Type 812 electronic reconnaissance ship
China's special electronic reconnaissance ships originated from the Type 812 electronic reconnaissance ship (later scrapped due to vibration problems), the Type 813 electronic reconnaissance ship 852 Kaixingxing Ship (retired), and the Type 814A mid-offshore electronic reconnaissance ship 900 North Diversion.
Vibration is the most common problem of this type of medium-speed ship. The 812 reconnaissance ship was scrapped because of severe vibration. Although the 813 electronic reconnaissance ship has better performance, its reconnaissance equipment is relatively simple. It is only equipped with a radio signal reconnaissance system, and it is not equipped with acoustic and optical systems. Therefore, the scope of use is limited. At the same time, due to the large tonnage of the ship and the high cost, considering the adjustment of the national economy and the reduction of the national defense budget at that time, it is difficult to build and carry out corresponding improvement design in a short period of time.
In 1979, the 812 electronic reconnaissance ship was designed. China designed the 812 reconnaissance ship on the basis of the 635 type channel survey ship.It was mainly used to track and monitor enemy ship activities and collect maritime information in the offshore. Due to the installation of more reconnaissance equipment, the full-load displacement increased to about 1,700 tons. The first ship was built by Tianjin Shipyard in 1983 and launched at the end of 1983.
Electronic reconnaissance ships are naval service ships used for electronic technology reconnaissance. Equipped with radio receivers, radar receivers, terminal demodulation and recording equipment, signal analysis instruments and receiving antennas in various frequency bands, and some are also equipped with electronic interference equipment. These collected data are used to decipher secret telegrams, study the law of communication frequency bands, and study interference methods. They are important materials for electronic warfare.
Unfortunately, the 812 ship suffered aft vibration problems during tests. Although the relevant structures were strengthened many times, the problem was still not resolved, which seriously affected the work of the ship's precision electronic equipment. China improved and designed the Type 812 reconnaissance ship on the basis of the Type 635 Yanlai channel survey ship. Due to the installation of more reconnaissance equipment, the full-load displacement increased to about 1,700 tons. The first ship was built by Tianjin Shipyard in 1983 and launched at the end of 1983. Unfortunately, the stern vibration problem that occurred during the Type 812 sea trial seriously affected the work of the ship's precision equipment. Later, after joint research by relevant departments, designers finally found out the reason was the exciting force of the propeller. The relatively large weight of a propeller positioned at the end of a cantilevered propeller shaft causes bending vibrations during propeller shaft rotation.
One problem facing ship designers is that of keeping hull vibration to an acceptable level. Excessive vibration not only causes unpleasant noise in the vessel but may also produce dangerous stressing of the ship's structure. In addition, the forces causing hull vibration also cause other undersirable effects. Propeller vibration can cause substantial lateral vibration when large horsepower engines operate at wide open throttle. Such vibration causes lateral forces on a propeller shaft, which forces substantially exceed the thrust developed by the propeller and transmitted to the propeller shaft. The vibrations in the propeller shaft can be caused by relative flexing between various parts of the hull. That is, it is common to mount the engine upon one section of the hull, and the propeller shaft suport bearings on another section. Although the hull is typically formed of steel, it is subject to flexing during vessel travel. As a result, the propeller shaft, engine and bearings can experience relative displacements, whereby undesirable stresses are imposed on the propeller shaft that can lead to damage of the bearings and excessive vibration of the propeller shaft.
A principal cause of hull vibration is pressure fluctuations in the water generated by the propeller which act on the hull above the propeller. Due to variations in the wake across the propeller disc, that is, the area swept out by the propeller blades, the blades undergo substantial changes in loading as the propeller rotates. With a conventional single screw stern construction, the maximum wake at the propeller disc may be as much as eight times the minimum wake there. One effect of the rapidly changing loading on the propeller blades as the propeller rotates is to produce the strong pressure pulses in the water which excite hull vibrations and cause serious cavitation errosion of the propeller blades.
Other effects of the uneven distribution of wake across the propeller disc are a reduction in the efficiency of the propeller and vibration of the tailshaft which increases the wear of the bearing linings and may damage the oil seals and add to the hull vibration. With a conventional hull, there is a heavy concentration of high wake at the upper portion of the propeller disc which gives rise to an eccentric loading of the propeller causing the tailshaft to run at the top of the stern bearing. Finally there is an increase in the dynamic bending stresses applied to the tailshaft which may cause fatigue failure of the tailshaft.
In the end, the 812 reconnaissance ship was scrapped. Later, after joint research by relevant departments, it was finally found out that the reason lies in the exciting force of the propeller. Through this effort, China's ship design department changed the past practice of increasing propeller efficiency, and instead designed and improved the propeller from the perspective of comprehensive efficiency, which brought China's ship propeller technology to a new level. The subsequent 814A electronic reconnaissance ship North Diversion 900 was developed on the basis of the 812 type.
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