Originally Posted by scraph
Rules of the Road, rule 5, states "Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision."
This method can even be used for extreme navigation conditions such as heavy fog. You might find, given heavily reduced visibility, that your safe look interval comes out to be a matter of seconds and that safe CPAs can't be maintained any less than visibility range itself.
Correct and interesting theoretical calculations.
Real life data shows that boats move at speeds of 30+ knots being 10feet high...
My rule of thumb is to keep a constant watch on board while close to the land (min. 2 people at night). When cruising >100NM from the land and knowing no fast ferry routes are going nearby we can all go down for lunch but with the radar alarm on and not more than 15 minutes without anyone on board.