How do you tell if you are on a collision course with another boat?

Watch the other vessel's bearing. If it stays the same while the distance between you closes, there is a risk of collision. If it moves steadily ahead or astern, she will pass ahead of you or behind you.

Rule 7 of the collision regulations says a risk of collision exists if the compass bearing of an approaching vessel does not appreciably change.

A simple way to check on a small boat:

1. Keep a steady course yourself.
2. Line the other vessel up against something fixed on your own boat, such as a stanchion, a shroud or a winch, while you stay in the same position at the helm.
3. Watch for a minute or two.
4. If she moves forward against your reference, she will pass ahead of you. If she moves aft, she will pass astern. If she stays in the same place and gets bigger, you are on a collision course.

A hand-bearing compass gives a more precise answer: take several bearings a minute or so apart and compare them. On boats with radar or AIS, the closest point of approach can be read directly, but keep checking by eye as well.

Some points to watch:

• Rule 7 warns that a risk may exist even when the bearing is changing, particularly with a very large vessel or a tow, or at close range. A ship 300 metres long can have a changing bearing on her bow while her stern still sweeps through your position.
• Your own course and speed must stay steady while you check, or the test means nothing.
• Look all round. The jib hides a large area to leeward, so look under it regularly.
• Assumptions based on scanty information are not allowed by the rules. If in doubt, treat it as a risk of collision.

Once you know there is a risk, work out who is the give-way vessel, and if it is you, act early and obviously.

More in the book: Chapter 13, The rules of the road