In this blog post, we’ll explore everything from the principles behind how ships generate thrust to move forward, to the characteristics of various types of resistance—such as frictional resistance and wave resistance—that hinder a ship’s speed, and finally, methods for reducing these resistances to operate more efficiently with less energy.
The modes of transportation we use in our daily lives include not only buses, subways, and cars, but also ships. It’s easy to assume that ships are generally slower than other modes of transportation, but that isn’t necessarily the case. A ship’s speed varies greatly depending on its type and purpose. In particular, a WIG craft—which flies close to the water’s surface using ground effect—can travel very quickly in a manner entirely different from that of a conventional ship. The term “WIG” is derived from “Wing-In-Ground” and refers to the use of ground effect generated by wings near the water’s surface or the ground. The International Maritime Organization (IMO) defines a WIG craft as a composite mode of transport that flies using ground effect without continuously contacting the water surface during its primary mode of operation, and certain types of WIG crafts are subject to the maritime regulatory framework. As mentioned in the original text, a ship’s speed varies greatly depending on its type, and ships transporting bulk cargo such as crude oil or iron ore often operate at relatively low speeds for economic reasons. In contrast, container ships and high-speed vessels, for which transit time is critical, operate at higher speeds. Therefore, when designing and operating a ship, speed is not merely a matter of “how fast it can go,” but a crucial factor that influences energy consumption, operating costs, and even the shipping schedule. The original text also uses the fact that speed varies greatly depending on the type of ship as a starting point to explain the relationship between a ship’s resistance and economic efficiency.
A ship requires propulsive force to move forward, and the force exerted by the propulsion system to push the ship forward is called thrust. Since ships move through water or along the water’s surface, they encounter various forms of resistance during their journey. For a ship to increase its speed from a stationary state, the forward force generated by the propulsion system must exceed the resistance; when operating at a constant speed, the propulsion force and resistance are in equilibrium. This is easy to understand if we imagine swimming forward through water in a pool. When a net force acts on a ship, acceleration occurs, which can be explained by Newton’s Second Law: F = ma. In a typical propeller-driven vessel, the propeller installed at the stern rotates, transferring momentum to the water; as a reaction, thrust in the forward direction is generated on the ship. This can be explained from the perspective of Newton’s third law—the principle of action and reaction—and the conservation of momentum.
To make an object move faster, one must either increase the net force acting on it to boost acceleration or reduce the resistance that hinders its motion. The same applies to ships. By generating greater propulsive force or reducing the drag acting on the hull, a ship can reach its desired speed more quickly. However, in actual ship design, there are limits to increasing speed simply by continuously increasing propulsive force. This is because as speed increases, the fluid resistance acting on the hull generally increases as well, and the required propulsion power also rises significantly. In practical ship design, after setting a target speed, factors such as the drag generated at that speed, the required effective power, and propulsion efficiency are all considered together. In ship resistance tests conducted by the International Towing Tank Conference (ITTC), the total resistance coefficient is analyzed in relation to the ship’s speed, water density, and wetted surface area, and speed is treated as a key variable in determining ship resistance and propulsion performance.
The resistance acting on a ship can be broken down into several components. Traditionally, frictional resistance was distinguished from total resistance, with the remainder described as residual resistance; however, today, various components—such as viscous resistance, frictional resistance, viscous pressure resistance, wave-induced resistance, air resistance, and appendage resistance—are analyzed in greater detail. The ITTC also defines frictional resistance as the resistance caused by fluid friction occurring at the object’s surface, and defines residual resistance as the portion of total resistance excluding frictional resistance. Simply put, frictional resistance is the resistance generated by friction between the hull and the water. Since water is much more viscous than air, a hull moving through water experiences significant frictional resistance. If we assume a ship is moving through a substance much more viscous than water, the resistance experienced by the hull’s surface would be even greater. Frictional resistance can be understood by considering the shear stress acting parallel to the hull’s surface across the entire wetted surface of the hull.
A ship’s hull below the waterline is in contact with water, while the hull and superstructure above the waterline are in contact with air. Therefore, during actual operation, resistance is generated not only by water but also by air. However, since frictional resistance due to the viscosity of water plays a major role in the submerged portion of the hull, friction between the water and the hull is a very important factor when analyzing a ship’s resistance. Conversely, above the waterline, air resistance—or wind resistance—acts on the hull and superstructure. Imagine two people traveling 50 meters in a swimming pool: one swimming underwater and the other running on dry land. This analogy makes it easy to understand just how much fluid resistance is encountered when moving underwater.
In the traditional classification of resistance, residual resistance refers to the portion of total resistance excluding calculated frictional resistance. Among these, wavemaking resistance plays a particularly significant role depending on the ship’s speed and hull form. “Wavemaking” literally means “creating waves.” Wavemaking resistance should not be understood simply as the resistance caused by a ship colliding with waves coming from the outside. Rather, it is the resistance generated as a ship creates surface waves on its own in calm water. The ITTC also defines wavemaking resistance as a resistance component resulting from the energy expended in generating gravitational waves.
If you observe a boat moving quickly at a beach or on a river, you can easily see waves forming around the hull and behind it as the boat passes. These waves do not arise without any energy expenditure. The energy supplied by the ship’s propulsion system is used not only to propel the ship forward but also to move the fluid around the hull and create waves. Therefore, not all of the energy generated by the propulsion system is used solely for the vessel’s forward motion. As the hull forms surface waves and transfers energy outward, this creates a force that impedes the vessel’s forward motion, which manifests as wave resistance. However, rather than viewing this as a “loss” where the energy itself disappears, it is more accurate to understand that a portion of the energy supplied for propulsion is transferred in the form of waves and fluid motion.
Vortices also form around the ship. A vortex is a swirling flow pattern created by the rotation of a fluid. It is easy to understand if you imagine the swirling motion that occurs around a drain when draining water from a bathtub or sink. Various vortices can form around the hull and stern due to complex fluid flow and pressure changes, and energy is required to generate these flows as well. In the past, the drag associated with these phenomena was sometimes described as “vortex drag,” but in modern ship hydrodynamics, it is common to analyze them using more specific concepts, such as viscous pressure drag and energy losses due to vortices. The ITTC also classifies the pressure components related to viscous effects and turbulence as viscous pressure drag.
So, what methods can be used to reduce the drag acting on a ship? First, to reduce frictional drag, factors such as the hull’s wetted surface area, surface condition, hull form, and operating conditions must be considered together. Since frictional resistance can increase if the hull surface becomes rough or marine organisms attach to it, proper maintenance of the hull surface is also crucial. The ITTC explains that resistance can increase due to the structural roughness of the hull surface, the condition of the paint coating, corrosion, and marine organism fouling. Therefore, in addition to efficient hull form design, hull painting, surface maintenance, and fouling prevention also play a vital role in maintaining the actual energy efficiency of a ship.
One of the most common hull design improvements is the bulbous bow. A bulbous bow refers to a structure at the ship’s bow—the front part of the ship—where the section below the waterline protrudes in a rounded shape. Rather than being a device designed primarily to reduce frictional resistance itself, the bulbous bow serves to reduce wave-making resistance under specific operating conditions by regulating the waves generated by the hull and the resulting pressure distribution. If the waves generated by the bow and those formed by the bulbous bow are in the proper phase relationship, they can interfere with each other, reducing wave height and, consequently, the energy the ship expends in generating waves. However, the effectiveness of a bulbous bow is not consistent at all speeds; it varies depending on the ship’s size, hull form, draft, and operating speed, so it is crucial to design it specifically for the target operating conditions.
To reduce wave-making resistance, the size and waveform of the waves generated by the ship must be effectively controlled. Wave characteristics depend on the hull’s geometric shape and the ship’s speed. Specifically, methods include adjusting the angle of the waterline, optimizing the shapes of the bow and stern, and reducing wave height by utilizing interference between waves generated at the bow and stern. In particular, when understanding a ship’s wave-making characteristics, the relationship between speed and ship length is crucial; the Froude number is a representative dimensionless number that characterizes this relationship. The Froude number is calculated by dividing the ship’s speed by a value derived from gravity and the characteristic length; it serves as a key criterion for analyzing how wave form and wave-making resistance change as the ship’s speed increases. In modern ship resistance testing, both the Froude number and the Reynolds number are considered to analyze viscous effects and wave-making phenomena.
So far, we have examined the principles of ship motion, the various types of resistance acting on a ship, and methods for reducing resistance to achieve faster and more efficient operation. A ship moves forward using the thrust generated by its propulsion system; when accelerating, the forward thrust must exceed the resistance, and when operating at a constant speed, thrust and resistance are in equilibrium. Therefore, reducing the drag acting on the hull and improving propulsion efficiency are just as important for enhancing a ship’s performance as simply increasing engine power. In particular, since the required propulsion power can increase significantly as a ship’s speed rises, optimizing the hull form, operating speed, and propulsion system together is key to reducing fuel consumption and operating costs.
The environment surrounding South Korea’s shipbuilding industry has also changed significantly from the past. While there were once widespread concerns about the growth of competing nations such as China and Japan, as well as the outflow of labor and technology from South Korea’s shipbuilding sector, the industry currently continues to hold a significant position in the global shipbuilding market and maintains strong competitiveness, particularly in the high-value-added vessel segment. Recently, South Korea’s share of global ship orders has reached the 20% range, and the country is demonstrating strong competitiveness in the high-value-added vessel sector, such as large LNG carriers. At the same time, it faces new challenges, including technological advancements by competitors such as China, the transition to eco-friendly ships, the application of artificial intelligence and digital technologies, and securing a workforce for shipbuilding sites. Accordingly, South Korea is expanding research and development investments focused on eco-friendly ships, artificial intelligence, and digital and smart shipbuilding technologies. Ultimately, future competitiveness does not lie simply in building faster ships by installing larger engines. Technologies that precisely analyze resistance from water and air and optimize hulls and propulsion systems to sail more efficiently with the same amount of energy—along with the research and development and specialized workforce needed to support these efforts—will be key factors determining ship performance and the competitiveness of the shipbuilding industry.
