Thrust Measurement and Stall Detection Techniques for WAPS Applications: From Model to Full Scale

Maritime transportation is the backbone of international trade, carrying approximately 80% of global freight and more than two-thirds of total freight transport within the European Union. Despite its high efficiency, the sector accounts for around 3% of global greenhouse gas emissions, with vessels above 5,000 GT responsible for more than 85% of international shipping CO₂ emissions (IMO, 2025). In response, the International Maritime Organization (IMO) and the European Union have introduced increasingly stringent regulatory frameworks, including the Energy Efficiency Design Index (EEDI) and the FuelEU Maritime Regulation, aiming to substantially reduce emissions by 2040. Within this regulatory context, Wind-Assisted Propulsion Systems (WAPS), such as suction sails, Flettner rotors, and rigid or soft sails, have demonstrated significant potential to reduce fuel consumption with relatively short payback periods (IMO, 2024). As a result, the number WAPS installations has increased steadily in recent years. Despite their increasing adoption, accurate and continuous assessment of WAPS performance remains a major challenge. Numerical models used to estimate the aerodynamic thrust produced by wind assisted propulsion systems under realistic operating conditions often exhibit substantial deviations from true full-scale values, owing to pronounced environmental variability (wind, waves, and currents) and time dependent effects such as hull and propeller fouling. While direct full-scale thrust measurement could offer improved accuracy, it is rarely implemented due to the absence of standardized methodologies and the strong sensitivity of measurements to ambient conditions. Consequently, real time performance assessment and the detection of critical aerodynamic phenomena, such as stall, remain limited, and existing approaches often rely on indirect estimations, empirical correlations, or comparative on-off analyses. This review aims to address current limitations in the accurate and continuous quantification of aerodynamic forces generated by wind-assisted propulsion systems under realistic operating conditions. The objectives are to systematically analyze and compare thrust measurement techniques at both model and full scale, considering the governing variables and the experimental methodologies used for their assessment. This study adopts a structured literature review and comparative assessment approach to analyze thrust measurement and stall detection techniques applicable to WAPS. The existing literature includes only a limited number of scientific papers that clearly describe the methodologies applied in model-scale or full-scale measurements. Relevant methods were identified through a comprehensive search of peer-reviewed journal articles, conference proceedings, classification society guidelines, and technical reports published by international maritime organizations. The search strategy combined keywords related to wind-assisted propulsion (e.g., “suction sail”), force and thrust measurement (e.g., “load measurement”), and aerodynamic performance monitoring (e.g., “stall detection”). Studies from adjacent fields, such as aeronautical and offshore engineering, were also included where measurement principles were transferable to maritime applications. Methods were screened for relevance to model- and full-scale applications and assessed based on accuracy, environmental sensitivity, scalability, installation complexity, robustness, and suitability for real-time monitoring. The present work examines a wide range of sensing technologies and methodologies, including load and force transducers, strain gauges, pressure tap systems, shaft power measurements, accelerometers, optical and particle-tracking systems, inertial measurement units, anemometers, and LiDAR. Particular attention is given to the coupling between aerodynamic, structural, and hydrodynamic phenomena, highlighting the importance of sensor selection, placement, and data quality (Prabowo, Tuswan, & Ridwan, 2021). Model-scale approaches such as wind tunnel and towing tank experiments are compared with full-scale sea trials, emphasizing their respective advantages, limitations, and scalability. The analysis demonstrates that, although indirect methods remain prevalent, no single technique currently enables direct and fully standardized thrust measurement at full scale. Strain gauges are the most widely applied solution (Ghorbani, Slaets, & Lacey, 2024); however, emerging technologies such as fiber-optic sensing exhibit strong potential, offering high robustness, repeatability, and suitability for harsh marine environments. The study concludes by identifying methodological best practices, assessing trade-offs between accuracy, cost, and operational complexity, and outlining key research gaps that must be addressed to achieve reliable and standardized performance assessment of wind-assisted propulsion systems.