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The Colregs do not specify exactly what you have to do to maintain a
proper watch at all times. That is purposely left for a court to
adjucate on a case by case basis.

According to Jim Austin in Ocean Navigator -
"ColRegs and Inland Rule 5 are identical in their description as to the
lookout requirement: "Every vessel shall at all times maintain a proper
look-out by sight and hearing AS WELL AS BY ALL AVAILABLE MEANS APPROPRIATE"
(emphasis added). The last phrase adds, by implication, both radar and radio
to the requirement. (Courts have looked at binoculars, the depth sounder,
radar and radio as included in the armamentarium for a proper lookout.)"

CM