Beam vs abeam: what do they mean on a boat?
A vessel’s beam is its greatest width. An object is abeam when it lies approximately at right angles—90°—to the vessel’s fore-and-aft line, on either side.
Related terms
How it works
Beam is a dimension: a 4 m beam means the boat is 4 m wide at her widest point. Abeam is a relative direction: a lighthouse abeam to port lies roughly square off the port side.
The distinction matters in the collision rules. Overtaking is approached from more than 22.5° abaft another vessel’s beam; restricted-visibility guidance also distinguishes targets forward of, abeam, and abaft the beam.
One cross-line, two ideas
Imagine drawing a crossbar through the boat at its widest point. The length of the crossbar is the beam. Continue the same line outward across the water and it points abeam on either side.
On the water
- A beam reach has the wind coming approximately abeam.
- A contact two points abaft the starboard beam is behind the beam line.
- A marina berth may specify a maximum permitted vessel beam.
Common mistake
Beam does not simply mean “the side,” and an object cannot have a beam of 90°. Beam measures the vessel; abeam locates the object.
Remember it
BEAM is the crossbar; ABEAM is where that crossbar points.