For many smaller boats, noise calculation is a simple matter of working out the power to displacement ratio and then calculating a Froude number. For boats with a power unit having an integrated exhaust, noise is the problem of the engine manufacturer rather than the boat builder.

For all other boats it is more complicated and the only practical way to demonstrate compliance is to carry out a pass-by test in accordance with ISO 14509 Part 1. The boat under test must be driven past a microphone at full speed at a distance of 25m. The maximum sound pressure level recorded at the microphone demonstrates compliance, less than 78dB(A) for twin engines and 75dB(A) for a single engine.

Pass-by testing can be difficult in practice. The Halyard led EU SoundBoat project resulted in the free floating SoundBuoy, which contains all the electronics necessary to measure noise levels and to transmit it back to the boat under test.

To carry out an ISO standard test, SoundBuoy is deployed from the test boat and left to drift. A built-in GPS receiver and radio link constantly transmits the buoy's location and sound level back to the test boat. The information is relayed to a small tablet computer next to the helm, with SoundBuoy appearing on the screen as a flashing circle on a chart, together with the boat's position and current sound level.

Knowing the boat's position and the buoy's position, the SoundBuoy computer calculates the course the boat must steer to pass exactly 25m from the buoy.

The course, together with the distance to run, is provided to the skipper through a 'Bluetooth' earpiece as a series of spoken instructions. The computer even tells the skipper how close he was to target and whether the run was sufficiently accurate.

SoundBuoy saves the data in an encrypted format to a removable memory device which is sent to one of a number of notified bodies which will check that all is in order and issue the relevant compliance documentation.

MJ Information No: 20209