Navigation and Seamanship

3.2.4. The Pitometer Log or Pressure Log or Pilot Tube

3.2.4. The Pitometer Log Pressure Log Pilot Tube

 

The pitometer log is similar to that of a pilot tube of an aircraft. A long tube called Rodmeter, is the main part of this log that protrudes from the hull of the ship. Rodmeter has two faces, one facing the direction of the sea water flow and the other is perpendicular to the flow. The face of the opening in the direction of sea water flow measures the dynamic seawater pressure where as the perpendicular face measures the static seawater pressure. A difference of both these pressures is used to measure the ship’s speed.

For measuring the speed of the ship, both the static and dynamic pressures are added and the velocity of water is found out. However, the result is not accurate when the ship is moving with slow speed, as the difference in pressure is meagre. T he speed is measured by the variation of the pressure in the tube. This is not a very sensitive device.

Principle

Two tubes with same diameter are immersed in to the water below the vessel hull. The first one is closed to one end and opens at the other, is immersed in water with opening facing down. In this tube, static pressure develops, which is proportional to the depth of the orifice or opening below the water. Another tube is similar to the first one, but with an orifice opening sideways facing to the direction of ship’s forward movement. It is also immersed to the same depth and in this tubes both static and dynamic pressures are developed during vessel’s motion. The control units for converting pressure indications into speed units consists of a sensitive bellows and its movements converts into rotary motion by suitable mechanical and electrical linkage.