MARINE STEAM TURBINES The advantages of the marine steam turbine which have led to its more general adoption during the past few years are as follows (a) There are fewer working parts, as no slide valves, pistons and connecting rods are required. This means also that fewer spare parts have to be carried; (b) the steam is supplied direct from the boilers with no intervening loss ; such as occurs through glands, etc., when a reciprocating steam engine is the prime mover ; (c) there is considerably less danger of breakdown, a big and for astern working, astern nozzles are fitted. This arrange ment is particularly suitable for light warships. The action of the impulse turbine and the reaction turbine is shown diagram matically (fig. 2).
Fig. 3 shows the general arrangement. These turbines are rapidly gaining favour for marine propul sive purposes. The chief trouble with the marine turbine arose with the propeller. It is necessary that a turbine for maximum efficiency should run at a high rate of revolution, whilst for pro peller efficiency, much lower rates of revolution are necessary. The solution for this was by the introduction of some form of gearing between the turbine and the propeller. Experiments have been made with several forms of reduction gearing, such as the electrical, hydraulic and mechanical. Mechanical gearing appears to show advantages over the other forms, an efficiency of over 98% being obtained in the case of the single reduction and over 97% with the double reduction. There is practically no limit to the ratio of such reduction. A complete unit of geared turbines is to be found in certain twin screw torpedo-boat de stroyers. Double-reduction gearing finds favour for the following reasons: (a) the type permits larger ratio between revolutions of turbines and propellers without excessive size of gear wheels; higher revolution of turbine permits, for the same power, smaller turbines, and increase in the number of turbines for the same power further reduces the size of each unit; (b) high revolutions or greater blade speed are possible, resulting in the nearest ap proach to the point where the blade speed in relation to the steam speed gives maximum economy; (c) a small turbine permits of a pivot-thrust block being used, as the dummy can be omitted, and all unbalanced load taken by the block; (d) small turbines have small rotors and can thus be made of a much more robust construction; (e) variations in temperature for any single tur bine are kept moderate, and these turbines are therefore suit able for use with superheated steam ; (f) the parts being smaller, the turbines can more easily be overhauled, repaired or renewed.
Plate H. (fig. 5) shows an example of double-reduction gearing.
The first application of elec tricity for the transmission of power between the prime mover and the propeller shafts in ships was first adopted in America in the year 1908. This proved most successful, and many of the world's warships are now so equipped. The electrical equip ment is an alternating current generator suitable for direct cou pling to a high-speed turbine, a motor of suitable speed for direct coupling to the propeller shaft, a direct current exciter or an auxiliary generator, from which direct current can be obtained, and suitable control gear. The whole can be regarded as a re duction gear, the ratio of reduction being proportional to the number of poles on the generator and motor. The main driving power is transmitted magnetically across large air gaps giving an elastic medium for absorbing shocks, and thus making it a simple and reliable form of speed reduction. The electric drive lends itself particularly to any speed reduction between the turbine and the propeller. The general advantages are perfect balance with no sliding surfaces and no reciprocating parts, thus making this method free from vibration and smooth in operation; the electric drive is noiseless, an important factor in passenger ships; there is perfect control in heavy seas without danger of the pro pellers racing; the rapid manoeuvring of the machinery is a very big asset in warships; and the upkeep costs are less than with any other form of marine propelling machinery.
all marine installation of gearing a Michell thrust block is essential to the satisfactory working of the gears. The whole of the gearing is an independent unit as far as attachment to the turbine is concerned, being driven from the turbine by a flexible coupling. It is necessary, therefore, to hold the gearing in a position from which it will not vary more than a predetermined amount. There being no collars for this purpose on the gear shaft, and as a slight lateral movement is necessary when the thrust collar moves over from driving ahead to astern, the whole of the gearing is located from the thrust block. The Michell block possesses marked advantages over the older form of multiblock. The device (fig. 4) consists of one collar on the shaft, the thrust being taken by a series of pads cap able of a slight rocking movement. These pads maintain a con tinuous flow of oil between the metal surfaces, which are thus kept apart by as many oil wedges as there are pads to produce them.