Ships are steered by rudders. These are moving plates at the back of the ship that twist. These work in two ways, firstly by pushing the water flowing away from the propeller away at an angle and secondly by deflecting the water as it moves past the ship. This puts a force on the rudder which acts on the bearings that hold it in place ( the pintle ) and this pushes the back of the ship sideways. The ship is long and thin and this force acting on the back causes the ship to turn in the direction of the rudder. However the front ( bow ) does not move sideways very much - in fact the back ( stern ) of the ship swings out away from the direction of turn and the ship then pivots about a point generally about one third of the way along the length. This has the effect of throwing the stern towards an obstruction before the ship starts to turn away from it ( That is what did for the Titanic ! )
These are Balmoral's rudders and propellers -photographed on a 1:48 scale model. You can see rudders and the pintle where they enter the hull, the propellers, the A brackets, tail shafts and stern glands. On this model the propellers are 'handed' so that they each turn a different way. This makes the ship more directionally stable as any twisting effect from the propellers are cancelled out. The real Balmoral has four bladed props !
HOW A RUDDER TURNS A SHIP.
In this diagram, the rudder is in red and the fulcrum or pivot point about which the ship turns is the red star. The yellow lines shows the path of the front of the ship and the blue line shows how the back takes a much wider path which tends to tighten as the turn progresses.
FORCES ON A RUDDER
This is an attempt to show the forces acting on a rudder ! The propeller is the blue disc with a yellow edge, the blue arrows show the water direction, the red arrow shows the resulting thrust ( pushing force ) generated by the propeller and rudder, and the orange arrow shows the direction in which the back of the boat will try to go. The black arrow shows the path of the ship through the water.
In the left hand diagram, the rudder is straight ahead and the propeller wash and thrust are both going in a straight line. The ship moves straight ahead.
In the right hand diagram, the rudder is turned to the left and is deflecting some of the water from the propeller to the left. This puts a sideways force on the rudder as the water changes direction when it strikes the rudder and reacts by pushing it the opposite way. This force which is trying to push the rudder straight acts at an angle giving a turning force which is transmitted through the rudder pintle to the back of the ship. However there is still water flowing directly away in a straight line behind the ship, so the turning effect is a combination of the sideways force acting on the rudder and that pushing the ship along. Because the rudder is at the back of the ship, the effect is to push the back, away from the direction in which the rudder is turned, and because the boat then pivots about a point towards the front, this causes it to turn in the direction the rudder is pointing.
There are additional ways to steer ships. Some have bow rudders to help push the front round, there are bow thrusters, ducted propellers that push water through a tube across the inside of the hull to move the front sideways , and it is possible to alter or reverse one of the engines ( if the ship has more than one ) to help pull the bow round. Most ships with two propellers will have two rudders, as a rudder in the wash from the propeller will have far more effect than one which relies on water passing it from the forward movement of the hull.
This is also a problem when trying to go backwards ( astern ). There is no speed of water over the rudder and no propeller wash to help as the water is flowing the wrong way ! Many ships have great difficulty going astern, particularly at low speed and in some cases if there is only one propeller and it is a big one, the turning effect of the screw can push the stern in one direction regardless of which way the rudder points !
Rudders are big and heavy to be able to handle the forces applied to them, a large ship like Balmoral needs steering gear. On early ships this was a wooden rod ( tiller ) mounted on the top of the rudder shaft ( stock ) but this was difficult to control , especially in a rough sea, so a steering wheel was introduced with ropes to link it to the tiller. This reduced the effort needed to turn the rudder but required a lot more turns of the wheel, and on sailing ships it could sometimes take several men to hold the steering steady. As ships developed and used engines, the steering position moved from directly above the rubber where ropes could be used to move it, to a position near the front. This required a linkage to take the movement from the steering wheel right along the ship to the rudder and ropes didn't work as they were too flexible. The force needed to turn a large rudder was huge and only massive chains and shafts could transmit the load - this wasn't practical, so someone had to invent the powered steering gear system so that the person steering (helmsman ) could turn the wheel easily, there would be small linkages running the length of the ship and a big powerful machine above the rudder to transmit the movements from the input shaft to the steering.
On Balmoral, the steering wheel is on the bridge in the wheel house. It is connected by gears to a series of shafts that run down from the bridge, through ceiling of the observation saloon and then into the engine room. The shafts then continue along the ship to the 'steering flat'. This the area under the rear poop deck which houses the top of the rudder stocks and the steering engine. This is a large hydraulically powered pump that does the same job as a power steering system on a car. It takes the control input from the steering shaft and applies it with greater force to the rudders. In this case as there are two rudders they are linked with a rod so that they both move together.
The steering wheel is on the bridge and turns a series of shafts that run through the ship .................
The input shaft from the steering wheel on the bridge is marked with red and white tape , here the shaft is coming down from the deck house into the engine room, through the red gearbox and out towards the side of the ship to clear the engines ......
..... then it runs right along the inside of the hull and reaches the steering flat where it has more gearboxes to bring the shaft back into the middle of the ship, then it enters the red gearbox before going aft..........
to the steering engine................
which amplifies the force from the man on the wheel and operates a hydraulic ram that is connected to the rudders................
You can see the Port side tiller arm, the linking rod and part of the control system painted red.....
........ and that system changes the direction of the rudders and controls the ship. The propeller wash is deflected by the rudders and this greatly improves the steering by pushing the water to one side. This is a photograph of Balmoral in dry dock and you can see the propellers and rudders.
If the steering system fails, there is a small emergency steering wheel on the poop deck and this can be connected to the rudder. The main steering engine can then be disconnected from the final linkage and the ship can be steered by hand. Its a hard job but better than having no directional control at all!
Here is the emergency steering wheel and part of the emergency steering system in the steering flat. The man at the wheel would be unable to see where the ship was going but would be told how to steer by a chain of people passing orders from the bridge - or he might be given a portable walkie talkie radio !
In the event of steering failure it might be possible to steer the ship by using the two engines at different power settings or even running one backwards - this is sometimes done to help maneuvering and is known as 'splitting the sticks' as the engine telegraph handles point in opposite directions. It is not a good method of steering as the rudders have a very powerful effect and unless they are dead straight they would have a huge effect on the steering.
Emergency steering wheel.
Emergency gearbox and steering rod.
In the right hand photograph, the bar with the red clip ( clevis ) on top of the gearbox is the part that is connected to the tiller arm on the right of the photograph when the emergency steering is needed.
All this is contained under the poop deck at the very back of the ship.