The first ROK naval vessel operated in 1947 was a 287-ton iron boat with two diesel engines and two shaft propulsion systems (up to 13 knots) and was a captured boat after the Japanese Empire defeat. However, the boat had retired in 10 years owing to maintenance limitations of the main equipment, such as engines.

The ROK Navy's first naval vessel for combat was the ROKS Baekdusan, which was purchased from the United States in 1950 through naval self-fundraising and national donations. ROKS Baekdusan was a patrol craft (PC) with a full load of approximately 450 tons and a maximum speed of 20 kts. It used two RB-99DA diesel engines (2,560 HP) of Hooven, Owens & Rentschler (HOR) company. The RB-99DA diesel engine installed on ROKS Baekdusan is believed to be the first propulsion engine directly introduced by the ROK Navy for naval construction. ROKS Baekdusan served during the Korean War and was disassembled in 1960.

Figure 2: 
ROKN’s first own diesel engine (RB-99DA) [5]

Four months after the outbreak of the Korean War, the Tacoma-class Patrol Frigates (PF) transferred from the US Navy between the year 1950 and 1951 were named Taedong, Imchin, Nae Tong, Apnok, and Duman. With a full displacement of approximately 2,200 tons, the vessels could achieve speeds of up to 20 kts using two external combustion vertical triple-expansion steam engines (compound steam engines) (Figure 3). The compound steam engine unit is a type of steam engine in which steam is expanded in two or more stages [6]. A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger-volume low-pressure cylinders. These vessels were decommissioned in the early 1970s after service.

Figure 3: 
Principle of vertical triple-expansion steam engine

After the Korean War, Canon class Destroyer Escorts (DE) (full displacement of 1,620 tons, maximum speed of 20 kts) were introduced by the US Navy in 1956 as part of the five-year naval reinforcement plan from 1954 to 1958. These naval vessels were named ROKS Kyongki, and Kangwon and these vessels carried out the task of escorting the fleet against the U-boat during World War II and operated an early-stage electric propulsion system with two electric motor drives and four diesel engines (Figure 4) [7]. The electric propulsion system that modern naval forces are increasingly adopting is not a new technology propulsion system. When sophisticated reduction gear manufacturing technology could not be developed, an electric drive was used to adjust the torque. For the DE, whose main mission was to detect submarines, electric propulsion system would have been an effective option for noise control. The US Navy has been applying electric propulsion to vessels since 1917 [8].

Figure 4: 
Configuration of diesel-electric tandem propulsion system

The newly introduced Rudderow-class DE (full displacement of 1,770 tons, maximum speed 24 kts) was named ROKS Chungnam and used a steam turbine-electric system instead of a diesel-electric system to secure increased tonnage and higher speed than the Canon-class. A steam turbine (Figure 5) is a machine that extracts thermal energy from pressurized steam and uses it to perform mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884 [9]. Unlike the expansion steam engine (Figure 3), which uses high-pressure steam to create a reciprocating motion and a crank to switch to torque, the steam turbine engine is an external combustion engine that creates torque directly.

Figure 5: 
Principle of steam turbine generator

In 1963, the ROK Navy began operating fully fledged destroyers. The ROK Navy acquired three fletcher-class destroyers (DD) (one in 1963 and two in 1968) commissioned in the United States during World War II. The first vessel was named ROKS Chungmu (full displacement of 2,500 tons, maximum speed of 36.5 kts) in honor of Admiral Yi Sun-sin, an iconic man of the navy, and operated until 1993. At that time, the ROKS Chungmu was 45% smaller in size than the destroyers currently in service with the ROK Navy (full displacement of 5,500 tons, maximum speed 30 kts) but approximately 1.2 times faster. When estimating the shaft horsepower using the propeller law with tonnage and speed as variables, it was determined that an equal shaft horsepower produced by the propulsion system installed on modern destroyers is necessary. Therefore, considering the limitations in terms of insufficient electric motor output and insufficient energy conversion efficiency, it seems that a mechanical propulsion connecting a reduction gear to the steam turbine was adopted. To this end, two 60,000 HP steam turbines with four oil-fired boilers producing 600 psi steam were operated for propulsion, and two 350 kW steam turbine generators and a 100 kW diesel engine generator (backup) produced electricity for weapon systems and cabins [10]. From 1972 to 1981, two Allen Melancthon Sumner-class DDs (full displacement of 3,535 tons, maximum speed 34kts) named ROKS Daegu and Incheon, and seven gearing-class DDs (full displacement 3,500 tons, maximum speed of 37kts) were acquired. These destroyers operated with mechanical propulsion systems with steam turbine-reduction gears, such as the Fletcher-class DD.

In the Middle East War of 1967, Israel's newly commissioned DD Eilat was sunk by a Styx surface to surface missile (SSM) launched from an Egyptian patrol guided (PG) missile boat. At that time, the DPRK Navy possessed more than 50 Osa-class PGs equipped with the Styx missile that sank the Eliat. The lesson learnt from the Middle East War made the ROK Navy aware of the need to change its naval power. In 1970, a ROKS broadcasting boat (120 tons) was kidnapped by a high-speed patrol boat of the DPRK Navy, further emphasizing its necessity.

Thus, in collaboration with the Korea Institute of Science and Technology (KIST), the ROK Navy built and operated two KIST-class boats, which were the first domestically produced high-speed boats, in 1972, followed by Baekgu-class patrol guided missile medium (PGM) and Haksaeng-class patrol killer (PK). The ROK Navy mass-produced 25 swallow-class PK (full displacement 76 tons, maximum speed 40kts) through technology accumulation during this period. These PKs were equipped with two German Motoren and Turbinen-Union (MTU) 16-cylinder V-type diesel engines for propulsion.

Sea eagle-class patrol killer medium (PKM) (full displacement 141 tons, maximum speed of 38kts, a total of 105 vessels produced), which is still in operation today, is equipped with improved MTU diesel engines and uses the same two-shaft propulsion system (Figure 6) as the conventional patrol boat's propulsion system.

Figure 6: 
Configuration of typical diesel propulsion system with power generation system in ROKN sea eagle-class PKM

Subsequently, the naval power of the ROK and ship building capability of naval ship builders developed dramatically. In 1980, the ROK Navy built an Ulsan-class Frigate Korean (FFK) based on its own technology. The Ulsan class with full displacement of 2,215 tons and maximum speed of 36 kts is equipped with two MTU diesel engines for patrol mode and two GE LM-2500 gas turbines for battle mode. These two types of prime movers were configured as the combined diesel engine or gas (CODOG) turbine propulsion system, which operates prime movers independently at each operation mode of the vessel.

Starting from the Ulsan-class FFK, the gas turbine (Figure 7) has operated as the main propulsion prime mover for the high-speed propulsion of ROKN's medium and large vessels for approximately 30 years. And now, the CODOG propulsion system (Figure 8) is the representative propulsion system for the ROK Navy combat vessels; such as the Pohang-class patrol combat coastal (PCC)(commissioned on the year 1983), the great Kwanggaeto-class destroyer helicopter (DDH) (commissioned on the year 1996), the Chungmugong Yi Sunsin-class DDH (commissioned on 2003) and the Incheon-class frigate guided missile (FFG) (commissioned on the year 2012).

Figure 7: 
Principle of gas turbine for propulsion

Figure 8: 
Configuration of CODOG propulsion system

During this period, for independent national defense capabilities, the ROK Navy expanded its missions in diverse domains. Therefore, the ROK Navy acquired amphibious warfare vessels, such as the landing ship tank (LST), landing platform helicopter (LPH), and combatant assistant vessels, such as surface ship rescue ships, submarine rescue ships, and salvage ships to subdivide vessel missions. In addition, the Aux. oiler (AOE) was acquired to improve the sustainability of naval operations. These vessels are primarily driven by diesel propulsion, and is combined with a diesel engine on each of the two shafts. In addition, these vessels have a sufficient margin for internal space compared to combat vessels. Therefore, they usually adopt a commercial diesel engine with improved economic efficiency instead of a gas turbine with high specific power or an expensive diesel engine for the military.

The ROK Navy submarine force was also established as a foundation at this time. Submarines operated by the ROK Navy in the past and present have used electric motors with diesel engine generators and batteries. In particular, the air-independent propulsion (AIP) mechanism using fuel cells has been applied to the Son won-il class submarine.

The ROK Navy, which has accumulated the technologies and know-how of naval ship building and operating through the independent national defense capability period, has built a balanced naval power with the coastal mission vessels against the DPRK Navy and the oceanic mission vessels for ballistic missile tracking, interception, merchant ship escorts, etc.

First, a Yoon Yeongha-class patrol killer-guided (PKG) missile (full displacement of 570 tons, maximum speed 44 kts) and a new type of sea eagle-class patrol killer medium rocket (PKMR) (full displacement of 250 tons, maximum speed of 40 kts) have been operating since 2008. These vessels are replacement vessels for the gradually aging sea eagle-class PKM to maintain the overwhelming naval power against the DPRK Navy on the coast and have increased displacement compared to conventional PKM owing to the guided missiles or rockets. In addition, these new combat vessels were equipped with water-jet-refined propellers (Figure 9). It is a choice to solve the problem of screw and rudder damage caused by floating objects and fishing nets on these vessels, which have many coastal voyages owing to the mission.

Figure 9: 
Principle of water jet refined propeller propulsion

These vessels are additionally fitted with one shaft driven by a gas turbine and two shafts driven by diesel engines. This measure satisfies the conditions of increased displacement and top speed. The propulsion system of these new vessels is called combined diesel engine and gas turbine–waterjet refined propeller (CODAG-WARP) in patrol mode. Only the left and right propulsion shafts that are directly connected to the diesel engine are activated, and when switching to battle mode, the central propulsion shaft that is directly connected to the gas turbine is additionally activated. These vessels are propelled by the pressure of seawater jetted by three water jets. By performing steering function in the seawater injection direction of the left and right water jet nozzles, the turning performance was improved when compared with the conventional PKM.

The great Sejong-class Aegis Destroyer-guided missile (DDG) (full displacement:of 10,600 tons, maximum speed of 30 kts), which carries out the mission of tracking and intercepting ballistic missiles, was introduced in 2007. To ensure a top speed of 30 kts and increased displacement of DDG, the combined gas turbine and gas turbine (COGAG) propulsion system that uses four gas turbines instead of diesel engines with low specific power was selected. This propulsion system operates one or two gas turbines in patrol mode (low speed) and operates all four gas turbines in battle mode at maximum speed.

During the period of independent national defense capability, the ROK Navy operated a diesel propulsion system on small combat vessels and combat assistant vessels, and the CODOG propulsion system on medium and large combat vessels. Later, it was confirmed that as the vessel spectrum was expanded to counter diversified threats. New propulsion systems, such as CODAG-WARP and COGAG were introduced to meet the various requirements of the operational capability (ROC) of the vessels. The propulsion system of the ROK Navy vessels up to this period corresponded to typical mechanical propulsion.

In 2010, the ROKS Cheonan (PCC) was sunk by the DPRK Navy submarine. Subsequently, radiated noise from the vessels emerged as an issue during the ROK Navy vessel building project. Submarines detect radiated underwater noise from the surface vessels. Therefore, the propulsion system for anti-submarine mission vessels, such as FFG and DDH operating in the sea area adjacent to DPRK must ensure a high level of quietness in the low-speed range. Therefore, an electric propulsion system with electric motors could be used as an alternative.

Incheon-class FFG, for which exploratory development was started before the ROKS Cheonan sinking, adopted the conventional CODOG system. However, Daegu-class FFG, for which exploratory development was started after Cheonan sinking, adopted the combined diesel electric or gas turbine (CODLOG) hybrid system (Figure 10) to minimize radiated noise. The CODLOG hybrid system drives two electric motors with electricity produced by four diesel engine generators in the patrol mode. In battle mode, the motors stop and a large gas turbine engine is activated and takes charge of all speed ranges from 0 to 30 kts. The ROK Navy operates eight Daegu-class FFG vessels.

Figure 10: 
Configuration of CODLOG hybrid system

With the exception of the vessels introduced during the US assistance period, the power generation and propulsion systems were clearly separated before the Daegu-class FFG. In other words, the purpose of the prime movers for power generation and the prime movers for propulsion was clearly distinguished. However, after the Daegu-class FFG, the power produced by the generator began to be used for low-speed propulsion of the vessel, and gradually, the vessel's entire mechanical system begins to be integrated without being divided into a generation system and a propulsion system. Subsequently, electric propulsion was applied to combat assistant vessels. Soyang-class AOE adopts (combined diesel electric and diesel engine (CODLAD) hybrid system. The Soyang-class AOE's CODLAD drives the propulsion motors with electric power produced by four diesel engine generators in the low-speed mode, and two diesel engines are additionally activated for the high-speed propulsion mode.

Another feature that can be identified with the propulsion and power generation system of vessels introduced since 2010 is the improved environmental friendliness of the system's prime mover. IMO emission regulation tier II came into effect on January 1^st, 2011, making environmental friendliness mandatory in the marine engine market. The engine market for naval vessels was also affected by this, and diesel engines adopted technologies, such as electronic control with common-rail injection, two-stage turbo charging, and variable valve timing to improve the environmental friendliness of ROKS.

ROKS Sejong, the great-class DDG, which was commissioned in 2007, operates more weapon systems, such as the Aegis combat system and an expanded vertical launch system (VLS), unlike the conventional combat vessels operated by the ROK Navy. The DDG was equipped with 3MW gas turbine generators to produce electric power needed to increase the weapon system operations. Typical combat vessels utilize highly economical (SFOC, g/kW) diesel engine generators to produce a maximum power requirements of 3-4 MW. However, if the DDG uses diesel engine generators, such as, typical combat vessels, very large diesel engines must be equipped to produce the required power of up to 9 MW. Thus, the design displacement could not be satisfied. Therefore, the ROKS Sejong, the great-class DDG falls under the category of vessels in which all prime movers mounted on the vessel are gas turbines only and is considerably disadvantageous in terms of economy (fuel consumption).

To overcome these shortcomings, Jeongjo, the great-class DDG launched in 2022, selected combined gas turbine electric or gas turbine (COGLOG) hybrid system. It maintains the optimum load (75-85%) of the gas turbine generators as much as possible, and a considerable amount of surplus electric power from the generators is used to drive propulsion motors in the patrol mode. In the battle mode, propulsion gas turbines are driven.

This change is also reflected in the Korean destroyer experimental (KDDX) program, and it is expected that the new DDH to be delivered in the next term will use an integrated fully electric propulsion system. The introduction of electric propulsion for ROK naval vessels has already begun.

All the requirements in the recent development process of ROK naval vessels can be satisfied with the electric propulsion system of the vessels. The enhancement of sensors, the increase in new weapon systems with extended range and improved accuracy, the decrease in human resources and the accompanying automation of vessels, improvements in computing technology, the increase in tactical information handled by a vessel, etc., make the naval vessel need higher electric power. In view of the future, introduction of electromagnetic weapon systems, in conditions with high instantaneous power demands, the electric propulsion system can provide high-density energy that can be produced by the prime movers of all vessels.

In addition, as the number of missions assigned to a future naval vessel increases, the space required for many weapons systems increases. In this situation, electric propulsion of vessels is an effective alternative that can respond to changes. Changing the propulsion system from mechanical to electric reduces the number of prime movers by up to 50% and eliminates the dependency between the prime mover and the shaft, thus increasing the flexibility of the propulsion system arrangement and saving space.

Furthermore, when applying an electric propulsion system, it is possible to shield the noise of the prime mover, which can improve the quietness of the vessel that emerged after Cheonan sinking in 2010.

However, electric propulsion system in the vessel is a strategy suitable for medium- and large-sized vessels, and its application to small vessels, such as PKMR, which is mainly used for high-speed operation and has a simple propulsion system, would increase the weight of the vessel and reduce its economic efficiency [11]. Therefore, a thorough review and analysis are required.

Second, the application of environmentally friendly technology to the propulsion systems of naval vessels is predicted. Global warming is a supranational and nonmilitary threat, and the response to greenhouse gases, the main cause of global warming, has become a global issue. The IMO's greenhouse gas emission regulations were applied to general ships in the process of promoting regulations and promotion issues for major greenhouse gas emission sectors, such as industries, power generation, and transportation. Naval vessels were not included in the scope of regulation. This is believed to be due to the restrictions on the monitoring of naval vessels, which are military assets.

However, countries that operate so-called advanced navies, such as the United States, the United Kingdom, and the European Union, have gradually sympathized with the need to reduce greenhouse gas emissions in the military sector and promote greenhouse gas mitigation policies [12][13]. The Ministry of Defense of the Republic of Korea has also promoted carbon neutrality tasks since 2021 to realize the government's carbon neutrality policy [14]. Practical tasks for the military sector, including energy conversion of military weapon systems, such as vessels and minimization of greenhouse gas emissions, are likely to be promoted.

For this reason, the necessity of applying alternative fuels, such as biodiesel and e-fuel, is increasing as a countermeasure for energy conversion in the military sector [15][16]. Unlike alternative fuels, such as hydrogen and ammonia, biodiesel and e-fuel have high applicability and can be operated within a range that does not significantly change the fuel storage and supply system currently operated in vessels. In addition, it is expected that active reduction measures, such as selective catalytic reduction (SCR) for the purpose of adsorption and removal of nitrogen oxides on general ships and exhaust gas recirculation (EGR), will limit the generation of nitrogen oxides in diesel engines.

Furthermore, the Navy will continue to reduce carbon emissions by minimizing vessel fuel usage through economy-enhancing technologies, such as electric propulsion and hull optimization. The electric propulsion vessel, which consists only of the prime mover for power generation, optimally distributes the power load between the prime movers and continues to operate under the most economical conditions. Techniques to satisfy the energy efficiency design index (EEDI) for general ships will gradually be applied to naval vessels.

The third change is the application of remote monitoring and control technologies for mechanical systems. As the various systems that constitute the vessels become automated and unmanned, advanced maintenance technology is required. The decrease in the number of vessel crews worsens, so self-unit maintenance in the naval vessel meets realistic limitations. To complement this, the navy actively uses data communication technology between the vessels and the onshore support forces. Onshore monitoring units that have field maintenance functions receive and analyze information, such as temperature, pressure, and flow collected in real time by various sensors in the vessel's main devices to support condition-based management using AI technology. For this purpose, data collection technology is used for the vessel's main engines, generators, electric motors, and aux. machineries, etc., will be developed, and a function to integrate and share them onshore will be installed in the vessel in real time. This technology is now in operation by a large merchant shipping company, and the new vessels of the ROK navy are also introduced to early stage functionality. After these kick into high gear, when a failure occurs, remote system checks and status diagnosis are first performed by the onshore monitoring unit supervisor, and self-unit maintenance by crews in the site proceeds by exchanging information with the onshore monitoring unit supervisor. As the situation worsens and limits the failure process of the site, the vessel enters the homeport and quickly turns into field maintenance. The onshore maintenance support unit that conducts remote monitoring applies life cycle support (LCS) to the vessels. The LCS method manages the main devices, such as the weapon system and propulsion system of the naval vessel, from the perspective of the total life cycle, improves the efficiency of field maintenance, and enhances the availability of the vessels. For these, modular maintenance concepts, such as the maintenance float (M/F) repair method, will be extended.

The fourth change is the integration of the weapon system and fully electric propulsion system. This was the ultimate electric warship. Owing to multiple factors, such as the decrease in military personnel, the development of automation and unmanned technology, and the development of remote monitoring and control technology, the number of vessel operators will continue to decrease. In the case of naval vessels built before 2010, three or four people were on duty in the engine room, but for vessels built after 2010, the number of people on duty was reduced to one or two people. Owing to this trend, the conventional three-station configuration is divided into three main stations, such as the bridge, the combat information center and mechanical control room will be turned over to one or two station configurations by integrating the functions of the mechanical control room into the bridge or combat information center. This change had already begun. Personnel operating the combat information center, especially officers, will required to have a broader range of job performance capabilities with additional duties, such as machine control and failure processes added to the navigation and combat information duties.

System integration, along with the remote management technology explained earlier, will become an indispensable core technology for powering and operating unmanned vessels in the future.