Radiated noise from a high-speed jet-propelled watercraft (the M/V Alakai, 1,646 tons, length 117 m) was measured at hydrophone depths of 3, 6, and 10 m while the ship passed by at speeds of 6.17333 m/sec (12 knots), 12.3467 m/sec (24 knots), and 19.0344 m/sec (37 knots). Noise spectra were similar for all speeds and hydrophone depths. Spectra peaked below 100 Hz and dropped off continuously at higher frequencies. Calculated source level noise was 10 to 20 dB lower than noise from propeller-driven ships and much lower than for ships of similar speed. Although exposure to noise radiating from the M/V Alakai over short time periods is unlikely to cause hearing damage to whales, the combination of low radiated noise levels and high transit speeds leads to a shorter closing time (defined as time between when source level of the ship at a stationary receiver is greater than ambient noise and time that a ship traveling directly toward the receiver arrives at its location) between ship and whale. Compared with other types of ships traveling at similar speeds, closing time for the Alakai ranges from 20 sec shorter (at 6.17333 m/sec [12 knots]) to 22 min shorter (at 19.0344 m/sec [37 knots]). Shortest closing time for the Alakai is 89.1 sec at a speed of 6.17333 m/sec (12 knots). Shortened closing time might reduce successful detection and avoidance of high-speed jet-propelled ships by whales, and increased speed shortens the time during which whales have the opportunity to respond to this detection.
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Vol. 69 • No. 2