Why Don't Penguins' Feet Freeze?: And 114 Other Questions - Mick O'Hare (2009)
Chapter 8. Troublesome transport
Why are the coloured lights in traffic signals universally arranged red over amber over green, as opposed to the universal practice of railway signals which have green over amber over red (for a three-aspect signal)?
Parkes, New South Wales, Australia
The difference between road and rail usage derives from the history of railways and the primacy of safety. The old mechanical railway signalling arms were designed so that failure, which would be in the ‘down’ position, meant stop. The illuminated part of the signal consisted of two coloured glass panels in the far end of the signal arm, beyond the pivot, which moved in front of a fixed lantern. Even though the higher of the two glass panels was the red panel, it showed when the signal was down and this meant stop. While railways retained mixed mechanical and electrical signalling the signals had to be compatible. Therefore, the new electrical signals showed red at the bottom so that train drivers always equated either signal in its down or bottom position with the order to stop.
Road signals had no mechanical forerunner and are designed so the most important light, the red, can be seen from the greatest distance. This means putting it as high as possible. Anyone who has used Junction 3 of the northbound M25 at night will appreciate this. Additionally, visibility for railway signals is not the same issue as it is on the roads. Railway signal sites are carefully selected.
Gerald Dorey is only partly correct in his historical explanation for the order of railway signal lights. Indeed, he overlooks the large parts of the country where lower quadrant semaphore signals (in which horizontal means danger and 45° down means clear) were used. In these signals the red light was therefore at the top.
The main reason for the red light being at the bottom in modern British signalling installations is the weather. To ensure visibility in bright sunshine, each colour light has a long cowl or hood above it. In the winter, however, snow can build up on these cowls and obscure the light above. Being at the bottom, the most safety-critical red light has no other light below it and therefore no cowl, so snow cannot build up to cover a red light.
There are two kinds of mechanical, or semaphore, signals. In the older lower quadrant type, the arm slopes downwards from the pivot to show clear or green, is returned to the horizontal by a counterweight, and the lamp glasses are red above green. In the newer upper quadrant type, the arm slopes up for clear, returns by its own weight (as in the scene in the classic film The Lady Killers) and the lamp glasses are in fact side-by-side. Red is nearer the pivot and green is to its right on the outside.
In both, the horizontal arm means stop but this is not synonymous with down, which means opposite things in the two cases. Red arms are always used in stop signals but distants (meaning warning) operate similarly. However, on distants the arm and lamp glass are yellow, not red, and these mean ‘pass with caution’.
The arrangement of multiple-aspect, coloured light signals has nothing to do with that of arm position. The red is at the bottom simply because it is the position nearest the driver’s eyes; yellow is above, then green and, in four-aspect signals, the second yellow is topmost, above the green.
C. C. Thornburn
Aston University, Birmingham, UK
Road users do not have to pass a colour vision test, and therefore the position of the red, amber and green lights must always be the same, so the light illuminated can be recognised by position as well as colour. Such signals are usually placed at sites where a speed limit applies and, because of the higher braking coefficient of rubber tyres, the driver can still stop safely even after identifying a red indication only by its position.
The train driver, whose colour vision is checked regularly, has to act on signals at a far greater distance to ensure the train can be stopped in time. On main lines the indication has to be identified accurately at long range, when it is impossible for the driver to see its position and where he must rely solely on its colour.
The original question was, in fact, incorrect, because there is no universal railway layout of green over yellow over red (in railway parlance, the caution indication is referred to as yellow, not amber). In the past, some signals only had a single lens, the different indications being given by interposing coloured filters over the beam. The only fixed rule with the layout of signals with multiple lenses is that the one which exhibits the red indication is mounted nearest to the line of the driver’s eyes. In some places, therefore, it may be at the top, as it is with a road traffic signal.
On high-speed lines, it is necessary to have a double-yellow indication, which is exhibited by the signal before the one showing a single yellow for caution. That, in turn, will be a further three-quarters of a mile or so before the one showing red for stop. This gives a run of two signals warning of a stop signal ahead. Such double-yellow signals normally have the two yellow lights separated by the green one, to maximise their visual separation when viewed from a distance.
P. W. B. Semmens
We are all familiar with the popping ears associated with takeoff and landing in an aeroplane. This is caused by changes in pressure. Given that the aircraft cabin is artificially pressurised, why isn’t the internal pressure maintained at one level throughout the journey?
For reasons of fuel economy, large civil transport aircraft have to fly at altitudes far in excess of those capable of sustaining life. Whereas 5500 metres is about the maximum altitude at which a person can live for any extended period, a subsonic passenger jet has the best fuel economy when flying at around 12,000 metres.
Aircraft manufacturers, therefore, have no choice but to pressurise the interior of a passenger aircraft. This poses huge technical problems. At 12,000 metres, where the pressure is about one fifth of that at sea level, the pressure inside is trying to burst the fuselage apart. This pressure has to be contained and all the stretching and flexing of the fuselage during a flight has to be kept within safe limits. So if the pressure differential between inside and outside is kept to a minimum, a cheaper and lighter fuselage structure can be used.
For civil airliners this means that the pressure inside during cruising is kept at the lowest possible safe level – 2500 metres. This is about the maximum altitude which a normal healthy person can be subjected to without ill-effects. Even so, unfit people, those with respiratory illnesses and those who have sampled a few too many duty-free drinks might still feel ill, even at this altitude.
There is another problem: all airfields are not at the same altitude. In an extreme case, a flight from Heathrow in England to La Paz in Bolivia would entail going from sea level to around 5200 metres, where the air pressure is about half that at sea level. Under these circumstances it is just not possible to maintain the same pressure throughout the flight. Imagine what would happen if the pressures inside and outside were not the same at the time the doors were opened: the effect would be quite spectacular and most undesirable.
As for the ear popping, nowadays, ‘for your safety and comfort’, the internal pressure is imperceptibly reduced, all under computer control, as the aircraft climbs. It is gradually increased (or, in the case of La Paz and other high-altitude airports, decreased) during descent so that, as the aircraft is coming to a stop on the runway, the pressure inside and out is the same. This is normally sufficient for your ears to adjust, but if all else fails, pinch your nose and gently but firmly increase the pressure in the nasal cavity until you feel the pressure equalise.
An advantage of flying by Concorde was that the fuselage had to be especially strong to fly at very high altitudes, so the cabin pressure did not have to be reduced below that experienced at 900 metres.
Alton, Hampshire, UK
Why are the windows of a ship’s hull round? And when did this design begin?
Oban, Strathclyde, UK
I assume that your correspondent is referring to old pictures and prints of wooden ships, where the portholes (usually gun ports) are square or rectangular, and is wondering why such ports are round in steel-hulled ships.
When ships were made of wood, the architectural material was fibrous and fairly flexible (wooden ships really did creak and this was caused by timber flexure from wave action). However, wood – especially wet wood – is highly resistant to fatigue stress. Try breaking a piece of wet willow by repeated reverse flexure, and then try the same process with a mild steel bar or rod of similar section. Ferrous metals (indeed, most metals), are highly prone to crystalline fracture as a result of changes to the grain structure arising from repeated stress reversal. The effect depends upon the section, heat treatment, carbon content and any alloying elements present.
Towards the end of the 19th century, steel hulls became universal for merchant vessels, and subsequently for warships. Naval architects found out pretty quickly that any rectangular or square hole in a ship, whether on a deck (a hatch) or in the hull (a porthole) was a source of metal fatigue, commencing at the corners. The hull or deck would literally rip, due to flexure cycles brought about by wave action; the rougher the seas, the greater the magnitude of the stress. The unlucky sailors found that their ship was most likely to fall apart when weather conditions were at their worst. Thus, naval architects specified circular portholes, and radiused corners for deck hatches. This left no sharp corners for stress concentration.
Aldershot, Hampshire, UK
The following paradox has puzzled me since I was a child. A fly is flying in the opposite direction to a moving train. The fly hits the train head-on. As the fly strikes the front of the train, its direction of movement changes through180°, because it hits the windscreen and continues as an amorphous blob of fly-goo on the front of the train.
At the instant it changes direction, the fly must be stationary and since, at that instant, it is also stuck on to the front of the train, the train must also be stationary. Thus a fly can stop a train. Where is the logical inconsistency in this (or does it explain something about British railways)?
Evanston, Illinois, US
You are right. A fly does stop a train, but not the whole train, just part of the small local area where the fly makes contact, and then not for very long.
All objects, no matter how rigid they seem, are flexible to some extent. So the train’s windscreen, on being struck by the fly, deflects backwards very slightly. That small piece of train not only stops for a short period but can actually move backwards.
It takes considerable force to do this (glass being fairly rigid) but it should be remembered that the forces involved in any type of impact are typically quite large.
The force exerted by the fly on the train is the same size as the force exerted by the train on the fly – a large force. And such a force acting on the small mass of the fly gives rise to a very large rate of acceleration. In fact, the rate of acceleration of the fly is so great that it accelerates up to the speed of the train in only the short distance by which the windscreen has been deflected.
Having got the fly up to speed, the windscreen then springs back to its original shape. Because it springs back very quickly the deformed part actually overshoots its original position and a vibration is then set up as it springs back and forth trying to regain its original form. This gives rise to the sound we hear when the fly hits the windscreen.
This simple picture is complicated by factors such as the crushing of the fly’s body and inertia effects in the glass, but it does demonstrate the principles that are involved.
Perth, Western Australia
The questioner is correct in the assumption that the fly must, at some point, be stationary. However, at this point it is not ‘stuck’ on to the front of the train.
As soon as the train’s front window touches the front of the fly (ignoring the effect of the wall of air that is pushed in front of the train), the fly is accelerated forward in relation to the train. During the very short, but finite, period of time, that it takes the train to cover the length of the fly’s body, the fly is being compressed and accelerated. Thus, in the instant that the fly is stationary, perhaps its front 10 per cent has become goo on the train’s window. The train has maintained a constant speed during this process. By the time the front of the train has completely caught up with the whole of the fly, some 2 × 10-4 seconds later at 200 kilometres per hour, the fly has been accelerated up to the speed of the train and continues, now completely flattened, to move with it.
A slightly more pedantic point is that, by conservation of momentum, the train will be very slightly slowed, although it will quickly build up to its original speed. The acceleration felt by the fly, if accelerated by 200 kph over 1 centimetre, is around 3 × 105 metres per second per second – about 30,000 g. The force felt by a 1-gram fly and the window is around 300 newtons.
Richmond, Surrey, UK
When the train hits the fly, the front few nanometres of the windscreen’s impacting surface stop momentarily, the next few nanometres suffer elastic deformation, and the rest of the train continues at full speed.
After the impact, the compressed windscreen material will recover, accelerating its front edge up to the full speed again, and showing virtually no sign of damage (unlike the inelastic deformation of the fly).
This is a slight oversimplification as, in practice, an elastic stress wave will propagate backwards into the train, and the front surface will oscillate until the motion is cancelled out, but such effects will be unimportant in the case of the fly and the train. Where the masses are more equal, as in the case of colliding cars, the additional motions within each structure may be important as, for example, they may determine the type of injuries suffered by the occupants.
M. G. Langdon
Farham, Surrey, UK
Readers’ explanations about the fly hitting the train cover many aspects, from the width of the fly to the pliability of the windscreen. (What if the fly hits the boiler instead?)
But they completely miss the implied point of the question, which is philosophical rather than physical. For ‘fly’ substitute ‘one atom of the fly’. This is just a rerun of the paradox posed by Zeno of Elea. Around 450 BC he said that a moving object is always in motion, and yet at any given time it is somewhere (that is, stationary). We humans cannot see, measure, or imagine an infinitely small time any more than we can truly imagine infinity. We never will.
R. K. Hendra
Through the hole
I recently did a parachute jump for charity and the one disconcerting thing about the jump (apart from a fear of heights) was the large hole at the top of the parachute. Why is it there? Does it help in any way to reduce the drag on the chute?
In the days before the apex vent (the disconcerting hole in the top of the parachute canopy), the only way that the air trapped underneath the parachute could escape was to spill out from one edge of the canopy, thereby tilting it and throwing the hapless parachutist to one side.
As the canopy swung back, more air would spill out from the opposite side, setting up a regular, pendulum-like oscillation (watch any footage of Second World War parachutists and you will see this).
As you can imagine, hitting the ground during a downswing was understandably hazardous, especially if it was also a windy day. The apex vent, by allowing the air to leak slowly out of the top of the parachute canopy, prevents this wild oscillation and makes for much safer landings.
Another benefit of the apex vent is that it slows down the opening of the parachute. Without the vent, air inflates the canopy much more abruptly, and it can damage the parachute or bring tears to the eyes of (particularly male) jumpers.
Why do aeroplanes have such small windows, and why are they positioned so low in the fuselage that most people have to bend down in order to see other aeroplanes on the tarmac?
New York, US
As with many things concerning the design of an aircraft, the final arrangement of various parts is based upon a series of compromises. An aircraft designer’s life would be so much easier if there were no windows at all, but so far the consensus seems to be that we should have them.
Britain lost the initiative in jet airliner manufacture when the development of the de Havilland Comet in the 1950s suffered a setback through a series of crashes, in part because metal fatigue around the windows led to structural failure.
While windows remain an accepted part of aircraft design, they have since been kept as small as possible. These days they are typically 33 centimetres high. The window has to have three panes: two pressure panes and one interior pane to prevent passengers getting at and damaging the vital ones. The panes are contained in a window unit which is fastened and sealed to the aircraft structure.
It is, of course, much heavier and costlier than the thin sheet of aluminium it replaces, and the structure of the aircraft needs to be reinforced to support it. All this extra weight means fewer passengers or less cargo can be carried, so it reduces airlines’ potential revenues.
Windows also present a maintenance problem. As well as getting scratched and broken, they are a source of air leaks from the cabin and they also suffer from condensation and icing.
The position of the windows varies depending on the aircraft but generally designers try to place them with their centre line a little below the eye level of seated passengers. On the ground this is perhaps too low, but in flight it gives an oblique view of the ground. Little would be gained by positioning the window higher. Because the seats are placed at the widest part of the circular or oval fuselage, the windows would end up angled upwards some 10 or 15 degrees. The only view the passenger would then have in flight would be of the sky. Also, if the top of the window were above eye level there would be a constant problem from the sun’s glare and dazzle. Passengers would just end up pulling down the blinds, which would negate the benefit of having a window in the first place.
It would be useful to have them deeper, but, as I have already said, the weight penalty makes this impractical.
It also has to be remembered that every civil aircraft flying today was designed at least ten years ago, and some actually started life on the drawing board 40 years ago. During this time people have changed and seat design has changed. When these aircraft were developed, the structural design – including the position of the windows – was fixed and the window line has traditionally been used as a convenient breaking point to bring pieces of fuselage shell together. This position having been determined, and production lines then set up with the correct tooling, it would be enormously costly to change it.
In the meantime, people have been getting bigger. Designers have to use what are known as ‘Dreyfuss criteria’ to determine seat sizes. These criteria are constantly changing, but a designer will typically make a plane’s seats big enough to accommodate 95 per cent of American males. If you are particularly tall, this is going to make the window seem lower for you – and people are generally taller than they used to be.
Finally, the present trend in air travel is away from luxurious, spacious layouts to high-density seating. In these circumstances, where the seat pitch is reduced to accommodate as many passengers as possible, the seat base has to be higher to provide leg-room for the person sitting behind. This also makes the relative window position lower still than was originally intended.
The windows on aircraft are so small to make them safe. The first major jet airliner, the de Havilland Comet, had large, rectangular picture windows through which the passengers had a great all-round view. But after a few years in service, the aircraft started to break up during flight.
To find out why, de Havilland put a new Comet into a tank of water and then pressurised and depressurised it repeatedly to simulate the conditions of flight. After the equivalent of two years’ worth of pressurisation cycles (which actually only took a few weeks in the water tank), the airframe was found to fail in the top corner of one of the large windows, which caused a catastrophic break-up in flight.
The windows had to be redesigned and small, round windows set low in the fuselage were created. This solved the problem and the position of the windows remains the same today.
Crowthorne, Berkshire, UK
On the turn
Why, when driving, does the steering wheel of a car straighten itself if you remove your hands after turning it? It doesn’t happen on my friend’s Lego Technics car.
The tendency of the steering wheel to return to the straight-on position is caused by the caster action of the front wheels. This effect is more clearly seen on a shopping trolley where the vertical swivel axis of each wheel is in front of the wheel-to-ground contact point. If you start pushing the trolley when the wheels are not aligned to the direction of the trolley motion, the wheels are pulled around into alignment by the drag force between ground and wheel.
The full explanation is that as the trolley moves forward, the drag force exerted by the ground on the wheel always opposes any relative motion (or slip) between the wheel and the ground.
Unless the wheels are aligned to the trolley motion, the drag force does not pass through the swivel axis, and therefore it produces a turning moment about that axis which always acts to bring the wheel back into alignment.
In a car, the same effect is achieved by inclining the steering axis and ensuring that the point where the axis intersects the ground is ahead of the tyre-ground contact point.
The same is true of a bicycle, as you can see if you hold a broom handle alongside the steering axis of the bike so that the handle touches the ground. You should see that this point is just in front of the tyre-ground contact point.
You can demonstrate the caster action on a bicycle for yourself by pushing the bike backwards and forwards by the saddle while the handlebars are left free. When going forwards, the bike is easy to push in a relatively straight line.
However, going backwards is almost impossible because the front wheel tries to turn round through 180 degrees just as would a shopping trolley wheel. You will also find when reversing a car that the steering wheel loses its tendency to centre itself.
Southampton, Hampshire, UK
Which way is up?
My whole class, including my mathematics teacher, is baffled. We cannot work out how an aircraft can manage to fly upside down without crashing into the ground. We understand that the wings are designed to provide uplift when the plane is flying horizontally. However, when the plane flies on its back as some smaller jets often do, surely the uplift is working in reverse and forcing the plane back down towards the ground. Yet most types of small aircraft seem able to maintain the upside down position for long periods of flight. How do they do this?
Although the aerofoil shape of an aircraft’s wing produces some of the lift in normal flight, the more important factor is the angle of attack – the angle at which the air strikes the wing.
The wings of an aircraft are normally inclined to about 4° to the horizontal when compared to the main body of the aircraft. This is known as the chord angle of the wing.
So even when the fuselage is level, the angle of attack into the oncoming wind is 4°. This produces lift in the same way that your hand experiences an upward force when you hold it at about 45° to the horizontal in a fast-moving stream of air. Your hand does not have an aerofoil shape but the lift that you feel is caused by the angle of attack of your palm to the oncoming wind.
It is this principle that allows an aircraft to fly upside down. The nose is pointed further upwards than in standard flight because of the need to offset the chord angle of the wing. But if the angle of attack is positive compared to the relative airflow over the wing, then an upward force will still be produced. It is this lifting force which overcomes the force produced by the shape of the wing, and holds the aircraft in the air.
The bigger problem that pilots should be concerned about when flying their aircraft upside down is the risk of the engine stopping, because both the oil and fuel systems in most ordinary light aircraft are fed only by gravity. Flying your aircraft upside down can easily cut off the fuel supply because the valve that is feeding fuel to the engine suddenly finds itself at the top of the tank.
On a recent flight, I was studying a card listing items that were prohibited by airlines. I was amazed to see that I couldn’t take a mercury thermometer on a flight. Why on earth not?
Cleckheaton, West Yorkshire, UK
Planes are largely made from aluminium and, surprisingly, a very small amount of mercury can destroy a large amount of aluminium. Despite its apparently inert behaviour, aluminium is actually a rather reactive metal which will combine violently with oxygen in air. However, this reaction quickly produces a thin, tough oxide layer which stops further attack. The process of anodising the aluminium thickens this layer to give better protection.
Mercury has the ability to disrupt this protective oxide layer, and the results can be spectacular. It can dissolve aluminium to form an amalgam which may break up the oxide layer from below – presumably the initial attack occurs through tiny faults in the oxide.
Many years ago a technician working for me spilled a few drops of mercury on his wooden bench, which had heavy aluminium angles screwed round the edges to protect it. Next morning large holes were eaten through the aluminium, the wood nearby was deeply charred, and large fragile towers of friable aluminium oxide had grown like strange corals.
This used to provide a fine chemistry experiment but it is now frowned upon because of the toxicity of the mercury.
On one occasion a passenger in front of me was prevented from carrying a barometer onto an aircraft because it was on the list of prohibited articles, even though this particular barometer was empty. With difficulty I persuaded the staff that it was harmless. They did not realise it was the mercury that was dangerous, they thought it was just barometers per se. I wonder what they thought an altimeter measures . . .
Department of Electronics and Computer Science
University of Southampton, UK
Given the mobility of liquid mercury, the corrosive amalgam may form deep within the structure. An aircraft in which mercury has been spilled must be put into quarantine until the amalgam makes its presence known. Ultimately, the aircraft is likely to be scrapped because the engineering textbooks state that the amalgam slowly spreads like wood rot to adjacent areas.
Air Medical Limited
Kidlington, Oxfordshire, UK
Mercury, along with many other common chemicals, is classified under ‘dangerous goods’ in international regulations developed by the International Civil Aviation Organization, which is part of the UN. You are not permitted to carry this substance, or any article containing it, aboard an aircraft in hand luggage or checked-in baggage. An exception is made for small clinical thermometers in protective cases for personal use.
Should mercury-containing articles need to be transported they must be consigned as air freight. The ICAO rules specify in detail how this must be done.
Don’t think that you can afford to ignore these restrictions. In Britain, endangering an aircraft by taking aboard dangerous goods could result in a charge and hefty fine under the 1982 Civil Aviation Act. In the event of a mercury spillage the aircraft would need to be taken out of service. The airline and/or its manufacturer may try to recover costs from you or your employer.
Freight Transport Association
Tunbridge Wells, Kent, UK
I have noticed that when I travel on an escalator, the handrail always moves at a different speed to the stairs. You would expect it to move at the same speed but it never does. Why not?
The stairs and handrail are designed to move at the same speed and are driven by the same electric motor. The motor connects to a drive gear that moves the steps, and from there a belt turns a wheel that drives the handrail. Although the handrail will ideally move at the same speed as the steps when first installed, it wears and stretches as it is used. As a result, it can change speed. Improper handrail set-up, seized rollers, flat spots or contaminants on the handrail drive surfaces can all affect the speed.
Bellville, South Africa
The handrail can travel at different speeds but it is not supposed to. The American National Standards Institute code ANSI A17.1 requires that the speed of the handrail shall not change when 444.8 newtons are applied against the direction of motion. To meet this requirement, the handrails are sometimes adjusted to move slightly faster than the step. Escalators that are installed under the ANSI A17.1-1990 code require that a handrail-speed monitoring device be installed. If the speed of the handrail changes by more than 15 per cent, all power is removed from the motor drive and the brakes are applied.
Richard A. Kennedy
Richard A. Kennedy & Associates
Maintenance auditors for elevators and other lifting devices
West Chester, Pennsylvania, US
Escalator handrails are moved by the friction of a rubber-tyred wheel operating against the inside of the handrail, and slippage is not uncommon, though seldom uniform. Most commonly a build-up of oil and dirt on the inside canvas of the handrail causes some slippage, although this can be cleaned off and the canvas roughed up to afford more traction. Passengers pulling on the handrail will also make it slip.
The handrail drive runs off the step drive, so the handrail should always match its speed. The typical diameter of a handrail drive wheel is between 1 and 1.2 metres, so 2 millimetres of wear on the drive tyre would result in the loss of roughly 4 millimetres of handrail travel per metre of step travel, which is hardly noticeable.
Other possible causes of handrail slippage include bald patches on the drive tyre or the handrail, either of which would make the slippage highly predictable. In rare cases, with particular manufacturers, the drive chain can stretch to such a degree that it is forced to jump over a cog or two. This results in a loud noise and a noticeable jerk in the handrail.
British Standard EN115: 1995 states that the handrail speed should match the step speed to within 2 per cent. The system driving both the step and the handrail are derived from the same source, so in theory they should run at the same speed. In practice the step system, consisting of precision-made metal components, allows the step speed to be easily and accurately controlled. In contrast, power is transferred to the handrail by friction, and the rubber and neoprene components make the system vulnerable to slippage and stretch from loading and frictional losses. These factors make it more difficult to accurately control the handrail speed, hence the necessity in British Standards to allow a 2 per cent tolerance.
In reality, a small degree of slippage actually increases the level of safety should anything obstruct the handrail system.
When driving a speedboat, why don’t you have to change gear when you change speed, as you do in a car?
Some powerboats do have gears, but these are the exception rather than the rule – Ed.
The difference between boats and cars lies in the way in which the power generated by the engine is translated into movement of the vehicle. In a boat the engine turns a propeller which pushes water backwards. The reaction to this rapidly moving stream of water pushes the boat forward.
If the engine and propeller are well matched there will be sufficient power to turn the propeller, even when the engine is running slowly. If the boat is large it may take some time to accelerate, during which water can be seen streaming away from the stern. Have a look next time you are on a ferry, you’ll see churning water at the back of the ship, even though the ship has yet to move.
In a car the wheels can turn only if the car moves – in contrast to the ferry – but it takes a lot of power to accelerate from standing. Unfortunately, internal combustion engines do not generate much power when they are running slowly, so if the engine were connected to the wheels without a gearbox, the inertia of the vehicle would stall the engine. The gearbox allows the engine to turn rapidly, generating power, even when the wheels are moving slowly. If it were not for the ingenuity of gearbox and clutch designers, the internal combustion engine would have had no future in road vehicles. In contrast, steam engines generate a lot of power from a standing start so steam locomotives can pull away without a gearbox.
On loose surfaces such as sand, a car’s wheels can turn without the vehicle immediately moving too. This is somewhat similar to the ferry, in that sand is thrown backwards as the wheels rotate. However, more sand does not immediately rush in to take its place, so the wheels are likely to dig themselves into the sand until the car is embedded to its axles.
Aberystwyth, Dyfed, UK
Speedboats experience a huge amount of drag. Typical full-speed drag force is a quarter of the weight of the boat, which is like driving a car up a slope of slightly over 25 per cent. Speedboats have to be low geared to overcome this drag, and a multi-speed gearbox would make very little difference for low-speed acceleration. The drag is so large that any gear change would have to be extremely fast or the boat would slow down too much during the changes. Because propellers slip through the water when the boat is starting up, there is no need for a clutch – the water acts like one.
It is possible to change gear on a boat by changing the propeller for one of a different pitch. A lower pitch gives better acceleration, and allows you to pull larger loads, while a higher pitch gives better top speed, if the waves are small, which reduces drag. However, the typical change that might be useful on a boat is less than the difference between adjacent gears on a car.
Nuneaton, Warwickshire, UK
The speed of a car is proportional to the engine speed for a particular gear. This is not the case in a boat because the propeller can ‘slip’ in the water, whereas a car tyre stays stuck to the road. In all engines an increase in revolutions means an increase in power, up to a certain point.
Most of us will have accidentally taken off from traffic lights in a car in third gear. The number of revolutions in third gear is much lower than that in first gear, so the engine does not generate enough power to move the car and it stalls.
This shows that a low gear is essential in a car for power at low speed. But if you open the throttle fully in a boat, the propeller spins freely in the water, the engine reaches a high number of revolutions and the boat moves off without stalling. The boat’s single gear is designed so that the propeller works most efficiently within the engine’s operating range. There is no need for additional gears.
In a boat, the drop in power while changing gear results in a large drop in speed, because the resistance in water is much greater than on a road, so a boat cannot pass through a gear change as easily as a car can.
Eltham, Victoria, Australia