The Intact Stability of Wind-Assisted Merchant Vessels
Pablo Albers, Hochschule Emden-Leer
This Master’s thesis analysed and compared existing intact stability criteria for vessels equipped with Wind Assisted Propulsion Systems (WAPS), which are becoming increasingly popular as sustainable alternatives to conventional propulsion systems. Ambitious energy-saving targets are driving the development of larger, high-performance WAPS installations. However, to fully exploit their potential and ensure the safe and efficient operation of WAPS-equipped vessels, the typical stability risks associated with wind propulsion technology require reassessment within the context of modern ship and system design.
Conventional merchant vessel stability is primarily governed by the International Maritime Organization’s 2008 Intact Stability Code. In addition, classification societies may impose supplementary criteria, and several include specific provisions for vessels using WAPS. This study analysed fourteen different intact stability standards, focusing on those relevant to WAPS, sailing yachts, and naval vessels, and evaluated the impact of variations and gaps across the respective criteria. Central to the analysis were the righting-arm curve criteria and the weather criterion, including the calculation methodology for WAPS-induced heeling moments and the influence of each system’s operating characteristics. The investigation further assessed the operational stability risks associated with the integration of WAPS on modern vessels, and the extent to which these risks are considered within existing stability criteria.
Four WAPS types—Indosail, Dynarig, Flettner rotor, and suction sail—were used to assess the criteria, each representing a different operating principle. These systems were applied to two case vessels with contrasting design profiles. The analysis supported both a comparison of applicable criteria for different WAPS–vessel combinations and an evaluation of the WAPS-associated operational stability risks.
The findings reveal a lack of clarity and uniformity across current stability standards with respect to how WAPS-related influences on stability are addressed. Gaps and inconsistencies were identified not only in the formulation of the criteria but also in their underlying rationale. Particularly noteworthy are the discrepancies observed in the weather criterion, especially regarding dynamic stability and the calculation of wind-induced heeling moments. These inconsistencies can significantly affect a vessel’s calculated stability and its compliance with regulatory requirements. In addition, several operational stability risks, such as the effects of rolling motions, vessel manoeuvres, and the use of anti-heeling systems, as well as the importance of effective WAPS depowering and its role in reducing wind-heeling moments, are inadequately considered within current regulatory frameworks.
Given the urgency to decarbonise maritime transport and the growing industry interest in wind propulsion technologies to support this goal, the findings of this study provide relevant and practical insights. The research highlights the need to incorporate the operational characteristics and depowering behaviour of different WAPS technologies when applying or formulating intact stability criteria, and recommends the development of type-specific criteria, applied consistently across stability standards and supported by a clear rationale. This would enable the safe and scalable integration of larger WAPS installations without compromising vessel stability.
