## 1 Introduction  
In marine propulsion systems, diesel engines prevail in the entire world's fleet [1, 2]. The most common arrangement is low-speed two-stroke diesel engines being used for propulsion [3, 4], while the middle- or fast-speed four-stroke diesel engines are used as auxiliary engines [5]. As such, many researchers are developing various systems that can improve the operation of these diesel engines or reduce the harmful emissions [6, 7]. Gas turbines in marine propulsion systems are usually used in various combinations along with other propulsion elements [8, 9], but rarely as stand-alone devices due to increased costs in comparison to other propulsion alternatives [10]. However, gas turbines are the components included in the development of new, complex, and dynamic marine propulsion systems [11, 12]. The share of steam propulsion in marine propulsion is low when considering the entire world fleet, but this propulsion is still dominant on liquid natural gas (LNG) carriers due to the specificity of its operation and the transported cargo [13, 14]. Notably, the proportion of propulsion alternatives for LNG carriers are continually increasing, so steam propulsion will likely lose its dominance in the near future [15]. Various auxiliary devices can be found inside each marine propulsion system (regardless whether it is based on a diesel engine, steam turbine, gas turbine, or combinations thereof), the operation of which plays an inevitable role in the entire propulsion system operation [16; 17; 18]. Many authors have studied several auxiliary components in one or more operation regimes [19; 20]. One auxiliary component inside many marine propulsion systems is a turbo compressor. In general usage, turbo compressors can be found as components of gas turbines that operate as stand-alone devices [21; 22; 23] or as a part of various complex systems [24; 25; 26]. They are also found in many heat-recovery systems that, in most cases, produce additional mechanical power [27; 28]. The literature provides various other examples of turbo compressor usage [29; 30]. Engineers and scientists agree that proper turbo compressor construction and maintaining its desired operation is usually a notable challenge [31] due to turbo compressors increasing the gaseous operating medium pressure and delivering it from lower to higher pressure. This operation principle is opposed to natural fluid flow (from higher to lower pressure) [32]. This operation produces many difficulties and problems during the operation of any turbo compressor [33]. Inside the propulsion systems of LNG carriers, turbo compressors are used to increase the pressure of boil off gas (BOG) from the cargo tanks and deliver it to main propulsion element inside the system (dual fuel internal combustion engines or steam generators). LNG carriers can also have a system for BOG re-liquefaction [34; 35] or a combination of re-liquefaction and compression to the main propulsion element. Studies about BOG turbo compressors, especially under various loads and/or in exploitation, are rare in the scientific and professional literature. One recent study examined the two-stage BOG turbo compressor that delivers evaporated gas from cargo tanks to a dual fuel main internal combustion engine [36]. The mentioned research and two-stage axial turbo compressor analysis were based on measured data from exploitation under various turbo compressor loads. We did not find any other detailed investigation of the same or similar BOG turbo compressors in the literature, which was the main motivation for the research presented in this paper. In this study, we conducted isentropic, energy, and exergy analysis of a low-duty BOG single-stage radial turbo compressor. The turbo compressor takes evaporated gas from the LNG carrier cargo tanks, increases its pressure, and delivers it into steam generators. In steam generators, evaporated gas is burned and superheated steam is produced, which is used for the main and all auxiliary steam turbine driving.