Navigation and Seamanship

3.2.4.1. Parts of pressure log

A typical pressure log consist of the following components,

1. Two openings outside the hull of the ship, static tube that provides static pressure and impact or Pilot tube that measures dynamic pressure or the water flow of pressure (Rodmeter)

2. Controller unit (pressure differentiator)

3. Speed and distance transmitter

4. Speed and distance recorder

Operations

The opening of the Pilot tube faces forward so that when the vessel moves forward, the water causes a pressure at the tube this dynamic pressure is proportional to the speed at which the vessel moves. The pressure differentiator measures the differential pressure. The Controller unit converts the pressure difference into speed and distance units. This type of log can give only speed through water and is greatly affected by the movement of the water which would induce an extra pressure giving rise to error in readings.