In this blog post, we’ll explain why shipyards are said to “sell time” and how the Tandem and Semi-Tandem methods improve construction efficiency.

What It Means for a Shipyard to “Sell Time”

The expression “selling time” is commonly used in the shipbuilding industry. This stems from the nature of shipbuilding as a made-to-order business. Shipyards do not sell pre-manufactured products; instead, they enter into contracts with shipowners promising to “complete the ship by a certain date” and carry out design and production to meet that deadline.
Because ships are large and requirements vary by order, delivery deadlines are critical. Since a delay of even one day can result in substantial liquidated damages owed to the shipowner (the amount varies depending on the ship type and size), strict adherence to schedules and trust are core competitive advantages in the shipbuilding industry. Therefore, shipyards strive to maximize profits by building more ships within the same timeframe, and to this end, they research and apply various construction methods and process optimization techniques.

Overview of the Tandem and Semi-Tandem Methods

Generally, a dock is the location within a shipyard where a vessel is finally completed, and the size and number of docks serve as key indicators of a shipyard’s production capacity. Traditionally, only one vessel was built in a single dock at a time, which limited the number of vessels that could be constructed simultaneously.
However, since vessels vary in size and often do not fully occupy a large dock, efforts were made to efficiently utilize the remaining space. The method of building multiple ships in parallel within the same dock is known as the Tandem method; however, in actual practice, the Semi-Tandem method—which involves building in parallel using only a portion of the dock due to space constraints and safety concerns—is more common.
In large docks, it is even possible to complete and launch (the process of floating a completed ship into the water and sending it out to sea) two or more ships almost simultaneously, and this is also referred to as Tandem. Although it may seem like a simple concept, its actual implementation involves numerous technical and planning challenges.

Dock Operation Methods and Construction Workflow

A typical dry dock is a structure created by excavating the coastline into a “C” shape and installing steel gates to allow seawater to be let in and out. During construction, the seawater is drained to keep the dock dry, ensuring the interior of the hull is not exposed to water. Once the ship is completed, the dock is refilled with water to launch the vessel.
In the Tandem or Semi-Tandem method, as soon as a space becomes available inside the dock, construction of another ship begins in that space. The first completed ship is partially or fully launched, and the unfinished ship that was in that spot is moved to allow for final work. The key is to continuously utilize these vacant spaces to minimize idle time in the dock.

Technical Requirements of the Tandem Method

To operate this method safely and efficiently, there are three main technical requirements. First, watertight technology is needed to protect the unfinished hull from seawater. Temporary water-stopping structures or sealing devices must be installed to prevent the partially open interior from becoming submerged.
Second, since an unfinished vessel lacks its own propulsion, facilities and procedures are needed to safely move the vessel within the dock or during the launching process. Technology for the precise movement and mooring of heavy structures, along with towing equipment and mooring plans, is essential.
Third, handling multiple vessels of different sizes and with varying delivery schedules simultaneously requires sophisticated scheduling and resource allocation. Efficiency is maximized by developing a three-dimensional process plan that goes beyond simple two-dimensional layout and takes into account block assembly, equipment placement, and workforce scheduling.

Two-Way Docks and Their Variations

Depending on the dock design, there are also two-way docks that feature a movable steel gate in the center, with vessels positioned on both sides.

In this configuration, seawater does not flow into the opposite side when a vessel is launched from one side, so separate watertight measures for the unfinished hull may not be necessary in some cases. The decision to adopt this design should be based on a careful evaluation of its pros and cons.

Disadvantages and Operational Considerations

While the Tandem and Semi-Tandem methods can significantly increase dock utilization rates, they also increase risks and complexity. For example, the launch of a single vessel or a delay in the process can have a cascading effect on the schedules of adjacent vessels, and the risk of safety accidents may increase in such a congested work environment.
Furthermore, complex movement and mooring operations inside the dock, additional costs for watertight devices, and more management resources for precise process planning are required. Therefore, before adopting this method, economic feasibility, safety, and process flexibility must be comprehensively evaluated.

Scope and Conclusion of Naval Architecture and Ocean Engineering

Naval architecture and ocean engineering is not merely the technology of building ships; it is a discipline that deals with various mechanical phenomena such as resistance, propulsion, structure, and noise and vibration, and it is a field where industrial applications and basic research coexist. Not only is research with a strong industrial focus—such as dock operations and process innovation—advancing, but theoretical research is also steadily progressing.
The tandem method is one practical solution aimed at reducing idle space in dry docks and increasing production volume; it can yield significant results when supported by appropriate technology and management. To the question, “Is further research on ships necessary?” one can answer that as long as the industry and technology continue to evolve, the need for such research will persist.