Hydrofoils Design and Multifidelity Optimization for a Flax Fiber Moth

Can sustainability enter the world of high performance sailing? Audace Sailing Team, a student group based at the University of Trieste (Italy), bet on flax fiber to create their own foiling prototype, an International Moth. In this paper, we present a hydrofoil design process that is arguably the first attempt of including flax fiber composite (more than 50% by weight) in hydrofoils design. A multifidelity approach was chosen and integrated in the broader concurrent engineering framework which had been developed for the Moth project. Starting with simple design tools, like XFOIL and XFLR5, both two-dimensional and three-dimensional analyses were performed in order to define the baseline (initial) design. A Static Velocity Prediction Program was also developed, to estimate hydrofoil performances in real conditions, embedding finite wing corrections, hull drag and sail thrust. Initial design was then further improved by integrating a design exploration software (modeFRONTIER): thousands of combinations of the main geometric parameters were tested, creating a multiobjective optimization which included also requirements on structural strength. Finally, a high-fidelity optimization – featuring parametric models of both foils, tested with CFD simulations – was carried out. Only the most promising ranges of parameters were analyzed, allowing for a noticeable reduction of the computational effort. The developed workflow proved to be effective and a first set of race-proof hydrofoils was manufactured. As a result, AST flax fiber boat won first place overall and best design at the 2023 SuMoth Challenge, showcasing natural fibers as a feasible alternative to full-carbon boats.