Some surprise and disappointment has been expressed by passengers on turbine-propelled ships that vibration, although it has been greatly reduced, has not been entirely eliminated from these vessels. In considering this subject, it may be as well to state at once that, no matter what kind of engine be used, vibration never will be eliminated from steamships driven by screw propellers. The hull of a steamship is a highly elastic structure and, therefore, peculiarly sensitive to any forces tending to set up vibration. These forces may be broadly divided into three kinds the impact of the waves; the unbalanced moving weights of the engines; and certain inequalities in the thrust of the propellers. Vibrations due to the shock of the waves may be disregarded as being too infrequent to cause any discomfort. It is only in heavy weather that they become of sufficient magnitude ta attract the attention of the passengers, and even then it is only at long intervals that the sea will strike a blow sufficiently powerful to cause the whole ship to vibrate. The second cause, unbalanced or imperfectly balanced moving weights in the reciprocating engine, is, or rather was, the most annoying trouble,, since it was responsible for that incessant pounding, and in some cases very violent vertical and lateral vibration, which was for many years the bane of a deep-sea passage. A few years ago, however, after a thorough investigation, Messrs. Schlick, Yarrow, and Tweedy devised a system of arranging the relative positions of the cranks and other moving parts of the engine which resulted in a great improvement; although in the latest high-powered transatlantic ship a considerable amount of engine vibration still remains, especially when the engines are racing. With the introduction of the steam turbine, however, vibration from the engine was absolutely eliminated, the moving parts being perfectly balanced and, therefore, incapable of producing those mechanical couples which,, in the reciprocating engine, send a rhythmical series of tremors through the whole structure of the ship. The public at large, on hearing that an absolutely vibrationless engine had been produced, jumped to the over-hasty conclusion that all vibration of the ship had at last been eliminated. In this they were not altogether to blame; for it must be admitted that the sponsors of the steam turbine, in speaking of its future benefit to marine navigation, had predicted an absence of vibration from the whole ship, which their knowledge of propeller action should have taught them was, in the very nature of things, impossible. The writer has stood in the engine room of the "Lusitania" at a time when there was perceptible vibration in the structure of the ship at a point some 200 feet farther forward, and failed to perceive the slightest sensible vibration of the engines, even when the hand was laid upon the casing of either the high-pressure or low-pressure turbines. For the causes of such vibration, then, as occurs in a turbine-propelled ship one must look outside of the hull itself; and it is to be found, as we have already remarked, in the uneven action of the propellers, whose effect does not consist, as it theoretically should, in a constant axial pressure on the ship, but in a thrust which varies from a maximum to a minimum, and is in reality a series of rhythmical impulses. Theoretically, a three-bladed propeller, rotating at a certain rate of speed in undisturbed water, should exert a, constant thrust. But, in the case of steamship propulsion, the propellers, so far from revolving in undisturbed water, exert their thrust upon water that is very much disturbed and flows past them in streams of varying velocity, full of eddies and more or less complicated motions. This movement is largely due to the friction of the water upon the sides of the ship. The layers of water in immediate contact with the hull tend to cling to it, and are dragged along with increasing velocity, until at the stern of a long ship they are traveling approximately at the same speed as the vessel. This drag on the water decreases with the distance from the hull, until undisturbed water is reached. Now, as a propeller rotates, its blades are alternately reacting upon dead water and water which is moving more or less swiftly forward against the thrust which the blades exert; and, consequently, the reaction against the blades is greater, or of a less yielding character, as they are passing through the water next the hull than when, on the other half of their rotation, they sweep through the still water fifteen or twenty feet away from the hull; in other words, each blade once in every revolution hits a hard spot, as it were, in the water, with the result that the impact sets up a series of tremors or vibrations throughout the whole structure of the ship, whose period will be equal to the number of blades in the propeller multiplied by its speed of rotation. Thus, in the case of the "Lusitania," whose three-bladed propellers make at full speed about three revolutions per second, one would expect to find, if this theory be correct, a frequency of vibration of about nine per second. Observations by recording instruments show that this is exactly what occurs. It is evident, then, from the above considerations, that although the steam turbine has entirely eliminated engine-room vibration, passengers on future highspeed boats must be prepared to submit to such limited discomfort as arises from vibrations which seem to be for the present entirely beyond human control. Evidently, if vibration is to be entirely eliminated, we must find some other means of propulsion than the propeller. There is one system which would bring about the desired result, namely, that of jet propulsion; but jet propulsion, in spite of the many ingenious efforts to develop it, has never proved a practical success, at least for high-speed vessels.
This article was originally published with the title "Vibration in Passenger Ships"