CHAPTER XIV. THE STEAM TURBINE APPLIED TO THE PROPULSION OF VESSELS. THE success of the Parsons steam turbine on land led to the formation of a company in the beginning of 1894 for applying the steam turbine to marine purposes. This pioneer syndicate the Marine Steam Turbine Co.-at once commenced experimental work, and the Turbinia was produced. It had often previously been proposed to use a steam turbine for the propulsion of vessels at sea; but, as far as the author is aware, no steam turbine was ever before fitted on board a vessel for this purpose. The same difficulty now arose with the marine steam turbine as had arisen with turbines previously made for use on land—namely, of running the turbine economically at a sufficiently low speed. In the driving of electric generators a high speed is usually an advantage, except when it becomes so excessive as to occasion dangerous stresses due to centrifugal force. With screw propellers, however, the case is very different. The existence of cavitation with high velocities of screw propellers was not unknown at the time the Turbinia was built; but the importance of it with propeller-blade velocities such as those tried in the Turbinia was not appreciated. The trials of the Turbinia, however, clearly demonstrated that an ordinary propeller could not be run with any degree of efficiency above a certain velocity. The propelling R gear of the Turbinia as first tried consisted of a single steam turbine driving a single propeller-shaft, on which were three propellers. The designed speed of the turbine was 3000 revolutions per minute, and the designed power was 2000 H.P. The power was obtained (as was proved by the use of a dynanometer); but, at the designed speed of rotation, only 18 knots could be got out of the vessel-the maximum efficient propeller velocity had been exceeded. Beyond this limiting velocity (the exact value of which depends on the size and form of the propeller) an almost perfect cylindrical vacuum is formed around the propeller, causing great loss of power. As a steam turbine could not be run economically except at a high velocity-above the limiting velocity of a propellerthe difficulty arose of getting an efficient combination. With a low velocity the steam consumption was excessive; with a high velocity the waste of power by the propeller was enormous. The designers of the Turbinia and her propelling gear, however, energetically and scientifically grappled with the difficulty. Trials were made with screws of various patterns, a spring torsional dynanometer was constructed and fitted between the turbine and the propeller-shaft to measure the actual torque, and a series of experiments were carried out in a tank with model propellers, which were illuminated by the light from an arc lamp thrown on to them for a single instant in each revolution. At length, after a great amount of labour, the efforts of the experimenters were crowned with success, a combination and arrangement of turbines and screw propellers being obtained which gave excellent results-results as good as the most optimistic of well-wishers had ever hoped for. The solution of the difficulty was found in dividing up the power into three turbines driving three propeller-shafts. Each shaft carried three propellers of a special form. As the economic speed of a turbine depends on the difference of pressure of the entering and exhausting steam, it will be obvious that, by dividing the total range of pressure into three parts—that is, in expanding the steam only about one-third in each turbine the minimum economic speed of each machine could be very much reduced-in fact, reduced to about one-half. The propeller-shafts could thus rotate at one-half the speed. In addition to this, the employment of so large a number of propellers-nine in all-allowed each to be of small size, and therefore allowed the tips of the blades to revolve in circles of small diameters. By thus reducing both the size and the angular velocity of the propellers, and giving them a suitable design, their efficiency was brought quite up to the normal. The result was that the Turbinia attained a speed-33 to 34 knots-never before reached by any vessel. The length of the Turbinia is 100 feet and the beam 9 feet. The displacement is 44 tons, which is made up as follows: Main engines, 3 tons 13 cwts. Total weight of machinery and boiler, screws and shafting, tanks, etc. ... Weight of hull complete 22 tons ... ... 15 Coal and water Total displacement .. ... ... 71, Steam is supplied by a water-tube boiler, and enters the first turbine cylinder at a pressure of 170 lbs. per square inch. The heating surface of the boiler is 1100 square feet, and the grate area 42 square feet. The stoke-holds are closed, and |