Log is an instrument that measures the speed of a ship. Logs used by modern ships include (1) a tachometer; (2) the Doppler sonar speed log; (3) the pitot-static log; and (4) electronic navigation devices.
A tachometer
can be used for determining the revolutions per minute (rpm) of a ship’s propeller. The speed of the ship often is directly related to the propeller’s rpm. Therefore, a person can use the rpm to calculate the ship’s speed. However, the accuracy of this method is greatly affected by weather, by the weight of the ship’s load, or by the build-up of seaweed or small shellfish called barnacles on the ship’s underside.
On yachts and smaller ships, a device called a knotmeter is often used to measure speed. The knotmeter determines the distance traveled by the ship by counting the revolutions of a small paddle wheel mounted on the ship’s hull. The speed of the ship can be found by dividing the distance by the amount of time traveled.
The Doppler sonar speed log
measures the apparent change in frequency of sound waves transmitted from a ship and then reflected from the bottom of the ocean. This change in frequency, or Doppler effect, is caused by the motion of the ship. See Doppler effect .
The pitot-static log
operates by the action of water pressure. The basic part of a pitot-static log is a pitot tube, which is mounted below the bottom of the ship. The pitot tube has an opening on the side facing the front of the ship. Another tube surrounding the pitot tube has openings that face the sides of the ship.
When the ship is at rest, the water pressure at the openings of the pitot tube and surrounding tube is the same. This is known as static pressure. As the ship moves, its speed increases the water pressure on the opening of the pitot tube. This additional pressure is known as dynamic pressure. But the openings in the outer tube still receive only static pressure. Devices inside the ship measure the differences in pressure between the tubes to determine speed and distance.
Electronic navigation devices
record the course and speed of a ship relative to stationary signal transmitters. For example, loran involves the transmission of radio signals from two stations, known as the master and slave stations, to a ship at sea. Receiver equipment on the ship measures the time interval between the signals from the two stations. The time difference places the ship on a loran line of position on a chart. The ship’s exact location on this line can be established by using a second loran line of position. This line is determined by measuring the time difference between signals from the master station and a second slave station. The second line will intersect the first, thereby marking the ship’s location. The ship’s speed can be determined by taking two readings of the ship’s position, with a known interval of time between the readings.
Logs used in the past
included the chip log and the taffrail log. A chip log was a thin board shaped like a quarter of a circle. A sailor threw the log into the water behind the ship, and a line attached to it unreeled as the ship moved. By noting the amount of line that ran out in a given time, a sailor could determine the ship’s speed. The taffrail log consisted of a rotator with spiral fins that caused it to turn as the ship pulled it through water. The rotator was connected to a recording device that showed the distance traveled. A sailor needed to make two readings, with a known interval of time between them, to obtain the speed of the ship.
