JRC JLN-900, Installation And Instruction Manual

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13. Water inlets and outlets in the nearby vicinity may disturb the water flow. Therefore the
Transducer should be located at least 2 metres forward of such openings.
14. For proper speed measurements the Transducer (sensor elements) must always remain
submerged, even at a minimum draught as well as in rough seas.
The Transducer is not affected or overheated by being powered when in air, e.g. during
dry docking, but recommended to turn-off system when ship is not in water.
15. The compass safe distance for the Transducer is: 5 m
Boundary Layer
A moving ship’s hull tends to drag a layer of water with it, causing a boundary region around
the hull. The water speed at the hull is almost the same as the ship’s speed, so that the relative
water speed is almost zero. With successive water layers, the relative water speed increases
until, at the boundary edge, the relative water speed becomes equal to the ship’s speed. The
thickness of this boundary layer is not uniform around the ship and can range from a few
millimetres near the bow to more than one metre at the stern on a large ship.
The relative speed of successive water layers is not proportional to distance from the hull, but
follows a theoretical curve as shown below:
Noise, Cavitations, Turbulences and Other Factors Affecting
Acoustic Speed Logs
The Transducer operates on the acoustic principle and is therefore susceptible to interference
from acoustic (noise) sources. In particular, the Transducer should be fitted well away from
the propeller and other sources of noise to minimise the effects of noise, vibration and
cavitations.
As well severe turbulences and diverted flow directions will affect, thus installation site and
hull surface conditions in the vicinity of the Transducer shall be carefully considered.
The Transducer and NJC-80 are designed to respond principally to the acoustic reflections
from water layer or sea bed denoting the speed of the ship. In some situations, however, the
system may respond to false signals that can arise under adverse conditions.
A majority of acoustic devices are affected in a similar way.