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How to tell if you are on a collision course with another vessel

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What is the issue?
One of the most perplexing problems for newcomers to sailing is trying to assess converging courses of surrounding vessels and knowing how to judge who's going to cross whom. Sailing novices tend to be familiar with driving automobiles where it is easy to detect the possible risk of a collision with implicit reference to the background (e.g., the roads, the scenery, the landscape, etc.) At sea, when vessels are borne in water, the sea surface removes all visual clues obsoleting to a large part terrestrial instinct.

Why address this?
It is all too easy to gauge that two vessels, which appear to be heading apart from each other, cannot collide. They very much can, as when a faster vessel is overtaking a slower one, a collision will occur even though the vessels are on substantially different headings. Getting this wrong, especially when operating under autohelm, can easily lead to a collision. Likewise, without a reliable system to identify the risk of collision courses, it is unnecessarily anxious being at the helm in busy waters.

How to address this?
Collision risk may be identified by observing the relative bearing to the other boat using 'constant bearing, decreasing range' (CBDR) navigation technique. In addition to revealing your duties under the Rules, it can help determine the risk of collision.

Constant bearing, decreasing range (CBDR) is a term in navigation which means that some object, usually another ship viewed from the helm, is getting closer but maintaining the same relative bearing.

Constant Bearing, Decreasing Range (CBDR)
Photo: Chiswick Chap via CC BY-SA 4.0

CBDR, as illustrated above, states that when an observer sees another vessel at a constant bearing and the range continually decreases, collision is imminent.

If the target vessel visually moves forward against the background, with relative bearing decreasing, then the target vessel will pass in front of the observing vessel and there is no risk of collision, provided both vessels maintain constant speed and course.

Similarly, if the target vessel visually moves backwards against the background, with relative bearing increasing, the target vessel will pass behind the observing vessel.

Observing the CBDR with a stanchion
Photo: Adventurejay via CC BY 2.0

In practice, all the helmsman or lookout needs to do is keep their head still and look where the vessel in question is relative to a fixed reference on your vessel, for instance over a particular stanchion, shroud, or another prominent fixed mark. Then continually observe it to see if it remains in place. If it does you should alter course as required by collision regulations.

Radar, AIS, and other electronics can also help gauge range and bearing to other vessels. But they all require you to take your eyes off the water and look at them. In normal boating situations, with decent visibility, CBDR is the best guide to the helmsman.

Cutting it fine in The Solent at the start of Fastnet Race
Photo: Martin Hesketh via CC BY 2.0

At night however, Radar and AIS will be more helpful, but you do still see the vessels lights and its certain pattern of lights. CBDR still applies if the two boats are on a collision course and the pattern of lights will not appear to change. If the boat turns, the vertical separation of the lights isn't altered, but they quickly form a wider or narrower horizontal pattern.

With thanks to:
Michael Harpur, Yacht Obsession.
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