Beam (nautical)

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File:Ship length measurements.svg
Graphical representation of the dimensions used to describe a ship. Dimension "b" is the beam.

The beam of a ship is its width at the widest point as measured at the ship's nominal waterline. The beam is a bearing projected at right-angles from the fore and aft line, outwards from the widest part of ship. Beam may also be used to define the maximum width of a ship's hull, or maximum width plus superstructure overhangs.

Generally speaking, the wider the beam of a ship (or boat), the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position.

Typical values[edit]

Typical length-to-beam ratios for small sailboats are from 2:1 (dinghies to trailerable sailboats around 20 ft or 6 m) to 5:1 (racing sailboats over 30 ft or 10 m).

Large ships have widely varying beam ratios, some as large as 20:1.

Rowing shells designed for flatwater racing may have length to beam ratios as high as 30:1,[1] while a coracle has a ratio of almost 1:1 – it is nearly circular.


The beam of many monohull vessels can be calculated using the following formula:

<math>Beam = LOA^{2/3} + 1</math>

Where LOA is Length Overall and all units are in feet.

Some examples:

  • For a standard 27 ft (8.2 m) yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27 ft monohulls is 10 ft (3.05 m).
  • For a Volvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around 18 ft (5.5 m).
  • For a 741 ft (226 m) long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around 82 ft (25 m), e.g. Seawaymax.

As catamarans have more than one hull, there is a different beam calculation for this kind of vessel.


BOC stands for Beam On Centerline. This term in typically used in conjunction with LOA (Length overall). The ratio of LOA/BOC is used to estimate the stability of multihull vessels. The lower the ratio the greater the boat's stability.

The BOC for vessels is measured as follows: For a catamaran: the perpendicular distance from the centerline of one hull to the centerline of the other hull, measured at deck level. For a trimaran: the perpendicular distance between the centerline of the main hull and the centerline of either ama, measured at deck level

Other beams[edit]

Other meanings of 'beam' in the nautical context are:

  • Beam – a timber similar in use to a floor joist, which runs from one side of the hull to the other athwartships.
  • Carlin – similar to a beam, except running in a fore and aft direction.



  • Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0-87033-549-9. 
  • Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87033-056-X.