S.L. 499.12 Art. 20Signals.

(1) Before entering and throughout such ship’s stay in port or harbour, the ship shall by day fly the International Code flag "B" and by night, in addition to any navigation lights which may be required by any other regulations, exhibit where it can best be seen, but in any case not less than 7 metres above the deck, a red light visible all round the horizon for a distance of at least two miles.

(2) In the case of a ship constrained by construction or size, which cannot normally comply with the provisions of subregulation (1), the Master may in lieu of such signals display by day in a conspicuous position above the deck, a red flag of metal not less than 0.5 metre square and by night an all round red light.