The beam of a ship is its width at its widest point. The maximum beam (BMAX) is the distance between planes passing through the outer extremeties of the ship, beam of the hull (BH) only includes permanently fixed parts of the
hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affin ...
, and beam at waterline (BWL) is the maximum width where the hull intersects the surface of the water. Generally speaking, the wider the beam of a ship (or boat), the more
initial stability Initial stability is the resistance of a boat to small changes in the difference between the vertical forces applied on its two sides. It is determined by the angle of tilting on each side of the boat as its center of gravity (CG) moves sideways as ...
it has, at the expense of
secondary stability A kayak is a small, narrow watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles used in water, including boats, ship A ship is a large watercraft that travels the world's oceans and other sufficiently d ...
in the event of a
capsize Capsizing or keeling over occurs when a boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carr ...
, 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.

Typical values

Typical length-to-beam ratios (
aspect ratio The aspect ratio of a geometric Geometry (from the grc, γεωμετρία; '' geo-'' "earth", '' -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space that are ...

aspect ratio
s) for small sailboats are from 2:1 (dinghies to trailerable sailboats around ) to 5:1 (racing sailboats over ). Large ships have widely varying beam ratios, some as large as 20:1.
Rowing shells In watercraft, a racing shell (also referred to as just a ''fine boat'' (UK) or just ''shell'') is an extremely narrow, and often comparatively long, rowing boat specifically designed for Rowing (sport), racing or exercise. It is outfitted with lon ...
designed for flatwater racing may have length to beam ratios as high as 30:1,Science News Online: Ivars Peterson's MathTrek (7/17/99): Row Your Boat
/ref> while a
coracle A coracle is a small, rounded, lightweight boat A boat is a watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system wit ...
has a ratio of almost 1:1 – it is nearly circular.

Rule of thumb - formula

The beam of many monohull vessels can be calculated using the following formula: :Beam = LOA^\frac + 1 Where LOA is Length OverAll and all lengths are in feet. Some examples: * For a standard yacht: the cube root of 27 is 3, 3 squared is 9 plus 1 = 10. The beam of many 27 ft monohulls is . * For a
Volvo Open 70 The Volvo Open 70 (sometimes referred to as a Volvo Ocean 70) is the former class of racing yachts designed for the Volvo Ocean Race. It was first used in the 2005–06 Volvo Ocean Race, 2005–06 race (replacing the Volvo Ocean 60 yachts which we ...
yacht: 70.5 to the power of 2/3 = 17 plus 1 = 18. The beam is often around . * For a long ship: the cube root is 9, and 9 squared is 81, plus 1. The beam will usually be around , e.g. Seawaymax. As catamarans have more than one hull, there is a different beam calculation for this kind of vessel.


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.



* * {{Ship measurements Nautical terminology Ship measurements