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Beam (nautical)
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 rightangles 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
Typical lengthtobeam ratios for small sailboats are from 2:1 (dinghies to trailerable sailboats around Script error: No such module "convert".) to 5:1 (racing sailboats over Script error: No such module "convert".).
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.
Formula
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 Script error: No such module "convert". yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27 ft monohulls is Script error: No such module "convert"..
 For a Volvo Open 70 yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around Script error: No such module "convert"..
 For a Script error: No such module "convert". long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around Script error: No such module "convert"., e.g. Seawaymax.
As catamarans have more than one hull, there is a different beam calculation for this kind of vessel.
BOC
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
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.
Notes
References
 Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0870335499.
 Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 087033056X.

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