Navigation and Seamanship

3.2.5. Impeller logs

An impeller or small propeller that projects outside the vessel’s hull below the bottom of the boat. The movement of the vessel’s hull through the water causes the impeller to rotate. The rotation of impeller produces the electric impulse that is proportional to the counts or revolutions of impeller and the is translated into distance and speed. However, the projection below the boat is liable to be damaged by gear and debris floating in water.

The impeller is driven by the water as the ship moves through it. The Electric impulse output of the generator is amplified and passed to a indicator for display in to speed and distance. The speed and distance readings are repeated at remote indicators by means of a electrical synchro-transmission. The problem with this log is that the projection below the boat is liable to be damaged by fishing gear and debris floating in water.

The Impeller Log is fitted in small sized vessels including fishing vessels. A typical impeller log contains the following components,

1. The log tube sensor assembly

2. The amplifier

3. Speed indicator and distance counter

The sensing device is a long-tube consisting of a small rotating device called impeller (the dynamic element -small propeller or paddle) which is projecting into the water. The tube has the opening facing forward. On the movement of vessel, the water flow drives the impeller and the rotation induces an electrical signal. The output is fed to the amplifier and is used to operate the speed and distance indicator.
When the speed measurement is required, the log tube 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 remotely operated arrangement operated from the wheel house. The log-tube may be blocked with foreign bodies such as small fish, seaweed, debris etc. This log requires periodical cleaning.