What Is Moulded Depth?
Moulded depth (standard symbol D; spelled molded depth in American English) is one of the principal dimensions of a vessel. It is the vertical distance measured amidships from the top of the keel to the top of the freeboard deck beam at side — in other words, the hull's internal height from its baseline up to the deck from which freeboard is assigned.
The authoritative definition is given in Regulation 3 of the International Convention on Load Lines, 1966 (ICLL 1966):
"The moulded depth is the vertical distance measured from the top of the keel to the top of the freeboard deck beam at side."
The same regulation sets out modified baselines for wood and composite ships and unusual midship sections (see below).
Because the measurement follows the vessel's moulded lines — the theoretical surface of the hull framing, excluding shell and deck plating thickness — moulded depth is a purely geometric, construction-based dimension. Fixed when the ship is built, it is recorded on the general arrangement plan and statutory certificates and never changes with cargo load or ballast condition.
How Moulded Depth Is Measured
Reference points
A moulded depth measurement has three fixed reference points:
Bottom (baseline): the top of the keel, which defines the vessel's baseline.
Top: the top of the freeboard deck beam at the ship's side, equivalent to the underside of the freeboard deck plating.
Location: amidships, at the mid-length of the load line length, measured vertically at the vessel's side.
Special hull forms
The ICLL definition and classification society rules add provisions for non-standard construction:
Wood and composite ships: measurement starts from the lower edge of the keel rabbet.
Hollow midship sections or thick garboard strakes: the baseline is where the line of the flat of the bottom, extended inwards, intersects the side of the keel.
Rounded gunwales: depth is taken where the freeboard deck line, extended as though the gunwale were angular, meets the moulded shell line.
Moulded depth vs extreme depth
Extreme depth follows the same vertical path but includes the actual thickness of bottom shell, deck plating and any projecting keel. Moulded depth is used for statutory freeboard, tonnage and classification work; extreme depth matters where the hull's full physical envelope is relevant, such as drydock blocking or clearance checks.
Moulded Depth, Draught and Freeboard
The three vertical dimensions of a ship are closely related but not interchangeable:
| Dimension | Symbol | Measured from → to | Changes with loading? |
|---|---|---|---|
| Moulded depth | D | Top of keel → top of freeboard deck beam at side | No — fixed at build |
| Draught (draft) | T | Top of keel / baseline → actual waterline | Yes — varies with cargo and ballast |
| Freeboard | F | Deck line (freeboard deck edge) → assigned load line | Yes — increases as draught decreases |
In simple terms, freeboard is the complement of draught within the ship's depth: the deeper a vessel loads, the less freeboard remains. ICLL 1966 defines assigned freeboard as the vertical distance amidships from the upper edge of the deck line to the upper edge of the relevant load line (the Plimsoll mark).
For freeboard calculations, surveyors use a "depth for freeboard" (D) — moulded depth amidships plus the stringer plate thickness and, where applicable, a deck sheathing correction. Moulded depth also enters the definition of a ship's load line length, which is based on a waterline at 85 per cent of the least moulded depth.
Why Moulded Depth Matters
Load line assignment and reserve buoyancy
Moulded depth is the geometric foundation of the load line system. Together with length, block coefficient, sheer and superstructure, it determines how deeply a vessel may legally load and how much enclosed volume must remain above the waterline as reserve buoyancy. A deeper hull, all else equal, provides a larger seaworthiness margin.
Tonnage measurement
Under the International Convention on Tonnage Measurement of Ships, 1969, a parallel moulded depth concept — measured from the top of the keel to the underside of the upper deck at side — is used when calculating under-deck volumes for gross and net tonnage. Because deck references differ slightly between conventions, surveyors apply the definition belonging to the certificate being issued.
Stability, strength and design
Naval architects use moulded depth in hydrostatic and stability calculations, including the vertical positions of the centres of gravity and buoyancy (KG, KB) and metacentric height (GM). A greater depth also increases hull girder section modulus, strengthening the vessel against hogging and sagging. The dimension likewise appears in classification rules, newbuilding specifications and drydock planning: alongside length overall (LOA), beam and design draught, it determines whether a ship fits a given dock, lock or berth.
FAQ
What is the difference between moulded depth and draught?
Moulded depth is a fixed structural dimension from the top of the keel to the freeboard deck beam at side. Draught is the distance from the baseline to the current waterline and changes as cargo, fuel and ballast are loaded or discharged. Depth describes the hull; draught describes how deeply it floats.
Where exactly is moulded depth measured?
Vertically amidships, at the vessel's side, from the top of the keel to the top of the freeboard deck beam — effectively the underside of the freeboard deck plating. ICLL 1966 Regulation 3 gives the full definition, including special rules for wooden, composite and rounded-gunwale hulls.
What is the difference between moulded depth and extreme depth?
Moulded depth follows the theoretical moulded lines and excludes shell and deck plating thickness. Extreme depth includes plating thickness and any projecting keel, representing the hull's full physical vertical envelope. Statutory freeboard and tonnage work use moulded depth.
How does moulded depth affect a ship's freeboard?
Freeboard is assigned from the freeboard deck, whose height above the keel is the moulded depth. Depth thus sets the vertical envelope shared by draught and freeboard: it feeds directly into the ICLL freeboard calculation and the reserve buoyancy retained at the assigned load line.
Is it "moulded depth" or "molded depth"?
Both are correct. "Moulded depth" is the spelling used in ICLL 1966 text, British English and most international maritime documentation; "molded depth" is the standard US English spelling used in US Code of Federal Regulations (46 CFR) literature. They mean the same dimension.