In view of the immensity of the sea, the relative smallness of ships and the range of speeds, the probability of a collision at sea between ships blindly following constant and random courses is very low. Even on high-density routes, the probability is low for intersecting courses if speeds and headings remain constant. The risk is higher for routes which are at angles of less than 20° from each other, and where a ship is overtaking another rather than following an opposite course. It is lower if the speeds are very different, and higher if they are similar.