A propulsion system based on tandem hydrofoils is studied experimentally and numerically. An experimental measurement system is developed to extract hydrodynamic loads on the foils and capture their twisting deformation during operation. The measured data allowed us to assess the efficiency of the propulsion system as a function of travel speed and stroke frequency. The numerical simulation of the propulsion system is also presented and involves 3D, full-scale fluid–structure interaction (FSI) computation of a single (forward) foil. The foil is modeled as a combination of the isogeometric rotation-free Kirchhoff–Love shell and bending-stabilized cable, while the hydrodynamics makes use of the finite-element-based arbitrary Lagrangian–Eulerian variational multiscale formulation. The large added mass is handled through a quasi-direct FSI coupling technique. The measurement data collected is used in the validation of the FSI simulation, and excellent agreement is achieved between the predicted and measured hydrodynamic loads and foil twisting motion.