bow thrust

compartment

Bow

rudder

compartment

Box Thrust Units Bow Rudder Fore Peak Tank

Frame spacing 685 mm Frame spacing 610 mm

Appendix C-H8530 4/9/07 10:07 AM Page 205