Lighthouse - 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 They needed to be withdrawn for overhaul every two or three years.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.

Buoys

Buoys are used to mark safe channels, important reference points, approaches to harbours, isolated dangers and wrecks, and areas of special significance. They also mark traffic lanes in narrow and congested waters where traffic routing is in force (i.e., where ships are being routed into designated lanes with entry and exit points).

Structure and operation

Usually constructed of quarter-inch steel plate, buoys vary in diameter from one to two metres (three to six feet) and can weigh as much as eight tons. They are moored to a two- or three-ton concrete or cast-iron sinker by a single length of chain, which is normally about three times as long as the depth of water. Size and type depend on the application. Buoys in deep water need to be large in order to provide buoyancy sufficient to support the extra weight of chain. Stability is provided by a circular “skirt keel,” with possibly a ballast weight. The light is mounted on a superstructure some 3 metres (10 feet) above the waterline. For more powerful lights on the open sea or in deep water, the light may be 4.5 or 6 metres (15 or 20 feet) above the waterline in order to increase the range. Known as “high focal plane” buoys, for stability they require a cylindrical tail tube extending downward some 10 feet from the bottom of the hull.

Buoys are also manufactured from fibreglass-reinforced plastic. They have the advantage of light weight, hence lighter moorings (often of synthetic cable), ease of handling, and resistance to corrosion. Fibreglass buoys are generally confined to sheltered waters.

In addition to the light, a buoy may be fitted with a radar reflector and a low-power fog signal. In earlier times acetylene gas was the only practicable illuminant, which restricted the power of the light. Modern electric buoy lights range in power from a few hundred candelas up to the region of 1,000 candelas, giving ranges of 8 nautical miles (15 km) or so. The lighting equipment consists of a drum lens, usually made of plastic and between 4 and 12 inches (10 and 30 cm) in diameter, along with a low-voltage lamp of 5 to 60 watts. Power can be provided by expendable primary batteries, which need to be replaced every year or two. In order to increase the service interval and also to accommodate more powerful lights, rechargeable batteries with onboard generators are used. Some tail-tube buoys, which tend to oscillate vertically with the motion of the sea, generate power from the oscillating water column in the tube. The water column produces an oscillating air column, which in turn drives a small air-turbine generator. However, the vast majority of buoys are solar-powered.

Buoyage systems

A buoy’s colour and profile and the colour and flash pattern of its light convey information to navigators. The beginnings of a unified system of buoy marking emerged in 1889, but it was not until 1936 that a worldwide unified system was agreed upon at the League of Nations in Geneva. World War II began before the agreement could be fully ratified, and in the aftermath of the war little progress was made. At one time there were nine different systems in use. By 1973, however, a number of spectacular marine accidents urged the international community into action, and by 1980 a new unified system had been agreed upon by 50 maritime countries.

Maintained by the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), the Maritime Buoyage System applies two nearly identical standards to two regions. Region A comprises Europe, Australia, New Zealand, Africa, the Persian Gulf, and most Asian states. Region B includes the Americas, Japan, Korea, and the Philippines. In both regions, the buoyage systems divide buoys into Lateral, Cardinal, and associated classes. Lateral buoys are used to mark channels. In region A a can-profile (i.e., cylindrical) red buoy with a red light indicates the port (left) side of the channel when proceeding in the direction of buoyage, while a conical green buoy indicates the starboard (right) side. The direction of buoyage is the direction taken when approaching a harbour from seaward or when following a clockwise direction around a landmass. Where necessary, it is indicated by arrows on charts. In region B the marking is reversed - i.e., red is to starboard when returning to harbour, and green is to port.

Cardinal buoys indicate the deepest water in an area or the safest side around a hazard. They have a plain hull with a lattice or tubular superstructure that is surmounted by a distinctive “topmark.” There are four possible topmarks, corresponding to the four cardinal points of the compass - i.e., north, south, east, and west. The safest navigable water lies to the side of the buoy indicated by its topmark. The marks consist of a pair of cones arranged one above the other in the following configurations: both points upward (north), both points downward (south), base-to-base (east), and point-to-point (west). The cones are painted black, and the buoys are coloured from top to waterline with two or three horizontal bands of black and yellow.

Buoys indicating an isolated danger with safe water all around carry two separated spheres and are painted with alternating horizontal red and black bands. Safe-water buoys, marking an area of safe water, carry a single red sphere and vertical red and white stripes.

Special marks are buoys that indicate other areas or features such as pipelines, prohibited zones, or recreational areas. These buoys are all yellow, with an X-shaped topmark and a yellow light. They may also have a hull shape indicating the safe side of passage.