Rules

Design of steering gears have been influenced over the years by the rules and regulations of national authorities and classification subjects. Any changes of real substance tend nowadays to originate from the international

Maritime Organisations(I.M.O.) conventions and regulations. Classification society requirements are as follows;
1. All ships to have power operated main gear capable of displacing the rudder from 35^o port to 35^o starboard at the deepest draught and at maximum service speed. Must also be capable of displacing the rudder from 35^o port to 30^o starboard in 28 seconds and vice versa.
2. The auxiliary gear must be power operated and capable of being brought rapidly into action. The auxiliary gear is only required to steer the ship at either 7 knots or half service speed
3. If the main gear comprises two or more identical power units, then a single failure of either power unit or piping must not impair the integrity of the remaining part of the steering gear
4. Each power unit must be served by at least two electrical circuits from the main switchboard. One circuit may pass through the emergency switchboard. All circuits to be separated as widely as possible throughout their length.
5. All power operated gears to be fitted with shock relieving arrangements to protect against the action of heavy seas.
6. An efficient brake or locking arrangement to be fitted to enable the rudder to be maintained stationary
7. the maximum power developed by the gear is proportional to T x S
   where T = rudder torque
   S = Speed of rudder movement
   also T = A x P x sinq x V2
   where A = rudder area
   P = centre of pressure
   q = rudder angle
   V = velocity of the ship
   

Special requirements

Owners may specify additional requirements such as faster hard-over to hard-over time, strength of components above that required by the Rules, additional control points and additional duplication,

New tankers of 100 000dwt and above-shall comply with the following

The main steering gear shall comprise of either
• two independent and separate power actuating systems each capable of meeting the hard over port to 30^o starboard in 28 sec requirements,
  or
• at least two identical power actuating systems which acting simultaneously in normal operation, shall be capable of meeting the hard over requirements. Where necessary to comply with this requirement inter connection of hydraulic power systems shall be provided. Loss of hydraulic fluid from one system shall be capable of being detected and the defective system isolated so that the other system shall remain fully operational

In the event of loss of steering capability due to a single failure other than the tiller, quadrant or components serving the same purpose (these are excluded from single failure concepts), or seizure of the rudder actuators. The steering capability shall be regained in not less than 45 seconds after the loss of one power actuating system.

Steering gear other than hydraulic should meet the same standards.

Example of suitable system permissible for all ships

The system shown consists of two sets of rams but could equally be two rotary vane units. With no power on the solenoids are in by-pass mode with oil being allowed to pass freely from one side to the other. When an electric motor is started the control pump supplies oil to the solenoid shutting it. High pressure oil from the main unit is now fed to the rams as required. The other unit remains in by-pass until the electric motor is started.
Low level alarms are fitted to the tanks. Low low changeovers may also be fitted so that in the event of oil loss from one system, the other system is started.

New tankers between 10 000gt upwards to 100 000tdwt

For these tankers the single failure criterion need not apply to the rudder actuator or actuators subject to certain requirements being fulfilled. These include a requirement that steering be regained within 45 seconds following failure of any part of the piping system or power units and a special stress analysis of non-duplicated rudder actuators.
The left hand unit is shown in operation.
For this basic arrangement the power units must be identical

New ships 70 000gt and upwards

system suitable for all ships except tankers of 10 000 gt and above

The main steering shall comprise two or more power units and that the main steering gear is so arranged that, after a single failure in its piping system or in one of the power units the defect can be isolated so that steering can be speedily regained.
'Speedily' is intended to mean the provision of duplicate hydraulic circuits or , for example, a conventional four ram steering gear with a common hydraulic circuit with appropriate isolating valves

New ships of less than 70 000 gt and tankers less than 10 000 gt

suitable system

Single failure is not applicable as a rule, however, attention is drawn to the requirement that auxiliary steering gear be independent of any part of the main gear except the tiller. There is no requirement that main and auxiliary power units be identical.
The auxiliary steering gear must be capable of putting the rudder over from 15^o from one side to the other in not more than 60 seconds with the ship at its deepest draught and running ahead at half maximum speed or 7 knots.

Existing tankers of 40 000gt and upwards

The steering gear shall be arranges so that in the event of single failure of the piping or one of the power units, steering capability can be maintained or the rudder movement can be limited so that steering capability can be speedily regained by
• An independent means of restraining the rudder
  or
• fast acting valves to isolate the actuator or actuators from the external hydraulic piping together with a means of directly refilling the actuators by a fixed independent power pump and piping system
  or
• An arrangement so that, where hydraulic power systems are interconnected any loss of hydraulic fluid from one system shall be detected and the defective system shut off either automatically or remotely from the bridge so that the other system remains intact

Requirements for all new ships
• Administrations must be satisfied in respect to the main and auxiliary steering gear provided for every ship that all components and the rudder stock are of sound construction
• Every component, where appropriate, utilise anti-friction bearings which will be permanently lubricated or provided with lubricant fittings
• Parts subjected to hydraulic pressures should be designed to cope with 1.25 maximum working pressure when the rudder is hard over at maximum draught and service speed
• special requirements for fatigue resistance( due to pulsating hydraulic pressure), relief valves and oil cleanliness
• Low level alarm to be fitted to each hydraulic reservoir.
• Fixed storage capacity sufficient to recharge on system

Auxiliary steering gear

The other set of steering (auxiliary ) may be an arrangements of blocks and tackles or some other approved alternative method. The auxiliary steering gear need only be capable of steering the ship at navigable speed, but it must be capable of being brought speedily in to action in an emergency. Navigable speed is one half of maximum service speed ahead or 7 knots whichever is the greater.
The auxiliary steering gear must be a power operated type if the rudder stock exceeds 230mm for passenger ships and 250mm for cargo vessels. No additional means of steering is required when electric or electro-hydraulic steering gear is fitted having two independent motors or two sets of pumps and motors.

Electrical Supply

Short circuit protection and overload alarm are to be provided in steering gear circuits. Indicators for running indication of steering gear motors are to be installed on the navigation bridge and at a suitable machinery control position. Each electric or electro-hydraulic steering gear shall be served by at least two independent circuits fed from the main switchboard. Cables for each circuit led through a separate route as far apart as possible so that damage to one cable does not involve damage to the other. A change over switch is fitted in an approved position to enable power supplies to be interchanged. One circuit may pass through an emergency switchboard.

Rudders

In passenger ships where the rudder stock exceeds 230mm, an alternative steering position remote from the main position is to be provided. Failure of one system must not render the other system inoperable. Provision made to transmit orders from bridge to alternative position. The exact position of the rudder must be indicated at principal steering positions. An efficient braking or locking device must be fitted to the steering gear to enable the rudder to be held stationary if necessary. Spring or hydraulic buffer relief valves fitted in steering gear system to protect the rudder and steering gear against shock loading due to heavy seas striking the rudder. Suitable stopping arrangements are to be provided so as to restrict the total travel of the rudder. Stops or cut outs on the steering gear are arranged so that it operates on a smaller angle of helm than the rudder stops.

Rudder restraint

Since failure of a single hydraulic circuit can lead to unrestricted movement of the rudder, tiller and rams, repair and recharging may not be possible. Difficulty arises with which the speed a restraint whether in the form of a mechanical or hydraulic brake can be brought in to use. Due to the possibility of considerable damage occurring before it could, regulations have concentrated on continuity of steering rather than a shut down and repair solution