Every year more and more people are taking up sailing. Boats of all shapes and sizes, manned by one-man or family-size crews, turn the world's waterways into colourful sights at weekends; single-handed crossings over vast distances, or around the world, are now almost commonplace, despite the awesome hazards that the sea can offer. There is one requirement that all these sailors, whether experienced or not, must share if they are to avoid disaster...