In this study, it is aimed to protect underwater vehicles against active sonar systems by anechoic coating. A matrix material with close acoustic impedance to water, resistivity to hydrostatic pressure, suitability for the marine environment, and high material loss factor is selected. At low frequencies, the inclusions in different shapes and sizes are added to the matrix material. Since solid inclusions will increase the coating mass considerably, air cavities are preferred as inclusions. More attention is paid to low-frequency absorption, especially below 1 kHz, because of advancing sonar technology. The acoustic performance of the designed models is compared in three frequency ranges: low (below 3 kHz), middle (3–6 kHz), and high (6–10 kHz). The designed models are constructed by considering hydrostatic pressure; hence, volume of air cavities is tried to decrease, while absorption performance is aimed to increase. Therefore, a conical cavity which commonly used in the literature is optimized by chancing its dimensions and location. Also, novel approaches, gong shape cavity, and sandglass cavities are introduced. The results show that, not only cavity shape, but also its location and dimensions are highly influential on absorption performance. High-volume cavities increase the absorption performance at the low-frequency range, but they are not effective at high frequencies. The gong shape and sandglass air cavities show broadband absorption; also, gong shape cavity volume is less than literature models. Thus, its usability increases at deep waters. The results of this study provide novel underwater acoustic coating models for various applications.

The maritime sector, responsible for approximately 3% of global greenhouse gas (GHG) emissions, is under growing pressure to transition toward climate-neutral operations. Significant progress has been made in developing sustainable fuels and propulsion systems to meet these demands. Although electric propulsion and fuel cells are highlighted as key technologies for achieving net-zero carbon targets, they remain an immature solution for large-scale maritime use, particularly in long-distance shipping. Therefore, modifying internal combustion engines and employing alternative fuels emerge as more feasible transition strategies, especially in short-sea shipping and port applications such as tugboat operations. Among alternative fuels, hydrogen (H₂) and ammonia (NH₃) have emerged as the most prominent fuels in recent years due to their carbon-free nature and compatibility with existing marine compression ignition (CI) engines with only minor modifications. This study explores the viability of hydrogen and ammonia as alternative fuels for CI engines in terms of technological, economic, and environmental aspects. Also, using a life cycle assessment (LCA) framework, this study examines the environmental impacts and feasibility of gray, blue, and green hydrogen and ammonia production pathways. The analysis is conducted from both well-to-tank (WtT) and tank-to-wake (TtW) perspectives. The results demonstrate that green fuel production pathways significantly reduce emissions but lead to higher economic costs, while intermediate blends offer a balanced trade-off between environmental and financial performance. Moreover, the combustion stage analysis indicates that H₂ and NH₃ provide substantial environmental benefits by significantly reducing harmful emissions. Consequently, a Multi-Criteria Decision Making (MCDM) approach is employed to determine the optimal blending strategy, revealing that a 24% hydrogen and 76% marine diesel oil (MDO) energy share yields the most favorable outcome among the evaluated alternatives.

RCCI is a promising combustion strategy that can improve the controllability of combustion phasing. This study evaluates fusel oil (an inexpensive industrial by-product) as a low-reactivity supplementary fuel in an RCCI diesel engine. Fusel oil was injected into the intake air at 4, 6, 10, 12, and 16 g/min (DF4-DF16), and experiments were conducted on a four-cylinder, four-stroke diesel engine at 1750 rpm under 40, 60, 80, and 100 Nm. In-cylinder temperature/pressure-based combustion behaviour, air excess ratio (λ), NO and smoke emissions were assessed. The influence of fusel oil on combustion was strongest at low load. At 40 Nm, the highest fusel-oil energy share increased peak cylinder pressure by 14% and peak in-cylinder temperature by 4% compared to diesel fuel tests, while at 100 Nm the corresponding increases were 4% and less than 1%. NO increased at 40 Nm, with a maximum rise of 17.9% at the highest fusel-oil energy share, but decreased at medium and high loads, falling by 7.13 to 13.54% between 60 and 100 Nm. Smoke increased consistently with fusel oil, reaching about 42% at 40 Nm and remaining below 22% at 100 Nm. A techno-economic assessment showed that although capital costs increased slightly, the low price of fusel oil decreased operating costs by up to 33% and reduced life-cycle costs by up to 42%. Overall, fusel oil-assisted RCCI operation can provide notable cost benefits and conditional NO reductions, though with a smoke penalty that should be considered in application and after-treatment strategies.