Page "Lightvessel" Paragraph 6

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 could be heard at a distance of, with a practical range in operational conditions of 1 – 3 miles.