Abstract Flettner rotors are increasingly explored as a sustainable hybrid propulsion technology in maritime applications, yet predictive tools for multi-rotor installations remain limited, particularly when rotors are independently controlled. This study presents an experimental investigation of a double Flettner rotor system, employing a fully automated wind tunnel setup with independent spin ratios. Two identical rotors (𝐿/𝐷 = 6, 𝐷𝑒/𝐷 = 2, 𝐷 = 0. 09 m) were tested over 𝜆1, 𝜆2 ∈ 0, 5, inflow angles 𝜑 ∈ 0°, 90°, spacings 𝑆/𝐷 ∈ 2. 2, 3, 4, and three free stream velocities (𝑅𝑒 ≈ 5×104−1×105). The sense of rotation was used only to cover angles of 𝜑 90°. The fully automated setup acquired 10164 configurations, each repeated three times. We map projected thrust and side force coefficients (𝑐𝑥, 𝑐𝑦) across (𝜆1, 𝜆2, 𝜑, 𝑆/𝐷) and distill design-relevant trends. Within the tested ranges: (i) increasing spacing from raises 𝑐𝑥 for all operative points; (ii) optimal operating points are frequently asymmetric, with the windward rotor commanded at up to ~50% lower spin ratio than the leeward; (iii) the downstream rotor in the wake can exhibit up to ~11% higher lift than the upstream rotor at comparable operating points; (iv) a lateral deck alignment achieves ~10% higher peak 𝑐𝑥 compared to a longitudinal alignment at 𝑆/𝐷 = 4 when each is operated at its optimal (𝜆1, 𝜆2) across 𝜑. We present compact 𝑐𝑥-𝑐𝑦 polars and lookup tables over the measured grid to enable simple performance prediction and control scheduling. Keywords flettner rotors; double rotor system; Magnus effect; performance prediction; sailing polar; wake effect
Hillers et al. (Thu,) studied this question.