In the basic planning of a ship, the shipowner’s requirements regarding ship speed, deadweight, and the maximum number of persons on board must be established in addition to the ship’s type and intended purpose.
This section explains the basic meaning of each of these terms.

1.2.1 Ship Speed

Even though the International System of Units (SI) is now widely used, ship speed is generally expressed in knots.
Ship speed varies according to the type of ship. Cargo ships commonly operate at speeds in the range of 10 to 19 knots, while some passenger ships and ferries operate at speeds exceeding 20 knots. Certain high-speed vessels can reach speeds of more than 50 knots.

Ship speed also varies with displacement. It is therefore necessary to specify the ship’s loading condition, such as the full-load draft or design draft, at which the stated speed applies.
A sea margin must also be established to account for the reduction in speed that occurs during actual navigation.

Unit of Speed

The knot is a unit used to express the speed of a ship. One knot is equal to a speed of one nautical mile per hour.
One nautical mile (M) is 1,852 m and corresponds to one minute of latitude, or one-sixtieth of one degree.

Service Speed and Sea Margin

Service speed (V_S) is the speed attained at the design full-load draft when the engine is operated at its normal output rating (NOR).

During actual navigation, ship speed is reduced by weather and sea conditions such as wind and waves, as well as by factors such as hull fouling.
The allowance in propulsion power provided to account for these effects is called the sea margin (SM).
A sea margin of 10% or 15% is commonly used.


The sea margin is expressed by the following equation.

P₁: Power required to attain the service speed in sufficiently deep water, with a clean hull and without the effects of adverse weather or sea conditions

Trial Speed (V_T)

After a ship has been constructed, sea trials are conducted to measure its speed, engine output, and other performance characteristics.
The speed measured during these trials is called the trial speed (V_T).

The contractual speed and other specified speeds are defined for prescribed draft and environmental conditions.
Because the actual conditions during sea trials may differ from these specified conditions, the trial results are corrected to account for the effects of draft, water depth, wind, waves, currents, and other factors.

1.2.2 Deadweight and Cargo-Carrying Capacity

Deadweight is one of the principal measures used to indicate how much a ship can carry.
However, the method used to express cargo-carrying capacity varies according to the type of ship and cargo.

For example, the capacity of a container ship is expressed in TEU or FEU, the capacity of a car carrier is expressed by the number of vehicles it can carry, and the capacity of an LNG carrier is expressed by the volume of LNG it can carry. These values also serve as indicators of the size and cargo-carrying capacity of each ship.

Methods of Expressing Capacity by Ship Type

TEU, a measure of the cargo-carrying capacity of a container ship, stands for Twenty-foot Equivalent Unit and represents the number of containers expressed in terms of standard 20-foot containers.

FEU stands for Forty-foot Equivalent Unit and represents the number of containers expressed in terms of standard 40-foot containers.

For other ship types, the number of vehicles carried by a car carrier and the volume of LNG carried by an LNG carrier are used as measures of their respective cargo-carrying capacities.

Deadweight and Gross Tonnage

Deadweight is an important value when the shipowner and shipyard are developing the basic plan for a ship.
In addition to deadweight and displacement, gross tonnage is commonly used as an indicator of a ship’s overall size.

Gross tonnage is used as a basis for calculating port dues and various other charges. It is also used under applicable rules and regulations as a criterion for determining matters such as the equipment required aboard a ship and its manning requirements.

Cargo Hold Capacity

Cargo hold capacity is the total volume within the cargo holds that is available for loading cargo and represents the ship’s cargo-carrying capacity by volume.

For ships carrying relatively low-density cargo such as wood chips, cargo hold capacity, as well as cargo weight, is an important measure of cargo-carrying capacity.

Cargo holds contain projecting structural members such as frames and beams.
Because the volume that can actually be used varies according to the form of the cargo, cargo hold capacity may be expressed as either grain capacity or bale capacity.

Grain Capacity

Grain capacity is the volume available when carrying cargo, such as grain, that can flow into the spaces around the structural members inside the cargo hold.
It includes the volume extending to the inner surface of the shell plating and is commonly used for bulk carriers.

Bale Capacity

Bale capacity is the volume available for carrying packaged cargo and is measured within the projections of the structural members inside the cargo hold.

1.2.3 Maximum Number of Persons on Board

The maximum number of persons on board is a planning requirement that specifies the number of passengers and crew members the ship is intended to carry.

For passenger ships, passenger capacity is an important condition in the ship’s basic plan.
Although the number of persons carried has less influence on the planning of a cargo ship than on that of a passenger ship, the number of crew members affects the required number of cabins and berths. It is also a relevant condition when considering the automation of onboard systems and the remote operation of machinery and equipment.

Because the officers and ratings may be of different nationalities, the number and composition of the crew are also specified as part of the shipowner’s requirements.

About This Article

References
・An Introduction to Naval Architecture in One Volume, edited by the Maritime College Career Education Research Association
・Ships: From Initial Inquiry to Scrapping, edited by the Editorial Committee of the Kansai Society of Naval Architects


Reference Material
・ITTC, “Preparation, Conduct and Analysis of Speed/Power Trials,” Recommended Procedure 7.5-04-01-01.1, Revision 08, 2024.
(Last accessed: September 10, 2026)


※This article was prepared with reference to the sources listed above and organized based on the author’s understanding. The ITTC recommended procedure was also consulted for the explanation of trial speed.

General Arrangement & Basic Planning