Chapter 16, Ocean Engines
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Ocean water is salty because the ocean basins are the final repositories for water that runs off the land. The salinity of ocean water generally ranges from 33 to 37 parts per thousand ppt , varying from place to place because of differences in evaporation, precipitation, and freshwater runoff from land and glaciers. Seawater also contains nutrients such as nitrogen and phosphorus that play essential roles in nutrient cycling. Another aspect of ocean chemistry is dissolved gas content, particularly the dissolved oxygen upon which gill-breathing marine animals depend.
Ocean water is vertically structured. Water density increases as salinity rises and as temperature falls, giving rise to different layers of water. The waters of the surface zone are heated by sunlight each day and are stirred by wind. The pycnocline is the region below the surface zone in which density increases rapidly with depth. The deep zone of the ocean lies beneath the pycnocline and is not affected by wind and sunlight.
The authors address the advantages and disadvantages of renewable energies, their appropriateness, and their socio-economic implications. Qty: Add to basket. The northeast Pacific sofar network figure consists of three monitoring stations located at 1 the U. Each station consists, essentially, of hydrophones planted offshore on the ocean bottom and connected by underwater cable to amplifying and recording equipment on the beach. All three sofar monitoring stations are equipped with two channels, each consisting of 1 a beach amplifier located in a beach hut, 2 land lines connecting the beach amplifier and main monitoring equipment, and 3 the main monitoring equipment.
The main monitoring equipment is composed, essentially, of the following units: 1. Western Electric Co. Power-level recorder for each channel; 3. Automatic switching unit; 4. Monitor-speaker amplifier and speaker, which may be switched to either channel; 5. Chronometer and related time-tick circuits for both channels; 6.
Signal generator and calibration set for putting known calibration signals into the beach amplifier. A simplified block diagram of one channel of a sofar monitoring station is shown in figure At the left are the sea cables terminating at the lower panel of the beach-amplifier rack. The hydrophone numbers correspond to the numbers appearing next to each jack on this panel. No other numbers are used to designate hydrophones.
The outstanding advantage that sofar has over other air-sea rescue systems of signaling is that it operates automatically. The only action required of the operator is removal of the cotter pin. Then, regardless of whether the signal bomb is thrown overboard or sinks with the wrecked craft, the bomb fires and sends the signal because it is armed and detonated by pressure.
Another application of sofar is for long-range submarine signaling. By means of the multiple-shot bomb, signals can be sent at any fixed time interval.
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In this way coded messages can be sent great distances from any craft at sea. Special equipment is not needed, and with a network of monitoring stations messages are sure to be received. To ensure detection of any vessel regardless of size or type, a number of different devices are used in the detection line.
Most vessels are built of steel and have magnetic properties; consequently, one device is used which detects a ship's magnetic field. Propeller and engine noises are transmitted to the water and provide another means of detection by listening devices. That part of a vessel below the water line provides a surface from which underwater sounds of short duration may be reflected, thereby providing the requisites for echo-ranging devices.
Harbor echo-ranging and listening devices are used to provide precise tracking information to the patrol vessels and are placed adjacent to the patrol area inboard of the listening and magnetic detection lines. Magnetic indicator loops are placed to seaward, because experience has shown that they usually are more reliable in detection ability than the other systems.
Because the magnetic loop is less dependent on the human element for its warning efficiency, it is very useful for the first warning. The radio sonobuoys or cable-connected hydrophone listening devices are placed just inboard of the loops where they serve to indicate what segment of the loop has been crossed and to provide additional information as to the direction, speed and type of vessel.
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Detection Tactics The purpose of fixed underwater detection is to eliminate the element of surprise from an enemy attack and allow necessary defensive action to be taken by patrol craft and harbor-defense batteries. The system, then, must provide a harbor-detection line which cannot be evaded and which is as firm as topographical factors and technical limitations of the equipment permit.
The magnetic indicator loop, which is laid on the ocean's bottom, records any distortions of the earth's magnetic field caused by the presence of an iron body over it. The cable-connected hydrophones detect underwater sounds generated by a vessel's propulsion machinery and transmit the resultant electric impulses to a shore station by means of a submarine cable. These hydrophones are placed behind the magnetic indicator loop for the second line of detection. Radio sonobuoys perform the same function as cable-connected hydrophones but send the underwater sound ashore by means of radio instead of through a cable.
They are used in place of hydrophones when water depths are excessive or when time does not permit the laying of submarine cable required for the installation of hydrophones. Tests have shown that a submerged submarine running at "silent speed" usually cannot be heard when it is more than yards from a listening device, so hydrophones and sonobuoys are installed less than 1, yards apart to force any vessel entering the protected area to pass within range of one of them.
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By noting the unit from which the signal is loudest, the operator can estimate the position of the target, and an experienced operator can usually determine the type of ship by the noises it emits. A third type of harbor-detection equipment is the herald. Because of the ability of the herald to obtain ranges and bearings, the target position can be pinpointed and harbor patrol craft can be directed to the exact location of the enemy. The harbor detection system, then, is usually composed of three lines of defense: 1.
Magnetic loops which are the most dependable and require the least attention of the operator. Cable-connected hydrophones or radio sonobuoys-listening equipments with which the operators can verify the contact and establish an approximate position for it. Heralds , which give the bearing and range of the target and allow precise positions to be given to the friendly attacking vessels. The loop is a very sensitive detection device when properly laid and operated. The distortion of the earth's magnetic field by a metal object crossing the cable causes magnetic unbalance between the two areas enclosed by the cable, generating minute currents which are indicated by a sensitive recording fluxmeter galvanometer in the shore station.
The loop itself consists of cables laid along the ocean bottom in the form of a figure "8". The average length of the loops is between 2 and 3 miles, but may be as short as 1 mile, or as long as 6 miles. In general the lengths should be kept as short as possible in keeping with the number of fluxmeters available. A shorter cable allows greater accuracy in localizing the target, and a reduction of ambient noise permits the equipment to be operated at higher sensitivities.
The spacing of the cables is usually yards, which is the average length of most craft that will be passing over it.
When the loop is designed for the detection of small craft and midget submarines the spacing may be made less. When the cable is laid, great care must be taken to provide the proper tension on the cable as it is paid out.
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If too much tension is kept on the cable, lengths of cable will be suspended between high spots of the ocean's bottom. These suspended portions of cable will move with the movements of the water and limit the usable sensitivity of the system. If the cable is laid as slack as possible it will conform closely to the contour of the bottom, and movement will be materially reduced.
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However, some tension will exist as the cable leaves the ship because of the weight of the cable hanging in the water. Discriminator In most magnetic-loop systems the maximum sensitivity of the fluxmeter cannot be realized because of interference caused by cable movement. Because the frequency of interfering signals caused by cable movement is much higher than the frequency of signals caused by a ship passing over the loop, it is possible to construct a filter that removes the unwanted signals caused by loop movement, yet does not interfere with those from a ship.
The discriminator has this function. The discriminator consists essentially of two circuits, or channels, as follows: 1. A filter circuit , which passes frequencies of from 0 cycles per second d-c to 0. This circuit is made up of a filter network and a two-stage amplifier with a gain of slightly more than one. A limiter and an output stage are also included by which the output of the discriminator can be controlled so that the recorder pen does not exceed the limits of the recorder tape. A limiter circuit which operates the recentering relay of the fluxmeter recorder to recenter the galvanometer.
This circuit is required because the output of the filter circuit is so delayed that the galvanometer coil would be out of control if the usual centering action operated by the pen controls were in effect.
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This circuit can be adjusted to operate the recentering relay of the recorder at any desired deflection of the fluxmeter galvanometer, applying a return voltage to the galvanometer coil. Figure shows the external appearance of a discriminator with the OS fluxmeter and recorder. The fluxmeter and recorder are devices to convert the minute changes in loop current to deflections of the pen in the recorder. The fluxmeter is mounted on a concrete block so that vibrations are not transmitted to the very sensitive galvanometer movement.
Multiturn Loops From time to time the use of multiturn magnetic detection loops has been suggested for obtaining greater sensitivity. The suggestion is based on the fact that the size of a signature the trace left by a passing ship on the recorder increases in proportion to the number of turns used in the loop. However, there are relatively few locations where any real gain in sensitivity can be obtained by this means.