Aft of the beam (also called “abaft the beam”) describes any bearing falling between abeam (directly to the side) and dead astern — covering the rear quarter-sectors on either side of a vessel.
The Bearing Range
- Dead ahead — 000° relative
- Abeam — 090° (starboard) or 270° (port) relative
- Aft of the beam — anywhere between abeam and dead astern (090°-180° or 270°-180° relative)
- Dead astern — 180° relative
Why This Range Matters
This isn’t just descriptive vocabulary — it’s the basis for defining navigation light visibility arcs under COLREGs. A vessel’s sternlight is specifically visible over the arc from dead astern to a set number of degrees aft of the beam on each side, and correctly understanding this range is necessary to correctly interpret what lights should be visible when assessing another vessel’s aspect and likely course.
Aft of the Beam vs Forward of the Beam
These are the two halves of a vessel’s side sectors, split at the beam:
- Forward of the beam — between dead ahead and abeam
- Aft of the beam — between abeam and dead astern
Together with “abeam” itself, these three terms cover the full range of relative bearings on either side of a vessel.
Exam Relevance
Aft of the beam, its bearing range, and its role in defining navigation light arcs are core Navigation and COLREGs topics in deck officer competency exams.