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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
Page 4 of 22
Getting near enough that a mistake means collision
Not defined numerically in the rules, deliberately, because an acceptable passing distance depends on the vessels, speeds, sea room and visibility. In open water with large ships, one to two nautical miles is often cited; in a crowded estuary a few hundred metres may be normal. In restricted visibility the obligation is to take action in ample time to prevent the situation developing at all, rather than to manage it once it exists.
Read definition →How near a vessel will pass and how long until it does
CPA is the minimum separation that will occur if both vessels hold course and speed, and TCPA is the time remaining until that moment; a negative TCPA means the point has passed. Alarm limits are set to suit the waters, and a small CPA with a short TCPA demands early and substantial action. Both figures assume no manoeuvre, and are invalidated the instant either vessel alters, so they must be re-evaluated.
Read definition →The nearest distance a target will pass if both vessels hold course and speed
Computed from the relative velocity vector between own ship and the target using their reported positions, courses and speeds over ground. Displays present CPA in nautical miles alongside time to CPA in minutes, with user-set alarm thresholds, for instance 1.0 nautical mile and 12 minutes offshore. Because AIS CPA uses ground-referenced data it can differ from radar-derived CPA in a strong tidal stream, and it assumes both vessels maintain their present motion.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.