Abstract: A training apparatus for improving the performance of a swimmer. In a first aspect of the invention, a propeller rotates about an axis which is aligned with the path of the swimmer. A permanent magnet driven by the propeller attached to the swimmer produces a rotating magnetic field which acts on a magnetic field transducer to produce a pulsating signal whose frequency varies directly with the swimmer's speed. The sensor's output is multiplied, amplified and fed to an earphone worn by the swimmer. Changes in frequency immediately inform the swimmer of whether his performance has improved or deteriorated.In a second aspect of the invention, permanent magnets are mounted on vanes of an impeller which rotate about a horizontal axis which is at right angles to the path of the swimmer. Rotating magnetic fields of the permanent magnets act on a magnetic field sensor to produce a pulsating signal whose frequency varies with the swimmer's speed.