Page 35 - Electronic Navigation Cyber Book

MODERN SPEED LOGS

THE IMPELLER LOG

This type of log is usually fitted in small crafts.

The standard equipment consists of the following
1. The log tube assembly

2. The amplifier
3. Speed indicator and distance counter

The sensing device is at the end of a long-tube or probes, consisting of
a small device called impeller (the dynamic element could be either a
small propeller or a paddle or a screw) at the end of the probe, which is
lowered into the water.
The tube is set with the port (opening) facing forward.

The water flow drives (or turns) the impeller and the rotation of the
impeller induces an electrical signal, which is picked up at the coils. The
output is fed to the amplifier and is used to operate the speed indicator.
The rotating impellor's signal could also be used to provide a distance
measurement.

When speed (or distance) measurement is required, the log is lowered
into the water, and when not in use, is retracted inside the hull. Retraction
of the log can be done manually or by a remote hoisting arrangement
operated from the navigating bridge or engine room.

The log-tube may become blocked or obstructed by foreign bodies such
as small fish, seaweed etc. The arrangement allows the whole tube to be
withdrawn inside the vessel for inspection and cleaning. In the event of
the log-tube being bent by hitting an underwater obstruction such as a
sand bank or a large fish or more often caused by a wire or a rope having
passed under the vessel, the log-tube must be jettison.

© 2018 Digital Galaxy Index 35