Because sound waves are subject to the vagaries of weather humidity, wind direction and speed , and temperature radio and radar beacons, called racons, have become a standard means of guiding ships. These are detected with special equipment. Lightships and Buoys Other aids to navigation, similar in function to lighthouses, are lightships and buoys. Lightships came into use in the 17th century in places where a lighthouse was impracticable. Usually manned by a crew of about seven, it carries all the equipment standard to a lighthouse.
Some lightships are unmanned floats with automatic acetylene lights.
These are adequate for sheltered waters where high-powered illumination is not necessary. Buoys are floating steel devices used to delineate channels in rivers, harbors, and estuaries. They also mark single, isolated dangers such as wrecks or rocks. They are moored to heavy concrete or cast-iron sinkers by a length of chain. The color, shape, and markings of a buoy convey information to navigators .