Fig. A1. Coordinate systems used to describe the undulating centreline. The description by prescribed curvature _s(s,t) is transformed to a description as x_s'(s,t) and y_s'(s,t) in the system (0',x',y') where the center of mass of the deforming body remains in (x',y')=0 and the total rotational impulse of the deforming body is 0. The fluid-body interaction is computed in the inertial system (0,x,y,z) reducing the forces and torque of the fluid to the center of mass and moving the local coordinate system (0',x',y') in order to satisfy the dynamic equations.