{{Short description|Width of a ship at its widest point}} thumb|right|300px|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''' (B<sub>MAX</sub>) is the distance between planes passing through the outer sides of the ship, '''beam of the hull''' (B<sub>H</sub>) only includes permanently fixed parts of the hull, and '''beam at waterline''' (B<sub>WL</sub>) is the maximum width where the hull intersects the surface of the water.<ref>{{cite web |title=ISO 8666:2016 |url=https://www.iso.org/obp/ui/#iso:std:iso:8666:ed-2:v1:en |website=International Organization for Standardization |access-date=31 March 2020 |language=en |date=July 2016 |archive-date=17 June 2016 |archive-url=https://web.archive.org/web/20160617031837/https://www.iso.org/obp/ui/#iso:std:iso:8666:ed-2:v1:en |url-status=live }}</ref>

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.<ref>{{Cite web|title=Definition of BEAM-ENDS|url=https://www.merriam-webster.com/dictionary/beam-ends|access-date=2020-06-05|website=www.merriam-webster.com|language=en|archive-date=2022-05-13|archive-url=https://web.archive.org/web/20220513061417/https://www.merriam-webster.com/dictionary/beam-ends|url-status=live}}</ref>

== Typical values == Typical length-to-beam ratios (aspect ratios) for small sailboats are from 2:1 (dinghies to trailerable sailboats around {{convert|20|ft|0|disp=or|abbr=on}}) to 5:1 (racing sailboats over {{convert|30|ft|-1|disp=or|abbr=on}}).

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,<ref>{{Cite web |url=http://www.sciencenews.org/pages/sn_arc99/7_17_99/mathland.htm |title=Science News Online: Ivars Peterson's MathTrek (7/17/99): Row Your Boat<!-- Bot generated title --> |access-date=2006-10-18 |archive-date=2011-06-29 |archive-url=https://web.archive.org/web/20110629162315/http://www.sciencenews.org/pages/sn_arc99/7_17_99/mathland.htm |url-status=dead }}</ref> while a coracle has a ratio of almost 1:1 &ndash; it is nearly circular.

==Rule of thumb - formula== {{Unreferenced section|date=January 2021}} {{Explain|reason=1) Why? 2) Which beam? The maximum beam? 3) So every boat of lenth L have the same beam?|date=January 2021}} The beam of many monohull vessels can be calculated using the following formula: :<math>Beam = LOA^\frac{2}{3} + 1</math> Where LOA is Length OverAll and all lengths are in feet.

Some examples: * For a standard {{convert|27|ft|m|1|abbr=on}} yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27&nbsp;ft monohulls is {{convert|10|ft|m|2|abbr=on}}. * For a Volvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around {{convert|18|ft|m|1|abbr=on}}.<ref>{{Cite web |title=VOLVO OPEN 70 “TELEFONICA BLUE” – FarrDesign |url=https://farrdesign.com/yacht/volvo-open-70-telefonica-blue/ |access-date=2026-04-09 |language=en-US}}</ref> * For a {{convert|741|ft|m|0|abbr=on}} long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around {{convert|82|ft|m|0|abbr=on}}, e.g. Seawaymax.

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

==Beam on centerline== {{Unsourced|section|date=July 2025}} 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== {{wiktionary}} Other meanings of 'beam' in the nautical context are: *'''Beam''' – a timber similar in use to a floor joist, which runs horizontally from one side of the hull to the other athwartships. *'''Carlin''' – similar to a beam, except running in a fore and aft direction. *'''Beam''' – the direction across the vessel, perpendicular to fore-and-aft; something lying in that direction is said to be ''abeam''.

==References== {{Reflist}}

==Notes== * {{cite book |last=Hayler |first=William B. |author2=Keever, John M. |title=American Merchant Seaman's Manual |year=2003 |publisher= Cornell Maritime Pr |isbn= 0-87033-549-9}} * {{cite book | last = Turpin | first = Edward A. |author2=McEwen, William A. | title = Merchant Marine Officers' Handbook | edition = 4th | year = 1980 | publisher =Cornell Maritime Press | location = Centreville, MD | isbn = 0-87033-056-X }}

{{Ship measurements}}

Category:Nautical terminology Category:Ship measurements