On the water there are no traffic lights, no lanes, and often no radio contact with the other boat. Even so, you need to recognize early on whether two vessels are on a collision course. The key is a simple method that is centuries old: the bearing, a core part of the right-of-way rules for the SBF Inland and the SBF Coastal. This article explains what a bearing is, why a constant bearing is a warning sign, and what the term CPA has to do with it.

What is a bearing?

A bearing is the direction to an object, expressed as an angle. On the water you might take a bearing on a light, a landmark, or another vessel. There are two kinds:

  • True bearing: the angle to the object measured from geographic north (0° to 360°). It does not depend on your own heading. A handbearing compass first gives you a compass bearing (relative to magnetic north); to turn that into a true bearing you correct it for variation, as explained in Compass, Variation and Deviation Explained.
  • Relative bearing: the angle to the object measured from your own bow direction. It changes as soon as you alter your own course, even if the actual collision risk has not changed at all.

In practice, for the question “is a collision developing?”, simply watching how the other vessel’s position shifts relative to your own boat is often enough.

The golden rule: a constant bearing means risk of collision

If another vessel is approaching and the bearing to it does not change noticeably over several minutes while the distance is decreasing, you are on a collision course. If the bearing changes clearly instead (the vessel appears to “move” forward or aft), it will likely pass ahead of or behind you.

This is not just a rule of thumb; it comes directly from the Collision Regulations (COLREGs). Rule 7 requires every vessel to use all available means to continuously determine whether risk of collision exists. It states that if there is any doubt, such risk must be assumed to exist. Watching the bearing is one of the most important tools for making that judgment, especially when no radar or AIS is on board.

The avoidance rules that follow once a risk of collision has been identified are a large topic of their own. Who has to give way and which rules apply on crossing or reciprocal courses is explained in the article Right of Way on the Water: Who Has to Give Way?.

How to take bearings in practice, even without radar

You don’t need expensive equipment to spot a risk of collision. These methods work on any recreational boat:

Bearing over a fixed reference point

Pick a fixed point on your own boat, for example a rail, a stay, or a window frame, and watch the other vessel against it. If the other boat visibly shifts forward or aft relative to that reference point, the bearing is changing and the immediate collision risk is decreasing. If the other boat instead appears to stay “in the same spot” behind your reference point while getting closer, caution is required.

Bearing with a handbearing compass

Use a handbearing compass to take the angle to the other vessel as a compass bearing. For assessing collision risk alone, you don’t need to correct that reading for variation, since all that matters is whether the angle you read changes over time. Repeat the bearing after one or two minutes. If the reading stays nearly the same while the distance shrinks, there is a risk of collision.

Take several bearings, not just one

A single bearing tells you little. Only comparing several bearings over time shows whether the situation is easing or getting worse. In unclear encounters, take the time for two or three bearings spaced one to two minutes apart instead of relying on a single glance.

A practical example

Imagine you are cruising on an inland waterway in your motorboat and you spot another boat approaching from your starboard side in the distance. You take a bearing on it over your own rail:

  • Minute 0: The other boat sits just in front of your rail post.
  • Minute 2: The other boat is still almost in the same spot, only slightly closer to the post, but the distance has visibly decreased.
  • Minute 4: Still the same relative position, and the distance has shrunk noticeably.

The bearing stays practically constant throughout the observation period while the distance keeps decreasing. That is a clear example of a constant bearing, and therefore of a risk of collision. If the other boat instead appeared a little further aft of your rail post at each observation, it would pass behind you and there would be no immediate danger.

Important: this observation does not replace the question of who has to give way. Once you recognize a constant bearing, the next step is to work out whether you have to give way yourself or whether you are the stand-on vessel; see the article on right of way rules.

CPA and TCPA explained simply

On larger vessels and in commercial shipping, the bearing method is supplemented or replaced by radar, ARPA, and AIS. That brings up two technical terms:

  • CPA (Closest Point of Approach): the shortest distance two vessels will have to each other if both maintain their current course and speed.
  • TCPA (Time to Closest Point of Approach): the time remaining until that closest distance is reached.

Modern AIS units and chartplotters automatically calculate CPA and TCPA for other AIS-transmitting vessels and warn when a configured threshold is undershot. How such devices work in general and what they actually give a recreational boater is explained in the article GPS, Chartplotter and VHF On Board: Understanding Modern Navigation.

For most recreational boats without such electronics, the classic bearing method with a fixed reference point or a handbearing compass is essentially a manual CPA check. If the bearing stays constant, the CPA is approaching zero, and you need to act.

What to do once a risk of collision is identified

As soon as you notice a constant bearing, it is time to act, not to wait:

  1. Determine who has to give way. Use the ranking of vessel types and the type of encounter (reciprocal, crossing, overtaking) to work out who must give way. Details are covered in the article on right of way rules.
  2. React early and clearly. A small last-second course change is hard for the other vessel to recognize. Alter course or speed early and in a way that makes your intention unmistakable.
  3. Stand on if that is your role. A vessel that does not have to give way must maintain course and speed so the other vessel can rely on it, unless a collision can only be avoided by taking action yourself.
  4. When the other vessel’s intention is unclear: use clear sound signals. What each signal means is explained in the article Sound Signals for the SBF Explained.

Common mistakes

  • Looking only once. A single bearing without comparison over time says nothing about how the situation is actually developing.
  • Relying on the other vessel’s size. Even though larger vessels have priority in certain situations, that does not excuse you from watching the situation yourself and acting if necessary.
  • Confusing relative and true bearing. If you change your own course, the relative bearing changes too, even though the actual risk hasn’t changed. Keep this distinction in mind, especially while you are maneuvering yourself.
  • Reacting too late. Anyone who only acts once the distance is already small has little room left for a safe maneuver.

Conclusion

A constant bearing combined with a decreasing distance is one of the most reliable warning signs of a risk of collision on the water, with no electronics required. Anyone who takes bearings regularly, whether using a fixed reference point on their own boat or a handbearing compass, spots critical situations early and can react in time and unambiguously. CPA and TCPA are simply the technical, radar-based version of the same basic principle. In the Boatpass app you can practice the questions on collision avoidance and right-of-way rules with the official ELWIS question catalog.