Sound signals
The limitations of purely visual navigation very early led to the idea of supplementary audible warning in lighthouses. The first sound signals were explosive. At first cannon were used, and later explosive charges were attached to retractable booms above the lantern and detonated electrically. Sometimes the charges contained magnesium in order to provide an accompanying bright flare. Such signals could be heard up to four nautical miles away. Bells also were used, the striker being actuated by weight-driven clockwork or by a piston driven by compressed gas (usually carbon dioxide). Some bells were very large, weighing up to one ton.
Compressed air
About the beginning of the 20th century, compressed air fog signals, which sounded a series of blasts, were developed. The most widely used were the siren and the diaphone. The siren consisted of a slotted rotor revolving inside a slotted stator that was located at the throat of a horn. The diaphone worked on the same principle but used a slotted piston reciprocating in a cylinder with matching ports. The largest diaphones could be heard under good conditions up to eight nautical miles away. Operating pressures were at 2 to 3 bars (200 to 300 kilopascals), and a large diaphone could consume more than 50 cubic feet (approximately 1.5 cubic metres) of air per second. This required a large and powerful compressing plant, 50 horsepower or more, with associated air-storage tanks.
A later compressed-air signal was the tyfon. Employing a metal diaphragm vibrated by differential air pressure, it was more compact and efficient than its predecessors.
Electricity
Modern fog signals are almost invariably electric. Like the tyfon, they employ a metal diaphragm, but in the electric signal they vibrate between the poles of an electromagnet that is energized by alternating current from an electronic power unit. Powers range from 25 watts to 4 kilowatts, with ranges from half a nautical mile to five nautical miles. Note frequencies lie between 300 and 750 hertz. Emitters can be stacked vertically, half a wavelength apart, in order to enhance the sound horizontally and reduce wasteful vertical dispersion.
Effective range
Propagation of sound in the open air is extremely haphazard, owing to the vagaries of atmospheric conditions. Wind direction, humidity, and turbulence all have an effect. Vertical wind and temperature gradients can bend the sound up or down; in the latter case it can be reflected off the sea, resulting in shadow zones of silence. The range of audibility of a sound signal is therefore extremely unpredictable. Also, it is difficult to determine with any precision the direction of a signal, especially from the bridge of a ship in fog.
Automation
The acetylene gas illumination system, being fully automatic and reliable, enabled automatic lights to be operated early on. Automation on a large scale, bringing considerable savings in operating costs, came after the advent of electrical equipment and technology and the demise of compressed-air fog signals. Unattended lights are now designed to be automatic and self-sustaining, with backup plant brought online automatically upon failure of any component of the system. The status of the station is monitored from a remote control centre via landline, radio, or satellite link. Power is provided from public electricity supplies (where practicable), with backup provided by diesel generators or storage batteries. Where solar power with storage batteries is used, the batteries must have sufficient capacity to operate the light during the hours of darkness. In tropical and subtropical regions, day and night are of approximately equal duration throughout the year, but in temperate and polar regions the days become longer and the nights shorter during the summer and vice versa in winter. In these areas, solar power has to operate on an annual “balance sheet” basis, with excess charge being generated and stored in large batteries during the summer so that a reserve can be drawn upon in winter. Canada and Norway successfully operate solar-powered lights of this type in their Arctic regions.
Floating lights
Floating lights (i.e., lightships and buoys) have an important function in coastal waters, guiding both passing ships and those making for or leaving harbour. They have the great advantage of mobility and can readily be redeployed to meet changed conditions. For example, submerged hazards such as sandbanks can move over the years under the influence of the sea, and, for vessels of very deep draft, safe channels must be correctly marked around these hazards at all times.
Lightships
Lightships originated in the early 17th century, arising from the need to establish seamarks in positions where lighthouses were at that time impracticable. The first lightship, established in 1732 at Nore Sand in the Thames estuary, was rapidly followed by others. These early vessels were small converted merchant or fishing vessels showing lanterns suspended from crossarms at the masthead. Not until 1820 were vessels built specifically as lightships.
In the 20th century, lightships were in most cases an alternative to costly seabed structures. Used to mark the more important hazards and key positions in traffic patterns, they were capable of providing a range of powerful aids. Power was provided by diesel generators. Lightships varied in size but could be up to 150 feet in length, 25 feet in beam, and 500 tons in displacement. They were not normally self-propelled, most often being towed into position and moored by a single chain and anchor. They needed to be withdrawn for overhaul every two or three years.
Lightships were costly items to operate and to maintain and were therefore prime candidates for automation. In the late 20th century, most lightships were unattended, and the power of their lights and fog signals was downgraded to a more appropriate level—e.g., 10,000 candelas for the lights, giving a luminous range of approximately 15 nautical miles (28 km), and sound signals with a range of 2 nautical miles (4 km). Lightships were replaced by automatic buoys.
