The beam of a ship is its width at the widest point as measured at the ship's nominal waterline. 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 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 ft/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.
The beam of many monohull yachts can be calculated using the following formula:
LOA is Length Overall.
All units are in feet.
- 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 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.
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.
- 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.
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