Black-ball day shape

The black ball is the day shape that signals a vessel at anchor: one black ball shown forward where it is best seen, under COLREGs Rule 30. A ball is at least 0.6 m in diameter on a vessel of 20 m or more, per Annex I. The same Rule uses balls in other combinations: a vessel not under command shows two balls vertically, a vessel constrained by draft three balls, and a vessel restricted in ability to maneuver a ball-diamond-ball. Day shapes carry by day the message the lights carry by night, so an anchored ship in daylight shows the ball, not the anchor light. A small anchored vessel under 7 m outside a channel need not show it.