Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow interpretation of some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design.

The flow around a circular arc is governed by the effect of the sharp leading edge and by the arc's curvature. There is a range of incidences where a leading-edge separation bubble (LESB) is formed on the convex side of the arc, and the reattached boundary layer separates further downstream. Akin to foils and cylinders, for increasing values of the Reynolds number, the boundary layer turns from laminar to turbulent resulting in a step change in the forces, here termed force crisis. This phenomenon is characterised experimentally for an arc with a camberto-chord ratio of 0.22 and for a range of the Reynolds number from 53,530 to 218,000. Forces are measured both in a towing tank and in a water tunnel, and particle image velocimetry is undertaken in the water tunnel. In stark contrast to cylinders, where the force crisis is associated with laminar-to-turbulent transition of the boundary layer, here it is found to be associated with the suppressed relaminarisation of the boundary layer. In fact, the LESB is always turbulent at the tested conditions, and relaminarisation occurs up to a combination of critical angles of attack and critical Reynolds numbers. The critical angle of attack varies linearly with the Reynolds number. These results may contribute to the design of thin cambered wings, sails and blades at a transitional Reynolds number, such as the wings of micro aerial vehicles, swept wings in subsonic flight, turbomachinery blades and the sails of autonomous sailing vessels.

The forthcoming publication of the revised BS EN ISO 12215-5 is set to transform the structural design of most small crafts for the next decade, with the implementation of significant changes to the scope and underpinning theory, such as an applicability extended up to 24 m Load Line, and the use of finite element methods as part of the compliance assessment process. This paper represents the first public release of the major changes and novelty in the standard, with a strong emphasis on high performance composite sailing yachts. The aim is to provide designers and builders with an insight into the technical background and practical applications of the new regulation for structural optimization, and how the marine industry will be impacted.

Glass fibre reinforced polymer composites are the most common materials employed for the manufacturing of small crafts. Their hull construction and scantlings are governed by the ISO 12215-5. The latest version of the standard introduced a new methodology to assess the ultimate strength of composites laminates. The regression equations based on the fibre weight fraction of the former version have been replaced by a more detailed ply-by-ply analysis. However, no validation data is available to ascertain the relevance of the default ultimate strengths provided by the ISO 12215-5:2019. This paper employs destructive testing to experimentally characterize the ultimate flexural, tensile and compressive strength of a quasi-isotropic glass-epoxy laminate. Experiments were undertaken in accordance with the ISO 178 for flexural properties, the ISO 527-4 for tensile properties, and the ISO 14126 for compressive properties. Two manufacturing techniques are investigated, namely hand lamination and vacuum bagging. The former represents a cheaper and therefore more common manufacturing option, while the latter is representative of an advanced manufacturing process. The key results show that the ISO 12215-5:2019 default ultimate strengths for the quasi-isotropic composite laminate tested are (i) conservative for the ultimate flexural strength, (ii) appropriate for the ultimate tensile strength, and (iii) optimistic for the ultimate compressive strength, especially for vacuum bagged samples, with the main cause identified as the value of the ultimate compressive breaking strain for chopped strand mat. These findings provide validation data for the ISO12215-5:2019. They may inform designers, compliance assessors and policy makers in the selection of relevant factors of safety for composite structures, and it is anticipated the results may contribute to future improvements in small craft regulations.