If the merchant-ship's officer has problems, so do you, especially if you are on your own. Watch-keeping on a clear summer's day is one thing. Watch-keeping on a foul night is quite another. You might be tired, bored, cold, wet, even seasick, and are looking forward to turning in. Your mind is in neutral, or your head is down and your hood pulled tight to keep out the rain. When it comes to situations like these, everyone occasionally fails to look around properly, especially when you're away from the coast or a shipping lane. The temptation to leave the watch for "just a few minutes to plot a fix, heat some soup, listen to the weather forecast or write up the log is great. Such chores can indeed take just a few minutes, but more often they run closer to 20 minutes than three. A merchant ship doing 20 knots can go from being hull down on the horizon to being on top of you in less time than that, especially in reduced visibility.