Lightvessels

A lightvessel, or lightship, is a ship which acts as a lighthouse. They are used in waters that are too deep or otherwise unsuitable for lighthouse construction. Although some records exist of fire beacons being placed on ships in Roman times, the first modern lightvessel was off the Nore sandbank at the mouth of the River Thames in England, placed there by its inventor Robert Hamblin in 1734. The type has become largely obsolete; some stations were replaced by lighthouses as the construction techniques for the latter advanced, while others were replaced by large, automated buoys.

A crucial element of lightvessel design is the mounting of a light on a sufficiently tall mast. Initially, this consisted of oil lamps which could be run up the mast and lowered for servicing. Later vessels carried fixed lamps, which were serviced in place. Fresnel lenses were used as they became available, and many vessels housed these in small versions of the lanterns used on lighthouses. Some lightships had two masts, the second holding a reserve beacon in case the main light failed.

Initially, the hulls were constructed of wood, with lines like those of any other small merchant ship. This proved to be unsatisfactory for a ship that was permanently anchored, and the shape of the hull evolved to reduce rolling and pounding. As iron and steel were used in other ships, so were they used in lightvessels, and the advent of steam and diesel power led to self-propelled and electrically lighted designs. Earlier vessels had to be towed to and from station.

Much of the rest of the ship was taken up by storage (for oil and the like) and crew accommodations. The primary duty of the crew was, of course, to maintain the light, but they also kept record of passing ships, observed the weather, and on occasion performed rescues.

In the early 20th century, some lightships were fitted with warning bells, either mounted on the structure or lowered into the water, the purpose of which was to warn of danger in poor visibility and to permit crude estimation of the lightship relative to the approaching vessel. Tests conducted by Trinity House found that sound from a bell submerged some 18 feet (5.5 m) could be heard at a distance of 15 miles (24 km), with a practical range in operational conditions of 1–3 miles.

...
Wikipedia