Great War Destroyers - Design
The opening of hostilities in World War I found the Navy strengthened with a new type destroyer design, much improved over Bainbridge's. The scope of the destroyer was much wider than that of the earlier type. The destroyer had in the first place absorbed the functions of the torpedo boat, but, unlike the latter, was not operated from local bases only. Destroyer divisions were attached to and worked in conjunction with fleets, and their duties included the important task of scouting, for which destroyers were peculiarly adapted by reason of their mobility, shallow draught, and small degree of visibility.
Turbine engines were economical only at high speed and only at approximately the speed at which they are designed to run. This introduces two problems into the design of turbine motive power for destroyers: - first, to harmonize the high speed which is necessary for the turbine rotor, with the lower shaft speed required for propeller efficiency ; and second to provide for reasonable economy both at the high speed for which destroyers are designed and the comparatively low speed at which they necessarily cruise under ordinary circumstances.
Various solutions were devised for these problems and widely differing arrangements were to be found in the destroyers in service in 1914. In recent destroyers of 35-knots designed maximum speed, a turbine speed of 2,500 RPM was converted by reduction gears to 450 turns of the propeller shaft. The problem of cruising efficiency is solved in some cases by the use of reduction gears and in others by the installation of special cruising turbines or reciprocating engines.
When a proper balance has been struck between maximum speed efficiency and cruising speed efficiency, it was found that the most economical speed at which a destroyer can cover a given distance was somewhere between 15 and 20 knots. Between these speeds, the fuel expended per mile varies but little, whereas at higher speeds it increases rapidly with the horse-power developed. Approximately twice as much fuel is required per hour at 25 knots as at 20 knots; and from 25 to 35 knots the consumption increased at an astonishing rate.
All destroyers of the United States Navy had four boilers, but it was only when making very high speed that more than two are required. A speed of 26 to 28 knots could be maintained with half boiler power, and some destroyers had reached thirty knots under two boilers. Speeds as high as 20 knots could be made with one boiler, but this was, of course, uneconomical as the boiler must be forced. Destroyers had made long cruises economically with only one boiler in use, at about fifteen knots. It was unwise to undertake harbor maneuvering under one boiler except under most favorable weather conditions, as the necessity to do considerable backing would probably draw all steam from the boiler, thus stopping the blowers and disabling the vessel. When engines were stopped and an ahead or astern signal is given, it was engineering practice to open throttles fairly wide in executing the signal, then to close them quickly to the pressure necessary for the speed demanded. Turbines of the thousand ton ships are direct connected, while those of the later type drive the shafts through reduction gears, and it was especially noticeable while warping to lines alongside docks that the newer type ships did not receive the quick "jolt" ahead or astern that was apparent with the older vessels immediately after ringing up an engine room signal. The propellers of the 1914 destroyer were larger than those of the older ships and they seem slower to "bite," but as soon as they took hold they are more effective so far as leverage is concerned.
The lightness of the ship, the large surface which she exposes to the wind, and the comparatively small draft, gave exceptional importance to the effect of wind, especially if this be on the beam. It was important to understand and keep constantly in mind the effect upon maneuvering which results from the limited backing power of turbine ships in general; the power available for backing being but little more than half of that for going ahead. This condition held for destroyers except that the power available for backing is ample and satisfactory when compared with the power that will ordinarily be used ahead in approaching a dock. However, if considerable headway is gathered the backing engines are slow to take charge: and if excess speed ahead is used in making a dock or in entering a slip, an error in judgment as to backing may have very disastrous consequences.
In maneuvering a destroyer alertness and sound judgment were required. It must be kept in mind that great power is available, (hat headway or sternway is very quickly gathered, that the hull of the destroyer is frail, and that in confined waters the effect of wind may be a very serious factor owing to the leeway and the poor manoeuvring qualities of the vessel. The tactical diameter of the destroyer with engines going ahead and rudder hard over is greater than that of the battleship.
The effect of screws of different types were more or less modified in destroyers by the great length of the vessel as compared with the leverage of the screws, the result being that under the most favorable conditions the maneuvering power due to the screws is less than with vessels of greater beam as compared with length. When one engine is turning ahead and the other astern, the ship being dead in the water, it would be found that the destroyer would swing very slowly, her bow moving to the side of the backing engine. If the conditions of wind were such as to assist the bow to swing no difficulty will be experienced in turning, but if the movement of the bow is opposed by any appreciable wind, the vessel will remain stationary and merely "tremble," or may fall off, meanwhile drifting bodily to leeward.
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