Pitometer log
Encyclopedia
History
All nautical instruments designed to measure the speed of a ship through water are known as logs.This nomenclature dates back to days of sail when sailors tossed a log attached to rope knotted at regular intervals off the stern of a ship. The sailors would count the number of knots that passed through their hands in a given period of time. Today sailors still use the unit of knots to express a ship's speed. The speed of the ship was needed to navigate the ship using dead reckoning
Dead reckoning
In navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course...
, which was standard practice in the days before modern navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
instruments like GPS.
During World War II, pitometer logs were often interfaced directly into warship fire control
Fire-control system
A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a weapon system in hitting its target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more...
systems. This interface was necessary to allow gunnery and torpedo fire control systems
Torpedo Data Computer
The Torpedo Data Computer was an early electromechanical analog computer used for torpedo fire-control on American submarines during World War II . Britain, Germany, and Japan also developed automated torpedo fire control equipment, but none were as advanced as US Navy's TDC...
to automatically track targets.
While the pitometer log is very commonly used today, there are a number of other logs that are also in use. These logs include:
- Impeller Log: A small propellerPropellerA propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
mounted in the water that passes by the ship hull. The rotation rate of the propeller is proportional to the speed of the ship. This device is similar in principle to an anemometerAnemometerAn anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...
, which is used to measure wind speed. A variation of this approach is known as a patent log, which is towed from the stern of a ship. The patent log includes a mechanical register that counts the log's rotations as it is being towed. The patent log was invented in 1688 by the English instrument maker Humphry Cole. The patent log is also known as a screw log or taffrail log.
- Propeller RPMRevolutions per minuteRevolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
: The speed of a ship is roughly proportional to the rotation rate of the propeller. This approach is most useful when the propeller has constant pitch. It is less useful on ships with propellers that have variable pitch propellers or no propellers (like sailing ships). A variation of this approach was used by submarine's during World War IIWorld War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
to determine the speed of targets. The sonar operator would listen to the sound of a target's propeller and would determine its rotation rate. Knowing the propeller rotation rate and visually identifying the type of ship, the target's speed could be estimated.
- GPS: Besides providing position information, GPS can also provide direction and velocity.
- Measurement of Doppler Shift: Some ships are equipped with sonar instruments that can measure the Doppler shift in a sound pulse generated by the ship's motion as it reflects off the ocean bottom. Unlike a pitometer, the speed measurements from a doppler instrument are of ground speed.
- Correlation Velocity Log (CVL): A CVL consists of a sound transmitter and a number of receivers. These very expensive devices measure the ship's ground speed by performing correlation processing between the signals received by the different receivers at different times. This correlation can be related to the distance moved by the ship. CVL's are used primarily by ships that need very precise measurement of low speeds, which a pitometer has difficulty measuring accurately.
The pitometer log was patented
in 1899 by Edward Smith Cole.
Principles of Operation
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
on an aircraft. Typically, the pitometer has a long tube that penetrates the ship's hull near the keel. The part of the pitometer protruding from the ship is sometimes called a pit sword
Pit sword
The pit sword is a blade of metal or plastic that extends into the water beneath the hull of a nautical vessel. It is part of the pitometer log, a device for measuring the ship's speed through the water.-External links:**...
or rodmeter. This tube usually has two openings: one facing the direction of seawater motion that is used to measure the dynamic pressure of the seawater and one at 90o to the direction of seawater motion that is used to measure the static seawater pressure. The dynamic pressure of the seawater is a function of the depth of the water and the speed of the vessel.
In early realizations of the pitometer log, mercury manometers
were used to measure the pressure differences (see Figure 1). Later realizations used approaches that would generate equalizing pressures within the pitometer that would balance out the dynamic pressure. This eliminated the need for mercury manometers.
Analysis
An expression can be derived for the velocity of water impacting the ship as a function of the difference in dynamic and static water pressure using Bernoulli's principleBernoulli's principle
In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy...
. The total pressure of the water in the tube with moving seawater can be described by Equation 1.
| (Equation 1) |  |
|---|
- where
- pTotal is the total fluid pressure.
- pStatic is the static pressure, which strictly depends on depth.
- pDynamic is the fluid pressure caused by fluid motion.
Since water is an incompressible fluid, the dynamic pressure component of the total pressure can be expressed in terms of the water density and the water velocity as is shown in Equation 2.
| (Equation 2) |  |
|---|
- where
- vWater is the speed of the fluid flow.
- ρ is the fluid density.
Equation 2 can be solved for the velocity of water in terms of the difference in pressure between the two legs of the manometer. Equation 3 shows that velocity is a function of the square root of the pressure difference.
| (Equation 3) |  |
|---|
Because the speed computed by the pitometer is a function of the difference between pressure readings, the pitometer does not produce an accurate result when the ship's velocity is low and the two pressure readings are nearly the same.
