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OK then ... with how long to go before impact do you consider that a risk of collision exists?
Identification of the risk of collision is often long before the concluded probability of collision. My view is that a complex balance of factors (including factors such as sea state, ability of vessel(s) to manoeuvre, sea room, other vessels in proximity and their likely action) considered in aggregate will determine one’s personal decision point as to when to take collision avoidance action.
I personally had this mental process severely tested at the north end of Kilbrannan Sound just the week before last. There was plenty of sea room and no other vessels in proximity. My vessel was motor sailing so could easily manoeuvre. I would say the two vessels were less than 30 seconds from collision when I took collision avoidance action.