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

The beam of a ship is its width at the widest point, or at the mid-point of its length. 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. Initial stability is the resistance of a Boat to a small amount of lateral tilting from its equilibrium 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 ^[1], while a coracle has a ratio of almost 1:1 - it is nearly circular. In Watercraft, a racing shell (also referred to as just a fine boat (UK or just shell) is an extremely narrow and often disproportionately long A coracle (cwrwgl is a small lightweight Boat used mainly in Wales but also in parts of Western and South Western England, Ireland, and

I have found as a general rule of thumb that most monohulls from 6m (20') to 250m (820') have a beam that can be very roughly calculated by the following forumla:

Beam = LOA (Length Overall) in feet to the power of 2/3

Some examples -

For an 8. 23m (27') long yacht: the cube root is 3, and 3 squared is 9. The beam will normally be about 2. 75m (9').

For a 19. 5m (64') long yacht: the cube root is 4, and 4 squared is 16. The beam will normally be about 4. 9m (16').

For a 226m (741') long ship: the cube root is 9, and 9 squared is 81. The beam will normally be about 24. 7m (81') e. g. Seawaymax.

Notes

References

• 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, Centreville, MD: Cornell Maritime Press. ISBN 0-87038-056-X.