We report on evanescently coupled Ge waveguide photodetectors that are grown on top of Si rib waveguides. A Ge waveguide detector with a width of 7.4mum and length of 50 mum demonstrated an optical bandwidth of 31.3 GHz at -2V for 1550nm. In addition, a responsivity of 0.89 A/W at 1550 nm and dark current of 169 nA were measured from this detector at -2V. A higher responsivity of 1.16 A/W was also measured from a longer Ge waveguide detector (4.4 x 100 mum2), with a corresponding bandwidth of 29.4 GHz at -2V. An open eye diagram at 40 Gb/s is also shown.