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Risk of collision: bearings and COLREGS Rule 7

Every collision avoidance decision starts with one question: is there a risk of collision? Rule 7 of the COLREGS tells you how to answer it, and Rule 8 tells you what to do next.

Rule 7 in plain English

  • Use everything you have to decide whether risk of collision exists: your eyes, a hand-bearing compass, radar and AIS. If there's any doubt, assume it does.
  • Use radar properly if you have it and it works, including long-range scanning for early warning, and plotting or equivalent systematic observation of contacts.
  • Don't make assumptions on scanty information, especially scanty radar information. One glance at a screen is not a plot.
  • A steady bearing means risk: risk of collision is deemed to exist if the compass bearing of an approaching vessel does not appreciably change.
  • A changing bearing doesn't always mean safety: risk may still exist, particularly when approaching a very large vessel, a tow, or a vessel at close range.

Constant bearing, decreasing range

The classic sign of a collision course is CBDR: constant bearing, decreasing range. If another vessel stays on the same compass bearing while getting closer, you're on course to meet.

How to check it on a yacht

  1. Take a compass bearing of the other vessel with a hand-bearing compass, and note the time.
  2. Take another a couple of minutes later, then another. One bearing tells you nothing.
  3. If the bearing is steady and she's getting closer, there's a risk of collision. If the bearing is drawing ahead or astern steadily, she's likely to pass ahead or behind.

No compass to hand? Line the other vessel up with part of your own boat, such as a stanchion, and keep your head in the same place. If she stays fixed against it while closing, treat it as a steady bearing. It's cruder than a compass, but much better than nothing.

Radar and AIS

On radar, put the electronic bearing line (EBL) on a contact and watch the range rings or variable range marker (VRM). A contact that slides down the EBL towards the centre is on a collision course. AIS can show a closest point of approach (CPA) and time to it (TCPA), but not every vessel carries AIS, and its data can be wrong. Use it alongside a proper lookout, never instead of one.

Then: Rule 8, action to avoid collision

  • Positive and in ample time: act early, while you still have room to choose.
  • Large enough to be readily apparent: one bold alteration the other vessel will notice, not a series of small ones she won't.
  • Course alteration is often best in open water, but slowing down or stopping is allowed too.
  • Check it's working by continuing to take bearings until she is finally past and clear.

Who should act is set by the steering and sailing rules. See the steering and sailing rules explained, and Rule 19 for what changes in fog.

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Questions

How do you know if there is a risk of collision?

Take a series of compass bearings of the other vessel. If the bearing doesn't appreciably change while the range closes, risk of collision exists (Rule 7). If there's any doubt, assume there is a risk.

Can there be a risk of collision if the bearing is changing?

Yes. Rule 7 warns that risk may still exist with an appreciable bearing change, particularly when approaching a very large vessel, a tow, or a vessel at close range.

What does Rule 8 say about avoiding action?

Action to avoid collision should be positive, made in ample time and large enough to be readily apparent to the other vessel, so avoid a series of small alterations. Then check it's working until the other vessel is finally past and clear.

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Last checked 4 November 2026. DeckPrep is an independent revision tool and is not affiliated with, endorsed by, or operated by the RYA. Always check current requirements with the RYA.