Wheeled vehicles steer from the front. So why do boats and most planes steer from the rear, even boats that have no propellers?
Steering is only a matter of generating appropriate forces on the vehicle in a stable manner and so, in principle, vehicles can be designed to steer at either or both ends, provided the designer understands the dynamics of the system.
The tandem-rotor Chinook helicopter, the SRN4 hovercraft, the roller skate and the skateboard all steer at both ends, as do a small number of cars and a smaller number of bikes and motorbikes. Most aircraft and helicopters steer from the front because the steering is carried out by ailerons, or hinged surfaces, on the wings or the lateral cyclic tilt of the main rotor. The rudder or tail rotor is only used to balance the turn and to keep hull drag to a minimum.
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鈥淭he Chinook helicopter, the SRN4 hovercraft, the roller skate and skateboard steer at both ends鈥�
In the case of motor boats, the rudder is placed directly behind the propeller, so some steering is available even at low speed. However, in yachts, rudders at the front are feasible and might be more efficient because they would operate in undisturbed water, free from turbulence caused by water passing over the hull. However, a forward rudder is more vulnerable than one at the stern protected by the hull.
In high-speed craft like speedboats, which skim over the surface, only the rear of the hull is in contact with the water, so the rear rudder dominates by necessity. Hydrofoils may steer from front or rear. Generally the most lightly loaded foil steers.
Boats that have no propellers, such as water-jet boats, steer by turning the jet 鈥� which clearly has to be at the back.
Fork-lift trucks steer from the back because this gives them a smaller turning circle and the heaviest laden wheels can be fixed. In practice, the end from which a vehicle steers will be a mixture of economics, tradition and special operating constraints.
Incidentally, there are some vehicles that steer from neither end. Tanks, bulldozers and other tracked vehicles steer from both sides, as do rowing boats.
John Watkinson, Burghfield Common, Berkshire, UK
鈥� When a boat or a car is travelling straight ahead and the pilot or driver wishes to turn, their problem is the same: they have to find a way to generate a side force. This side force can only be generated where the vehicle connects with the medium it is travelling through or over. A car touches the ground at four points and can conveniently generate a side force on its front wheels by making them point right or left of the direction of travel. The front wheels then move sideways and the rear wheels will follow.
Unlike a car, a boat connects with the water along its entire length. It is more appropriate to liken it to one of the car鈥檚 wheels rather than the whole car. To generate a sufficiently large side force, the whole boat must be made to point right or left of the direction of travel. This can鈥檛 be done with an externally applied torque, or turning force, as it is for the car鈥檚 wheels, because there is no external fixed body against which the torque can react. The method is more subtle: a rudder is deflected to generate a small side force well behind the boat鈥檚 centre of gravity. The entire boat then rotates in response to this small side force and, once rotated, the entire boat generates the larger side force required to turn it.
This technique works whether the rudder is placed at the front or the rear. But in either case the steering force is relatively small: the actual turning force is generated from the entire boat surface in contact with the water.
鈥淩udders are usually at the rear because they give a naturally stabilising steering effect鈥�
Rudders are usually placed at the rear rather than the front, however, because there they produce a naturally stabilising steering effect. Forward rudders can be inherently unstable, so they are usually only used where there is good reason: for example, on some missiles, where a rocket motor takes up the rear portion, and it is convenient to package the target seeker, autopilot and control system together in the missile鈥檚 nose.
Chris Jermey, Gumeracha, South Australia
鈥� Wheeled vehicles travel over the ground and depend on friction between the tyres and the ground for propulsion and steering. To test this, add ice or a deep puddle to the road and watch what happens to the vehicle 鈥� it can now be steered from either front or rear, or even by all four wheels together.
Aircraft and ships鈥� hulls are driven through a medium rather than over it and so their method of turning exploits the flow of water or air created by their movement through it. Altering the angle of the rudder blade to the centre-line of the ship redirects the water flow to push the stern left or right. It also creates a certain amount of lift on the lee side which amplifies this turning effect. On a propeller-driven ship, the wash from the propeller augments the flow over the surface of the rudder blade.
A rudder hung at the stern and hinged at the leading edge needs relatively little turning force to move it. This arrangement is also self-correcting, and will always swing back in line with the keel because that is where the flow pressure on either side of the blade is in balance. It is also protected from damage by wave action, which is most likely at the bow when a ship is moving.
Tugs and other vessels operating in sheltered and confined areas may have rudders at both ends. But because the foremost edge of the bow rudder is not supported, there is a risk that it will be jammed hard over or ripped off if used at high speed. This is why most motor vessels when travelling astern do so slowly and are hard to steer.
Aircraft rudders operate in a similar way and so are positioned at the rear of the aircraft. Some early pusher-propeller aircraft experimented with steering planes at the front, but like ship-bow rudders these were difficult to control and easily damaged.
Jim Munrot, Strathaven, Lanarkshire, UK