A common problem in small vessels is the high levels of low frequency sound, usually produced by the engines and/or the propellers. Due to the low frequency character of the noise, the primary concern is usually not the potential risk of hearing damage, but the masking effect that reduces speech intelligibility. Another aspect is loss of sleep due to noise, which increases the potential risk for accidents during work onboard caused by tiredness. For this reason, a multiple reference, multiple input, multiple output active control system was implemented and tested in a small patrol boat. The boat was 13 meters long and powered by two 800 hp MTU Diesel engines. The primary tonal components in the drivers cabin were reduced by 20 dB.