Hempel competed from the late 1970s to the mid-1980s. He won nine medals at the ICF Canoe Sprint World Championships with three golds (K-4 500 m: 1983, 1985; K-4 1000 m: 1981), three silvers (K-1 500 m: 1982, K-4 1000 m: 1982, 1983), and three bronzes (K-1 500 m: 1978, 1979; K-4 1000 m: 1985).