Shipbuilders are proverbially conservative, a trait doubtless acquired from shipmasters, who with lives and property at stake, and frequently far removed from assistance, can least afford to take chances in the way of experiments. This conservatism is especially to be remarked in this country, where the principal plants are maintained largely by contracts for vessels of war, whose design and construction are limited by government specifications that permit little latitude to the contractor in the way of originality or experiment, if he wishes to insure the small profit that will protect the stockholders and keep his yard going. But the world marches, and in the last decade, the restless and constant demand for progress has compelled designers to look about them and take advantage of the strides of invention, necessary to keep pace in the sharp competition of industrial life.
On board ship, the reciprocating engine steadily increasing in expansion and power, from one cylinder to five, finds itself to-day struggling in the last ditch, with its final weapon of forced lubrication, against its stalwart young rival, the modern turbine. The new type, however, has come in spite of a handicap, and one so serious, that the designers of modern turbines are to be sincerely congratulated that they have brought them to a point of superiority to the reciprocating engine. To accomplish this result, it has been necessary to design for speeds within the maximum allowable propeller speed, necessitating a turbine of about five times the weight, with its corresponding complications and loss of economy over the normal design capable of developing the same horsepower. Fig. 1 indicates the size of turbine used to transmit power at 3000 r. p. m.: Fig. 2 at 1500 r. p. m.; and Fig. 3 at 300 r. p. m., illustrating in a striking manner the increase in size and complications necessary to obtain reasonable propeller speeds from a direct connected turbine drive. Fig. 3 shows also the addition of the reversing element.
As an illustration of the lengths to which designers have gone in search of economical turbine operation—the battleship Wyoming has an installation requiring 10 turbines and 4 propellers, an arrangement necessary with the type of turbine used, in order to obtain reasonable economies at both cruising and maximum speeds.
It is natural that the engineering world should not be long content with this partial solution of so vital a problem, and this dissatisfaction has manifested itself in some quarters by a return to the reciprocating engine—the obvious desideratum of operating both the turbine and propeller at the most efficient speeds leading to many visionary and half-baked schemes, that have borne only the fruit of doubt and delay.
However, it can now be safely said, that this much desired result is to-day an accomplished fact, with every point of ship propulsion provided for, and one of the largest companies in the country prepared to fully contract for the machinery and guarantee its operation. In other words, there is at hand a ready solution of the problem of ship propulsion, the leading features of which were developed independently to meet the demands of economy in commercial life.
A technical paper entitled, "Application of Electricity to the Propulsion of Naval Vessels," by W. L. R. Emmet, describing this scheme, appeared in the INSTITUTE for March, 1910, under Professional Notes, page 296—the method referred to being described under "Electric Drive" on page 299.
There has been so much comment in and out of the service, and in technical journals, based on misconceptions and misinformation concerning this scheme, that a further examination of its features seems necessary to rescue it from hopeless confusion with other methods that have been suggested to accomplish the same purpose.
The Emmet drive consists briefly of an alternating current generator, driven by a direct connected Curtis turbine, the current being used to drive the propellers through induction motors on the shafts. For a battleship installation requiring 30,000 H. P., two of these turbine alternating current generators of 13,000 H. P. each, would operate four motors, two on each shaft—one of these to be of simple squirrel-cage type, and the other having resistances connected with the rotor circuit for use in backing. One of the motors on each shaft has a double pole winding, which permits of a change in the speed ratio at a desirable point. Water consumption, or coal consumption (which is the same thing) is thus kept within narrow limits throughout the speed range of the ship, first by use of one generator, with one motor on each shaft, next by increasing speed through changing the number of poles of the motor by means of a switch, and next by two generators operating two motors on each shaft. It must be distinctly understood that there are no abrupt changes in speed, but that the propeller speed for every intermediate number of revolutions is obtained by controlling the speed of the turbine, as is done on the straight turbine drive. The economies shown by exactly similar apparatus built and installed throughout the world, that is, large turbine generators operating motors under the same loads, are fully 40 per cent better than the best straight turbine drive can show at 12 knots speed. The weight of this equipment can be shown to be at least 12 tons less than the design of the straight turbine equipment. The voltage of the driving current would be in the neighborhood of 2000, depending on the speed of the generator at any moment. Objections have been heard to this point, on account of the difficulty of insulation and consequent danger to life. This feature needs some explanation. The armature of the generator is stationary, the rotating field carrying only the exciting current at 125 volts. The entire generator is enclosed, so the current-carrying parts are inaccessible. The cables will be insulated to any desired thickness and carried in armored conduits. All switching is done by insulated levers operating oil switches, with inaccessible current-carrying parts. The motors will be completely enclosed. In short, the installation is designed "fool-proof," interlocking switches rendering an accident through ignorance impossible. The generator circuits are designed to be capable of complete short circuits under full load, without burnout or danger.
Many officers have been misled through unconsciously having in mind direct current apparatus with its commutators and brushes requiring constant attention to guard against sparking, overloads and grounds on circuits extending over the entire ship. No such troubles are possible in this scheme, and there is no wiring except the straight cable leads to the motors. The revolving field of the generator would be excited from the ship's circuits at 125 volts, with an auxiliary self-exciter on the shaft of the generator to be used when desirable or necessary.
With this arrangement, the actual operation of the sets would reduce itself to the last elements of simplicity, and any intelligent machinist, after a few minutes instruction, could operate the set with perfect confidence and safety. Stopping and backing involves simply the throwing of a switch, and the backing torque developed is greater than that in the design of the straight turbine drive, and would act more quickly. The large currents developed in the rotors of the motors through this sudden reversal, are taken care of by rugged indestructible resistances carried in piping, water-cooled from the main injection. As there are no wearing surfaces involved in this installation, outside of the main bearings of the generating units and motors, the efficiency of the sets would remain practically constant, a condition never attained in reciprocating engines, with their large area of rubbing surfaces affecting friction and leakage of steam.
As motors and generators will be cooled by artificial ventilation, objection has been made that injury might occur from shipping seas through the ventilators in rough weather. It is sufficient answer to such an objection, to state that the entire apparatus could be submerged, and when free from its water, would operate without injury. As the sets would be totally enclosed, and watertight from water in the engine room, and would be warm and ventilated when running, no trouble can be anticipated from dampness.
The operation of induction motors from large generating units belongs to the A B C of electrical installation, and there is no new development involved, the efficiency of the combination is known to every power plant in the world, and it is only a question of time, when its economy will force its use on the most conservative builder.
The combination of turbine generator and motor is simply a speed-reducing bond, comparable to a mechanical gearing, with the important exceptions, that it is an electrical bond permitting wide and variable ratios of speed reduction that enables the turbines to keep near their best efficiency, regardless of the speed of the propellers, and the elimination of backing turbines, to say nothing of the constant anxiety and watchfulness imposed on the engineer of the watch in transmitting such powers through spur gearing, where the failure of the forced lubrication for a short time might spell disaster. The situation has been summed up by the best engineering opinion as follows:
If it is possible to devise and construct some form of safety reducing gear, which will be as reliable as the rest of the steamer's machinery, and at the same time will be of such efficiency that the overall efficiency of the plant will be so increased as to more than compensate for the extra capital caused by the upkeep and weight involved, the case of the gear will be made out.
The reliability of electric transmission is beyond question as demonstrated by its universal use under the severest conditions in commercial life. Its efficiency for ship propulsion is greater than either the reciprocating engine or turbine, and there is nothing to compensate for, because it is cheaper to construct, because lighter, cheaper to maintain on account of having no wearing surfaces, except main bearings, and, as stated, of much less weight, amounting at least to 100 tons in a battleship. In other words, the case is made out, not because of statements like the above, but because a contract may now be drawn, guaranteeing every feature of the scheme claimed.
Under these conditions, are we justified in waiting till some more enterprising country points the way?