What Are the Beams of a Ship?

What Are the Beams of a Ship - Merchant Navy Info - Blog

The structure of a ships breadth is always defined by its basic dimensional parameters: length, width, depth, and draft. Even many people who are not familiar with seafaring jargon can understand these simple terms. However, on closer inspection, these terms have a fairly clear meaning. And also classification that many people need to get used to. For example, length can be further divided into Overall Length (LOA), Length Between Perpendiculars (LBP), and Waterline Length (LWL). All these pieces of information are slightly different from each other and require a minimum of maritime knowledge to understand. 

Similarly, the draught or depth of a ship is nothing but the vertical distance. From the ship’s baseline to the waterline. And is classified as summer draught, winter drought, freshwater draught etc. Hence, every ocean-going ship has markings called loading lines/waterlines/plimsoll lines on the side hull of the ship and depending on their length. They are called summer loading line, winter loading line, North Atlantic, fresh water, etc. The corresponding waterline values ​​are expected for different draughts. Similarly, depth is the vertical height from the main deck or strength deck. Of the hull structure to an bottom of the keel plate or baseline.

What is Breadth?

What is the first thing that comes to your mind when we talk about ships breadth ? The width of a three-dimensional body is its span or associated transverse width. Simply put, this is a very perspective term. Consider a three-dimensional box or cube. While height can be easily distinguished as the vertical elevation from the plane. In which it is maintained, the definitions of length and width are open to debate. This is because length and width can be used interchangeably for the same or similar dimensions on the same plane. 

In the case of a box that is 100cm x 100cm x 50cm and 50cm high. The 100cm interval can be said to be the width or length, respectively. If the difference in dimensions is significantly small, for example, 100 cm and 120 cm. Then they can be called the width or length, respectively. However, if there is a clear difference between dimensions on the same plane. For example, one distance is 500 cm and the other 50 cm, then the difference becomes more noticeable. Thus, the distance 500 cm becomes the length, and 50 cm becomes the width or ships breadth measurement. In a ship, even someone completely unfamiliar with the subject. And also seeing a ship for the first time can distinguish between length and ships breadth .

Length Is The Visible Width From Front To Back 

When the ship is viewed from the side, and width is the width of the ship when viewed from the side, either forward or aft. When a designer or builder defines a ship, the ships breadth value indicates the maximum width of the ship, along with other measurements. As we know, in most ships, the ships breadth varies along its length. It is smallest at the front and rear and largest near the amidships, also known in shipbuilding as the parallel-centred hull.

However, in rare cases, such as patrol tugs, some research vessels and icebreakers, and some warship designs, the maximum breadth slopes forward of the amidships area and gradually decreases aft. In such cases, the defined breadth is the maximum width forward of the amidships. This maximum breadth is technically defined as the latitude of the ship. Therefore, when talking about ships in general, it is more practical to talk about breadth rather than maximum width. The breadth of a ship can now be defined variably, just like its length.

The Maximum Breadth 

It is the width measured between the two outermost points of the cross-section. For all practical purposes, this is taken to be the transverse or horizontal distance between the two ends of the exposed or reinforced deck in the centre of the ship. However, older ships sometimes had tumblehomes, and the width at the height of the main deck was less than the width below, so the maximum breadth of the main deck was not measured. As a rule, the greatest distance between the two side plates or outer ends of a ship’s hull, viewed from the side, is taken to be its maximum width. 

The value of width used to define a ship at any point in its design or construction is, for all practical purposes, taken to be this maximum width. The width at the waterline or BWL is similar to the length at the waterline or LWL. BWL is the maximum width or breadth of a ship’s hull measured at a particular waterline. In practice, BWL may be less than the maximum width, but in some cases, it may be less than the maximum width. 

Let’s Consider A Simple Example

For a barge whose hull shape is more or less uniform both lengthwise and transversely, the maximum width corresponds to the BWL. Another term, beam on centerline, or BOC, is commonly used for multihulls. For a catamaran or twin hull, the BOC is measured as the transverse distance between two hulls from their respective centerlines at the level of the exposed or starched deck. Similarly, the BOC of a trimaran is measured as the distance between two outermost hulls estimated at their respective centerlines at the level of the main deck. Now, all width and depth parameters have what are called shape and extreme dimensions.

The form dimensions do not take into account the thickness of the skin, but the extreme dimensions do take into account the thickness or dimension as well. Thus, the width or breadth of a form is the measured width of the fuselage part from the inside of the side skin at one end to the widest point or area on the inside of the panel at the other end. The extreme width also takes into account the thickness of the side skin. For most design purposes, the extreme width is taken into account. At this point, it is important to note that width is something quite different from circumference and is often confused with it. As we just explained, width is the straight line distance measured from one side to the other.


The perimeter of a particular section, on the other hand. Is the measurement of the circumference from one end of the particular section to the other. For a transverse section of a hull, the circumference is measured as the total length or extent from one deck end to the other deck end, taking into account the shape of the section. Simply put, if the outline of the hull were to be straight like a thread and projected onto a straight line, the measured length or span of that section would be the circumference. 

Circumference is directly proportional to width and vice versa. Another very simple way to understand circumference is to imagine a necklace. The width measured gives the distance from one chain to the other. However, when the necklace is unfolded, it is laid straight on a table, and the distance measured from one end to the other gives the length, in this case, the circumference.

Beams In Ship Design 

Ship beams are fundamental in every stage of design and construction. However, the length-to-beam ratio or L: B is often used to define ship designs and hull shapes. Slimmer or slender vessels such as fast passenger ferries and warships like frigates and corvettes have a higher L: B ratio. Their maximum width is less than their length. They usually have higher speed characteristics. Fuller-shaped vessels such as tankers, bulk carriers, etc., have a higher beam and, hence, a comparatively lower L: B ratio. They are usually slower ships. Container ships are halfway between slim and large ships. Since both the speed and the volume of the hull must be optimized to ensure maximum cargo loading. 

The width of a ship is also a direct measure of the lateral stability of the ship. The larger the width, the more lateral stability is due to the larger water surface. But on the other hand, a wider beam creates greater problems after capsizing. Or deck flooding since more energy is required to return the ship from its static, stable state to an upright position. A ship with a larger beam also has the option of having a larger deck area and internal tank space. The width of a ship is an important factor when navigating canals and canals. As narrow spaces and ship passage are often an issue for wider ships.

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