IMO COLREG Rule 8

Rule 8 governs action to avoid collision: any alteration must be positive, made in ample time, and large enough to be readily apparent to another vessel observing visually or by radar. A succession of small changes is specifically discouraged. Where there is sufficient sea room, an alteration of course alone may be the most effective action provided it is made in good time, is substantial, and does not result in another close-quarters situation. Rule 8(f) sets out the obligations of a vessel required not to impede the passage of another.