Marine Glossary

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close quarters

A separation distance between two vessels that is crucial to the application of the Navigation Rules, but not specifically defined within the Rules. A good working guideline is to think of it as the space you need about you in order to maneuver on your own to avoid a collision, regardless of what the other vessel might do suddenly and unexpectedly. Passing slowly in a narrow channel it might be yards; passing high-speed traffic in the open ocean it is more likely measured in miles, depending on conditions.

Generally "close quarters" refers to that space about you that you require in order to maneuver on your own to prevent collision regardless of what the other vessel might do, suddenly and unexpectedly. If you let a vessel get that close, you have given up control of your fate. Note the shape and extent of this space depends on the circumstances. Passing slowly in a narrow channel, it might be yards. Passing at high speeds in the open ocean it is more likely a mile or more. It also depends on the weather, sea state, and visibility, and obviously the types of vessels interacting.

Note that there is both a CPA aspect to this evaluation as well as a time to CPA. If the time to CPA is small, then the size of the area could well be larger.

In modern writing (after 70's or so) this region is sometimes referred to as the "ship's domain," and you could search journals or web with those terms to see related discussions. There are numerous people working on theoretical models of this concept, especially as it might apply in confined areas like narrow channels. It is also valuable to have some computational insight to this that might be applied to ARPA systems that are evaluating risk of collision. (The latest Farwell's Rules of the Nautical Road, 2005, does not use this term, however.)

Nevertheless, this will always be a judgement call, up to the navigator at the helm.