Marine casualties usually arise through a combination of complex conditions. To trace any one of the conditions involved, and its reaction on others, not only helps to reveal the cause of an accident but is also important in understanding the nature of such casualties.

Like accidents on land, marine casualties do not arise spontaneously but have ascertainable causes, rarely a single cause but rather a combination of causes. Thus in a collision in fog in a narrow channel we must consider not only faults in carrying out a manœuvre but also the natural phenomena of fog and the narrowness of the channel.