Photo of a zodiac with outboard engine. Photo Copyright Tom Kieckhefer
Outboard motors are found on small boats that are popular in coastal waters. The propeller of an outboard motor is what creates sound. On smaller boats, like a zodiac, the small propeller produces a cavitation noise which is at higher frequencies than larger vessels. The smaller propellers also produce higher rotation rates which also causes the propeller to make higher frequency noises. A zodiac, for example, can produce frequencies as high as 6300 Hz.