**Antonio Saporito 1,\*, Adam Persson 2,3, Lars Larsson <sup>2</sup> and Antonio Mancuso <sup>1</sup>**


Received: 12 August 2020; Accepted: 26 August 2020; Published: 3 September 2020

**Abstract:** Parallel-sided foil sections are used for centerboards and rudders in sailing dinghy classes and also for struts placed in a fluid flow. The objective of this work is to create a systematic series of parallel-sided sections to be used under different conditions, with an emphasis on the sailing dinghies 470, 420 and Optimist. The loss, and surprisingly the gain, in performance relative to 4-digit NACA sections are also investigated. A 2D Reynolds-averaged Navier–Stokes solver is used with the k-ω SST turbulence model and the gamma transition criterion. A verification study is carried out based on four grids of systematically varied density, and results compared with experimental data on a NACA 64-006 section. The parallel-sided sections are modeled with rational Bézier curves whose geometrical parameters permit to link the shape of the profile to physical variables, which are systematically varied. Three Reynolds numbers and two angles of attack are investigated. Systematic plots show the influence of the trailing edge angle and nose radius for the different section families, and the optimum combination is presented in a table. Physical explanations of the trends, and of the exceptions, are given in the paper, using flow visualizations as well as pressure and friction plots.

**Keywords:** parallel-sided; NACA; CAD; systematic investigation; low Reynolds number; sailing; centerboard; Bézier curves; gamma transition criterion
