Beam (nautical)

Beam (nautical) image

Graphical representation of the dimensions used to describe a ship. Dimension "b" is the beam at waterline.

The beam of a ship is its width at its widest point. The maximum beam (BMAX) is the distance between planes passing through the outer extremeties of the ship, beam of the hull (BH) only includes permanently fixed parts of the hull, and beam at waterline (BWL) is the maximum width where the hull intersects the surface of the water.

Generally speaking, the wider the beam of a ship (or boat), the more initial stability it has, at the expense of secondary stability in the event of a capsize, where more energy is required to right the vessel from its inverted position. A ship that heels on her beam ends has her deck beams nearly vertical.


Typical values

Typical length-to-beam ratios (aspect 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, while a coracle has a ratio of almost 1:1 – it is nearly circular.

Rule of thumb - formula

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

B e a m = L O A 2 3 + 1 {\displaystyle Beam=LOA^{\frac {2}{3}}+1}

Where LOA is Length OverAll and all lengths 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

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