We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M2<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.