keeping the seas safe
A light vessel, or lightship is used in waters that are too dangerous or too deep to build a lighthouse. Although there are records of Romans using fire beacons the first modern light vessel was moored off the Noore sandbank, at the entrance to the Thames in 1734.
The most important function is to have a light mounted high enough, so it can be seen clearly from as far a distance as possible. Early vessels had oil lamps that could be raised and lowered for servicing, later these were fixed and employed fresnel lenses, as used in lighthouses.
In addition to the light was a large foghorn, as can be seen on the North Carr ship. It must have been absolutely deafening for those on board, particularly before the advent of efficient ear defenders.