LOA / Beam / Depth

What are a vessel’s principal dimensions?

A vessel’s principal dimensions are length overall (LOA), length between perpendiculars (LBP), beam (breadth), depth, draught and air draught. Together they fix what berths, channels, locks and bridges a ship can use, and they are the first numbers a charterer checks for fit.

These six measurements are the dimensional fingerprint of any merchant ship, and for a dry bulk carrier they decide far more about the commercial fit than the deadweight figure alone. The classification societies that survey and certify the hull, principally Lloyd’s Register, DNV and ABS, record the principal dimensions on the ship’s particulars and class certificate, and these are the figures owners and brokers quote when offering a vessel. Where a vessel-class page such as Capesize reads the dimensions down a single class, this page reads one set of dimensions across every class, because the same six terms describe a Handysize and a Capesize alike. Get the terms right and a fixture clears the load-port and discharge-port constraints on the first pass.

The principal dimensions defined

The table below sets out the six principal dimensions, how each is measured and the convention that governs the figure. The “reference” column names the authority that defines or records the measurement.

Dimension	Definition	How measured	Unit	Reference
Length overall (LOA)	Extreme length of the hull from the foremost to the aftermost point	Tip to tip including bulbous bow and any stern overhang	m	Class society ship particulars (Lloyd's Register, DNV, ABS)
Length between perpendiculars (LBP)	Length from the forward perpendicular to the aft perpendicular at the summer load line	Measured along the waterline between the two perpendiculars	m	Class society hull definition; basis for many tonnage and freeboard calculations
Beam, moulded	Maximum breadth of the hull measured to the inside of the shell plating	Widest moulded section, frame to frame	m	Class society moulded line; the figure quoted for canal-lock fit
Depth, moulded	Vertical distance from the top of keel to the freeboard deck at side	Moulded depth amidships	m	Class society moulded line; basis for freeboard and capacity
Draught, summer	Vertical distance from the waterline to the lowest point of the keel at the summer mark	Read at the summer load line, fully loaded in salt water	m	Load Line Convention summer mark, assigned by class society
Air draught	Vertical distance from the waterline to the highest fixed point of the ship	Top of mast or accommodation to the waterline at a given draught	m	Ship particulars; varies with loaded condition

A practical note on each: LOA is the figure that must fit a berth and a canal lock chamber end to end; LBP is the engineering length that underlies tonnage, trim and stability work; moulded beam is the breadth quoted for canal-lock fit; moulded depth governs how much freeboard the hull can be assigned and therefore how much cargo it can carry; summer draught is the deepest a vessel may legally float in the temperate summer zone; and air draught is the one dimension that grows smaller as the ship loads deeper, because the waterline rises towards the masthead.

Length overall vs length between perpendiculars; moulded vs extreme

Three pairs of terms cause most of the confusion when dimensions are read off a particulars sheet, and they are worth disambiguating directly.

LOA versus LBP. Length overall is the physical extreme length of the steel, bulbous bow and stern overhang included, and it is the number a berth or a lock chamber must physically accommodate. Length between perpendiculars is shorter: it runs between the forward perpendicular (where the summer waterline crosses the stem) and the aft perpendicular (conventionally the rudder stock axis). LBP is the calculation length used for tonnage, freeboard and stability. A vessel quoted with an LOA of 229m may have an LBP nearer 222m. Use LOA for berth and lock fit; use LBP for naval-architecture calculations.

Moulded versus extreme breadth. Moulded beam is measured to the inside of the shell plating, frame to frame. Extreme beam adds the thickness of the shell plating and any permanently attached fittings such as fendering or rubbing strakes. The difference is small, often a few centimetres, but it matters at a canal lock or a tight berth where the binding clearance is the extreme breadth, not the moulded figure that appears first on the particulars.

Depth versus draught. Depth is a fixed structural dimension of the hull, measured from keel to the freeboard deck, and it does not change with loading. Draught is a variable operating dimension, the depth of water the hull needs at a given load, and it rises as cargo goes in. The gap between depth and loaded draught is, broadly, the freeboard. Confusing the two is the single most common dimensional error, and it is treated again in the confusions section below. See draught for the operating side of this.

Where the dimensions sit on the hull

The diagram below places each principal dimension on the hull so the relationships are visible at a glance rather than read off a list.

Image Placeholder Bulk carrier principal dimensions: side elevation and plan view Side elevation labelling LOA bow-to-stern, LBP between forward and aft perpendiculars, depth keel-to-deck, summer draught keel-to-waterline, and air draught waterline-to-masthead; plan view labelling moulded beam at the widest section.

Read the side elevation first. LOA spans the full steel from the tip of the bulbous bow to the aftermost point of the stern. LBP sits inside it, running between the forward and aft perpendiculars at the summer waterline. Depth is the vertical span from keel to freeboard deck, and summer draught is the lower portion of that span, from keel to waterline, when the ship floats at her summer mark. Air draught is the vertical span above the waterline, from the surface to the highest fixed point, usually the masthead or the top of the accommodation block. The plan view shows moulded beam at the widest section amidships. Because air draught is measured upward from the waterline and draught downward from it, the two move in opposite directions as the ship loads: deeper draught means lower air draught.

Why dimensions govern port and canal access

The principal dimensions are not academic. Each one maps to a specific access constraint that can stop a fixture, and the binding constraint changes from port to port.

• Beam sets canal-lock fit. The width of a canal lock chamber is a hard limit. The Panama Canal Authority publishes a maximum beam for the Neopanamax locks, and the Suez Canal Authority publishes a beam-and-draught combination rather than a single beam cap. A vessel one centimetre over the published beam cannot transit, regardless of deadweight. Beam is therefore the dimension that defines the Panamax and Neopanamax classes by name.
• LOA sets berth length. A berth can only take a vessel whose overall length fits between the mooring dolphins, with an allowance for ranging. Where LOA exceeds the berth, the vessel works at anchor or is turned away.
• Draught sets channel and berth depth. The available water depth at the approach channel and alongside the berth, adjusted for tide, sets the maximum draught a vessel may arrive at. Where the receiver cannot take full summer draught the cargo is part-loaded or down-tiered to a smaller class, with a freight-per-tonne penalty.
• Air draught sets bridge and shore-crane clearance. A fixed bridge across an approach, or the lift height of a shore crane or loader spout, caps the air draught. A lightly loaded ballast vessel can sit too high to pass under a bridge that a laden vessel clears comfortably.

Draught and air draught interact, because loading the ship deeper lowers her air draught, and the summer mark that caps draught is set under the international load line regime. See draught for how draught is surveyed and read, and load line for how the summer mark that caps it is assigned. The wider vessel-class catalogue sits on the bulk-carriers hub.

How dimensions vary by bulk carrier class

The same six terms describe every dry bulk class; only the numbers change. The table below reads LOA, beam and summer draught across the mainstream classes, with each class cell linking to its full specification page.

Read the table down the beam column and the class structure falls out of one dimension. The Panamax beam clusters tightly around 32.2 to 32.3m because it is dimensioned to the original Panama Canal lock width, the canonical case of a single principal dimension defining a commercial class. Above that beam a vessel is excluded from the old locks and becomes, by name, a Capesize. Draught grows roughly in step with beam and length, which is why the deepest receivers and the deepest-cut channels are the gating constraint for the largest classes. The relationship is monotonic but not linear: a Capesize is not simply a scaled-up Handysize, because beam grows faster than length once a class is freed from canal-lock width caps.

Common confusions about vessel dimensions

• Moulded versus extreme breadth. Moulded beam is measured to the inside of the shell plating; extreme beam adds the plating thickness and any fittings. The particulars usually quote moulded. Canal and berth clearance bind on extreme, so the operative figure for a tight transit is the larger one.
• Summer draught versus scantling draught. Summer draught is the deepest the vessel may load in the temperate summer zone under the assigned load line. Scantling draught is the structural design draught the hull is built to, which can be deeper than the assigned summer draught. Quoting scantling draught as if it were the operating draught overstates how deep the ship may legally float.
• LOA versus registered length. Length overall is the physical extreme length. Registered length is a separate legal figure recorded on the registry document for tonnage and administrative purposes, and it is not the figure to use for berth or lock fit.
• Depth versus draught. Depth is fixed hull structure, keel to freeboard deck. Draught is variable and depends on loading. The two are routinely conflated. Depth never changes; draught changes every voyage with the cargo loaded.

Scope and what this page does not cover

This page defines the principal dimensions of a dry bulk carrier, explains how each is measured and which access constraint it governs, and reads the dimensions across the mainstream classes. It does not give the assigned dimensions of any specific named vessel, perform stability, trim or tonnage calculations, or state the current published lock and channel limits of any individual canal or port. For named-vessel particulars work from the class certificate and the owner’s ship particulars; for current canal and port limits consult the Panama Canal Authority, the Suez Canal Authority or the relevant port authority directly; and for a fixture, work with a desk broker against verified ship particulars.