Measuring a Vessel's Beam

Container Ship Bow
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When describing the hull of a vessel three basic measurements give a rough outline of the shape of the hull. These are Length, Beam, and Draft.

Beam is a measurement of a vessel's width. It is always measured at the widest point because it is often used to determine if passage can be safely made near an obstacle.

Beam is important in determining the handling characteristics of a ship design. A narrow beam hull will run fast but will not perform well in heavy waves because of the narrow cross section.

A hull which has a wider beam will be less efficient in cutting through the water because of the larger mass of water that is being displaced. This larger mass also tends to roll less.

Beam can also be measured at specific points on the hull like the pilot house or cargo area but these measurements will be designated with the names of these structures. The main measurement of beam is taken at the widest point of a vessel.

Naval architects use length, beam, and draft measurements to shape a hull for a specific job by using the concept of Deadrise. The three main hull measurements along with deadrise give the hull a specific shape and handling characteristics.

The Origin of Beam in Ships

Origin of the word comes from early wooden ship design. The large timbers that sit on top of each rib as they extend up from the keel span the whole width of the ship for strength. On top of this was a deck made of smaller boards that also acted as the ceiling for the first level cabins.

From the inside, the ship resembled a house with its floor beams and exposed underside floor decking.

A common way to talk about a ship was by the size of her roof beams which would tell you how wide the vessel was and how that proportion related to her length and rig. You could tell everything about a ship from the dimension of this single element of construction.

How Beams Are Used Today

Today, in modern ship construction, wooden beams are replaced with steel boxes which are much wider than the beams. Wooden beams may have been as wide as a person, steel beams called torsion boxes which are as wide as twenty people are set across the hull. Once this is welded together the ship becomes much more rigid because of something called a stressed skin design which makes ships strong and light. Modern cars use the same idea and use the floor pan and body to make a stiff structure that doesn't need the weight of a heavy solid frame.

Another benefit of a stressed skin design is a wide open interior. In wooden ships, two interior posts rose from the chine at each rib to help support the beam which made the interior cramped. In warships, these posts were used to lash down the cannons when they weren't in use. They also held the hammocks which really were used on ships of the era

The space below the deck was damp and only the lower ranked men slept there. Officers and the Master had better cabins with the junior officers in the bow and the Master's cabin at the stern and raised above the deck by one or more levels.


You may hear someone refer to a vessel as "Beamy".

This means that a vessel has a wide beam in proportion to her length.