117:. 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 90° 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.
98:
31:
643:(DVL): 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 or off particles in the water. The speed measurements from a Doppler instrument can therefore be relative to the ground, or to the water.
622:
measurements yield only a position, and neither speed nor direction of movement. However, most GPS receivers can automatically derive velocity and direction from two or more position measurements. The disadvantage of this principle is that changes in speed or direction can only be computed with a
120:
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
53:
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
652:
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
609:
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 or “blade count”. Knowing the propeller rotation rate and visually identifying the type of ship, the target's speed could be
580::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
601:: 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
555:
562:
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.
45:(also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.
427:
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.
398:
239:
280:
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.
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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.
109:
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
439:
672:
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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
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790:
623:
delay, and that derived direction becomes inaccurate when the distance travelled between two position measurements drops below or near the
38:
US Navy submarine pitometer. This unit uses a mercury-based manometer to measure the difference in static and dynamic water pressure.
571:
While the pitometer log is very commonly used today, there are a number of other logs that are also in use. These logs include:
826:
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of position measurement. To counter this effect, more advanced navigation systems use additional sensors like a compass or an
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133:. The total pressure of the water in the tube with moving seawater can be described by Equation 1.
74:
550:{\displaystyle v_{Water}={\sqrt {\frac {2\cdot \left(p_{Total}-p_{Static}\right)}{\rho }}}\,\!}
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to express a ship's speed. The speed of the ship was needed to navigate the ship using
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727:. Standards and Curriculum Division, Training, Bureau of Naval Personnel. 2000-07-11
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through their hands in a given period of time. Today sailors still use the unit of
35:
791:"Simulation of the Correlation Velocity Log Using A Computer Based Acoustic Model"
73:
During World War II, pitometer logs were often interfaced directly into warship
55:
393:{\displaystyle p_{Total}=p_{Static}+\rho \cdot {\frac {v_{Water}^{2}}{2}}\,\!}
106:
63:
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701:"The Photo-Pitometer: An instrument for measuring velocity in water mains"
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The basic technology of the pitometer log is similar to that of the
675:. Penn State Naval Reserve Officers Training Corps. Archived from
101:
Figure 2: Illustration of a
Mercury Manometer-Based Pitometer Log.
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29:
705:
American
Artifacts: Scientific Medical & Mechanical Antiques
748:. USS Bowfin Submarine Museum & Park. 2002. Archived from
619:
605:). A variation of this approach was used by submarines during
67:
84:
The pitometer log was patented in 1899 by Edward Smith Cole.
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systems. This interface was necessary to allow gunnery and
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is the static pressure, which strictly depends on depth.
62:, which was standard practice in the days before modern
673:"Naval Science 203: Navigation and Naval Operations I"
442:
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234:{\displaystyle p_{Total}=p_{Static}+p_{Dynamic}\,\!}
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725:"Submarine Underwater Log Systems, NAVPERS 16168"
584:. The patent log is also known as a screw log or
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771:"Arctic Corsair - Taffrail Log or Patent Log"
274:is the fluid pressure caused by fluid motion.
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27:Device measuring a ship's speed
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567:Alternatives to the pitometer
256:is the total fluid pressure.
79:torpedo fire control systems
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629:inertial navigation system
746:"Underwater Log Systems"
93:Principles of operation
827:Navigational equipment
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842:Watercraft components
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421:is the fluid density.
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131:Bernoulli's principle
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641:Doppler Velocity Log
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121:mercury manometers.
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34:Figure 1: Photo of
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43:Pitometer logs
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837:Shipbuilding
805:. Retrieved
798:the original
785:
774:. Retrieved
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754:. Retrieved
750:the original
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729:. Retrieved
719:
708:. Retrieved
704:
681:. Retrieved
677:the original
667:
625:random error
607:World War II
586:taffrail log
582:Humphry Cole
578:Impeller log
570:
561:
426:
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128:
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75:fire control
72:
52:
42:
41:
36:World War II
821:Categories
807:2006-08-26
776:2006-08-28
756:2006-08-26
731:2006-08-26
710:2006-08-26
683:2006-08-26
660:References
610:estimated.
597:Propeller
107:pitot tube
88:Technology
64:navigation
540:ρ
506:−
476:⋅
352:⋅
349:ρ
111:pit sword
125:Analysis
115:rodmeter
271:Dynamic
49:History
18:Pit log
406:where
262:Static
247:where
801:(PDF)
794:(PDF)
412:Water
253:Total
56:knots
620:GPS
599:RPM
113:or
68:GPS
823::
703:.
692:^
70:.
810:.
779:.
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514:S
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492:o
489:T
485:p
480:(
473:2
466:=
461:r
458:e
455:t
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449:W
445:v
419:ρ
410:v
384:2
379:2
374:r
371:e
368:t
365:a
362:W
358:v
346:+
341:c
338:i
335:t
332:a
329:t
326:S
322:p
318:=
313:l
310:a
307:t
304:o
301:T
297:p
269:p
260:p
251:p
225:c
222:i
219:m
216:a
213:n
210:y
207:D
203:p
199:+
194:c
191:i
188:t
185:a
182:t
179:S
175:p
171:=
166:l
163:a
160:t
157:o
154:T
150:p
20:)
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