Ship Steering Gears
Large commercial ships are steered using rudders, but this is not a simple task given that a rudder may weigh several tonnes. Most ships use some sort of electro-hydraulic system to turn the rudder, although the traditional -and much older- method was purely mechanical. This article focuses on a common type of steering gear system, the four-ram steering gear system.
Four-Ram Type Steering Gear
Marine Steering Gear Types
Common steering gear types include the ram type and rotary vane type. Ram type steering gears typically use two or four rams. Both system types are electro-hydraulic because they use electric motors for pumping of a hydraulic fluid. The hydraulic fluid used is almost always oil based, although a water glycol mixed fluid could also be used.
Note: Water glycol systems are generally used for very large hydraulic systems only.
Hydraulic System Parts
Hydraulic oil systems can be quite complicated, but will often contain the following common parts:
- Storage tank – a vessel where the hydraulic fluid is stored.
- Pump – used to pump fluid throughout the steering gear system. Most steering gears use some sort of piston pump for the pumping of hydraulic fluid.
- Valves – for regulating, starting, and stopping, of hydraulic fluid flow. Valve designs include non-return (check), directional, and relief types.
- Directional control valves – for directing the flow of hydraulic fluid within the system.
- Relief valves – to relieve hydraulic system pressure in an over-pressure scenario. Over-pressure may occur for various reasons, including external factors such as rough weather conditions acting upon the rudder.
- Actuator – a piston, ram, or vane, which is actuated by hydraulic oil pressure. The type of actuator used depends upon the steering gear’s design.
Steering Gear Parts
- Tiller – converts the linear motion of the ram to rotary motion at the rudder stock.
- Cod piece – a cylindrical shaped piece connected to the rams; the cod piece is located between the forked tiller.
- Rudder stock – transfers the motion of the tiller to the rudder. In its simplest form, the rudder stock may be just a long cylinder to which the tiller is connected at the top, and the rudder at the bottom.
- Rudder – the item used to control the direction of the ship.
- Rams – rams installed within hydraulic cylinders. Each ram is installed opposite to its counterpart, along a linear axis.
- Local rudder angle indicator – a piece of metal plate installed onto the steering gear tiller. The plate is marked with numbers indicating the angle at which the rudder is currently at.
- Main electric motor – hydraulic fluid is circulated through the system using a three-phase electric motor; the three-phase electric motor is usually connected to a piston pump.
- Pump – a positive displacement pump used for circulating fluid throughout the system.
- Servo motor – a servo motor and associated pump are used to control the position of the directional control valves.
- Control box and repeater – used to relay the position of the rudder to the wheelhouse and to ensure the rudder position is known -and correct- at all times.
How Ship Steering Gears Work
Ram Type Steering Gear Operation (Basic Working Description)
Hydraulic fluid is drawn into the steering gear main pump via the suction inlet. The fluid is then pumped out of the discharge port, through a directional control valve, and to its designated cylinder. Main steering gear pumps are typically mounted onto the hydraulic oil storage tank(s), although they may be installed remotely and connected via hydraulic hoses.
Four Ram Type Steering Gear Hydraulic System Diagram
Directional control valves change position to deliver hydraulic fluid to different cylinders. These types of valves are often electrically actuated using solenoid valves (electromagnetic valves), or hydraulically actuated using fluid from a servo pump. Springs can be used to return a directional control valve back to a fail-safe position. Actuating the directional control valve(s) changes the flow path of hydraulic oil through the system, thus allowing different rams to be actuated as a single pair (two ram design), or several pairs to be actuated simultaneously (four ram design). When no actuation of the rudder position is required, directional control valve(s) can be changed to a position that allows for recirculation of the hydraulic fluid from the pump directly back to the tank. When fluid is recirculated, the hydraulic circuit controlling the rams is ‘locked’, which means no fluid moves into or out of the system (thus the rams are ‘locked’ in position).
Four Ram Type Steering Gear Working Principle
When hydraulic oil is delivered to a cylinder, it forces the ram within the cylinder to move in a linear direction towards the opposing cylinder. As the ram moves, it forces hydraulic oil in the opposing cylinder to be discharged back to the steering gear pump, or steering gear pump tank (design dependent). It’s possible to change the direction in which the ram moves by changing the position of the directional control valve(s), thus changing the flow path of the fluid through the system.
The most common type of pump used for steering gears is of the piston design. Piston pumps can be split into two main categories, the axial piston pump (also known as a hele-shaw pump), and the radial piston pump. Piston pumps are a type of positive displacement pump and are able to pump air.
Axial Piston Pump
Steering Capability
A steering gear should turn the rudder to a maximum of 35 degrees to remain efficient. Turbulence and Eddy currents begin to occur above 35 degrees, which makes the rudder inefficient; the turning effect is also not increased when operating the rudder above 35 degrees, although its braking effect (slowing down) increases.
Safety Features
Almost all critical engineering systems have some form of redundancy built into them. For example, a steering gear may require only a single hydraulic powerpack (which consists of a pump, motor, valves, and tank) to operate, but two are usually installed. The reason for this setup is because a safety factor is built into the system. If one powerpack should fail, the other can still remain in operation, which means the rudder can still be operated and the vessel safely manoeuvred. This type of setup is referred to as a ‘fully redundant system’, or an ‘n+1 system’.
Online Video Course Description
In this Ship Steering Gear Fundamentals Online Video Course, we will explore how hydraulics are used to steer large seagoing vessels. We will start by looking at the evolution of ship steering mechanisms, from the days of sailors at the helm of wooden ships, to the fully-automated systems we use today. We will then review the major parts of modern ship steering gears, including rams, cylinders, tillers, and rudder angle indicators. We will learn how pumps, motors, directional control valves, and relief valves work together to manoeuvre a vessel with precision, even across long distances.
This course provides practical insights using real-world demonstrations. By the end of this course, you will be able to:
- Recognise the technological advancements that have shaped the field of marine steering.
- Comprehend the working principles of hydraulics and their application in controlling a ship's rudder.
- Gain an in-depth understanding of the mechanical components that govern ship steering.
- Understand how hydraulic technology enables precise control and safety in maritime navigation.
- Appreciate the critical role of redundancy in ship steering systems, ensuring reliability and safety during navigation.
Whether you're an engineering enthusiast, deck or engineering cadet, or simply curious about the inner workings of maritime vessels, this course is your gateway to unlocking the mysteries of marine steering gears and how they work.
Join us on this educational voyage!