The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship (or boat), the more initial stability

Initial stability

Initial stability is the resistance of a boat to a small amount of lateral tilting from its equilibrium position. It is determined by the difference in the volume of water displaced on each side of the boat as it tilts and is relatively independent of hull shape for small angles of tilt.The wider...

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

Coracle

The coracle is a small, lightweight boat of the sort traditionally used in Wales but also in parts of Western and South Western England, Ireland , and Scotland ; the word is also used of similar boats found in India, Vietnam, Iraq and Tibet...

 has a ratio of almost 1:1 - it is nearly circular.
The beam of many monohulls can be calculated using the following formula:

LOA is Length Overall.

All units are in feet.

Some examples

- For a standard 27' (8.23m) yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27' monohulls is 10' (3.05m).

- For a Volvo Open 70

Volvo Open 70

The Volvo Open 70 is the current class of racing yachts designed for the Volvo Ocean Race. It was first used in the 2005-2006 race and use a Canting keel which is capable of canting transversely up to an angle of 40 degrees.According to the VO70 rule, the yachts can be made...

 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around 18' (5.5m).

- For a 741' (226m) long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around 82' (25m) e.g. Seawaymax

Seawaymax

The term Seawaymax refers to vessels which are the maximum size that can fit through the canal locks of the St. Lawrence Seaway.Seawaymax vessels are in length, wide, and have a draft of and a height above the waterline of . A number of lake freighters larger than this size cruise the Great...

.

Other beams

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.