In the basic design of a ship, various terms are used to describe its principal dimensions, such as length, breadth, depth, and draft, as well as coefficients representing hull form and terms indicating the size and weight of the ship.

This article explains the basic terminology used in ship design, including principal dimensions, hull-form coefficients, capacity tonnage, and weight tonnage.

1. Principal Dimensions of a Ship

Length

Length Overall (L_OA)
Length overall (L_OA) is the horizontal distance measured from the foremost point of the bow to the aftermost point of the stern.

Length Between Perpendiculars (L_PP, L_BP, or L)
Length between perpendiculars is the horizontal distance between the Forward Perpendicular (F.P.) and the After Perpendicular (A.P.).
The forward perpendicular is a vertical line passing through the intersection of the designed full-load waterline and the outer line of the shell plating at the forward face of the stem.
The after perpendicular is a vertical line passing through the after face of the rudder post for a ship fitted with a rudder post, or through the centerline of the rudder stock for a ship without a rudder post.

Midship
The point located halfway between the forward and after perpendiculars is called midship.

Registered Length (L_R)
Registered length is the length of a ship used for official vessel registration and documentation.
Its precise definition and method of measurement may vary depending on the applicable national regulations.

Waterline Length (L_WL)
Waterline length is the length measured from the forward end to the after end along the line at which the hull intersects the water surface.
It is used when evaluating matters such as hull resistance.

Breadth

Molded Breadth (B_MLD, B)
Molded breadth is the horizontal distance measured between the inner surfaces of the shell plating at the widest part of the hull.

Extreme Breadth (B_EXT, B_MAX)
Extreme breadth is the horizontal distance measured between the outer surfaces of the shell plating at the widest part of the hull.

Depth and Draft

Molded Depth (D_MLD, D)
Molded depth is the vertical distance measured from the top of the keel to the point where the inner line of the side shell plating intersects the underside of the upper deck.
Where the deck edge has a rounded gunwale, the underside of the upper deck and the inner line of the side shell plating are extended, and the vertical distance is measured to the point at which the two extended lines intersect.

Full-Load Draft
Full-load draft is the draft of a ship when it is loaded to the maximum permissible limit at which sufficient reserve buoyancy is maintained for safe navigation. The corresponding waterline is called the full-load waterline.
The full-load draft is determined by taking into account factors such as hull form and structural strength, and varies according to the navigation area and season.

Molded Draft (d_MLD, d)
Molded draft is the vertical distance measured from the top of the keel to the waterline.
For design purposes, the molded draft measured to the designed summer load waterline is referred to as the design full-load draft.

Extreme Draft (d_EXT, d_MAX)
Extreme draft is the molded draft plus the thickness of the keel.

2. Hull-Form Coefficients

The principal coefficients used to represent hull form include the block coefficient, midship section coefficient, prismatic coefficient, vertical prismatic coefficient, and waterplane area coefficient.

Block Coefficient (C_B)
The block coefficient is the ratio of the molded displacement volume $\nabla$ to the volume of a rectangular block defined by the length between perpendiculars $L_{PP}$, molded breadth $B_{MLD}$, and molded draft $d_{MLD}$.

Midship Section Coefficient (C_M)
The midship section coefficient is the ratio of the molded midship section area $A_M$ to the area of a rectangle defined by the molded breadth $B_{MLD}$ and molded draft $d_{MLD}$.

Prismatic Coefficient (C_P)
The prismatic coefficient is the ratio of the molded displacement volume $\nabla$ to the volume of a prism whose cross-sectional area is the molded midship section area $A_M$ and whose length is the length between perpendiculars $L_{PP}$.

Vertical Prismatic Coefficient (C_V)
The vertical prismatic coefficient is the ratio of the molded displacement volume $\nabla$ to the volume obtained by multiplying the molded waterplane area $A_W$ by the molded draft $d_{MLD}$.

Waterplane Area Coefficient (C_W)
The waterplane area coefficient is the ratio of the molded waterplane area $A_W$ to the area of a rectangle defined by the length between perpendiculars $L_{PP}$ and molded breadth $B_{MLD}$.

All of these hull-form coefficients are dimensionless quantities.

3. Capacity Tonnage

One method of expressing the size of a ship is capacity tonnage, which is calculated on the basis of the ship’s internal volume.

Traditional Gross Register Tonnage and Net Register Tonnage

Gross Register Tonnage (G.R.T.)
Traditional gross register tonnage was calculated on the basis of the internal volume below the tonnage deck and the measurable volume of enclosed spaces above the tonnage deck.

It was expressed in register tons, with 100 cubic feet (approximately 2.83 m³) equal to one register ton.

Net Register Tonnage (N.R.T.)
Traditional net register tonnage was calculated by deducting spaces required for the operation of the ship—such as machinery spaces, crew accommodation, and ballast tanks—from the volume used to determine gross register tonnage.

It was used as an indicator primarily representing the volume of spaces available for carrying cargo and other revenue-earning loads.

Gross Tonnage (GT)

The International Convention on Tonnage Measurement of Ships, 1969 was adopted in 1969, introducing an internationally standardized system for measuring ship tonnage. The Convention entered into force in 1982.

Gross tonnage (GT) is a numerical value representing the overall size of a ship, calculated on the basis of the total volume of all enclosed spaces of the ship.

$V$: Total volume of all enclosed spaces of the ship [m³]

$K_1$: Coefficient determined from $V$

Net Tonnage (NT)

Net tonnage (NT) is a numerical value calculated on the basis of the volume of cargo spaces, taking into account the ship’s draft, depth, number of passengers, and other factors.

$V_C$: Total volume of cargo spaces [m³]

$D$: Molded depth amidships [m]

$d$: Vertical distance from the lower terminal of the molded depth to the reference waterline [m]

$N_1$: Number of passengers in cabins containing no more than eight berths

$N_2$: Number of other passengers

The following conditions apply when calculating net tonnage:

・If $N_1+N_2$ is less than 13, both $N_1$ and $N_2$ are taken as zero.

・$\left(\frac{4d}{3D}\right)^2$ must not exceed 1.

・$K_2V_C\left(\frac{4d}{3D}\right)^2$ must not be less than 25% of gross tonnage (GT).

・Net tonnage (NT) must not be less than 30% of gross tonnage (GT).


Although the current net tonnage (NT) is similar in name to the traditional net register tonnage (N.R.T.), it is calculated under the 1969 Convention and therefore differs from the former method based on volumetric register tons.

4. Weight Tonnage

In addition to capacity tonnage based on internal volume, weight tonnage is used to express the weight of a ship and the loads carried aboard it.

Full-Load Displacement (Δ_F)
The displacement of a ship at its full-load draft is called full-load displacement.
The density of seawater may be taken as 1.025 t/m³.

Lightweight (Δ_L, LW)
Lightweight is the weight of the ship itself in its completed condition.
It includes the fixed weight of the hull, machinery, equipment, statutory equipment, and other permanent items.

Deadweight (D/W, DWT)
Deadweight is the difference between the full-load displacement and the lightweight.
Deadweight includes not only cargo but also fuel, water, crew, consumable stores, equipment, and other variable loads. Therefore, the actual cargo-carrying capacity is obtained by subtracting these weights from the deadweight.

About This Article

References
・Ship Basic Design Theory, by Dr. Tetsujiro Tomita
・Merchant Marine College Career Education Research Association (Ed.), Introduction to Marine Engineering & Naval Architecture
・Illustrated Introduction: The Latest Fundamentals and Mechanisms of Ships, 4th Edition, by Toyohiko Kawasaki

Reference Materials
・International Maritime Organization (IMO), “International Convention on Tonnage Measurement of Ships”

(Last accessed: August 29, 2026)


※This article was prepared with reference to the sources listed above and organized based on the author’s understanding.

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