International Regulations for Preventing Collisions at Sea — the rules that matter most on watch: look-out, risk of collision, steering rules, lights, and sound signals.
Every vessel must maintain a proper look-out by sight and hearing, as well as by all available means appropriate to the circumstances — including radar and AIS where fitted — to make a full appraisal of the situation and risk of collision.
Every vessel must proceed at all times at a safe speed, allowing proper and effective action to avoid collision and to be stopped within a distance appropriate to the prevailing circumstances and conditions.
Factors to consider include visibility, traffic density, manoeuvrability, background lights at night, wind/sea/current, draft in relation to depth, and radar characteristics if fitted.
Risk of collision must be deemed to exist if the compass bearing of an approaching vessel does not appreciably change — this is the classic "steady bearing, decreasing range" indicator.
Assumptions must never be made on the basis of scanty information, especially scanty radar information.
Any action to avoid collision must be positive, made in ample time, and with due regard to good seamanship.
Alterations of course and/or speed should be large enough to be readily apparent to the other vessel, avoiding a succession of small changes.
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