THE British naval authorities and the officials of the White Star Line are seemingly bent upon withholding from the public the essential facts regarding the collision between the. Hawke and the Olympic on the 20th of September. Were the commander of either ship undoubtedly at fault in the handling of his vessel, it is not at all likely that the present policy of silence would be preserved, and this naturally suggests that there were conditions, contrib-utive to the mishap which were, in a degree, either unsuspected or beyond control of the commanding officers. This suggested a line. of investigation which, while not exactly new, is at present not thoroughly explored. In April of 1908 the “Prinzess Irene” and the steamer “Parima” were outward bound in the lower bay of New York The “Prinzess Irene” was bound for Europe, while the “Parima” was steaming for the West Indies. The two ships were at least 200 feet apart, but following a comparatively narrow channel. Just before reaching a certain buoy, the “Prinzess Irene"—the bigger craft—began to lap upon the stern of the leading “Parima.” Suddenly, the latter vessel was drawn to starboard and toward the “Prinzess Irene,” while her bow fell off to port despite her helm and engines, and her head grounded. There was no collision. Some hours later the “Parima” was gotten off, but she was found to be leaking, and repairs costing about $40,000 were needed before the ship could put to sea. It was claimed at the time that the mishap to the “Parima” was due to suction set up between the two vessels in motion while proceeding on the same course. As a result of the unusual mishap Naval Constructor D. W. Taylor, U. S. N., carried on a series of experiments at the Government Model Basin in Washington. In substance, Naval Constructor Taylor found that when the overtaking ship began to overlap the other the disturbing force was of slight moment: there actually appeared to be a repulsion at both bow and stern, and, strange to say, the repelling force acting upon the stern appeared to be greater than that exerted upon the bow of the following vessel— causing the stern of the latter to tarn in toward the leader. As the over-hauling craft continues to creep up, the bow 'suction grows stronger, and the repulsion of the stern of this ship lessens-—becoming, in fact, attraction when the two boats are abreast. Carrying the action through its natural sequence—the two boats proceeding—Naval Constructor Taylor discovered that there was a reversal of effects as the overtaking vessel assumed the lead: the bow suction diminishing rapidly while the stern pull became stronger—the latter reaching a maximum when the midiength of the overhauling boat was something like two-tenths its length ahead of the midships of the overtaken vessel. These disturbing forces are particularly potent in shallow waters, and the experiments at the navy yard in Washington were made under circumstances of depths which were calculated to diminish rather than magnify the measure of the suction as compared with the circumstances of ships passing through the ordinary channels approaching harbors. Naval Constructor Taylor found by his model trials, when the two boats were in the position described by Prof. Reeve, that the forces of attraction and repulsion were, in effect, directly opposite, i. e., the bow of the overlapping boat turning in toward the midlength section of the craft istill in the lead. The correctness of Naval Constructor Taylor's conclusions can easily be verified and understood even if one has not at his disposal a model basin for actual tests. In Fig. 1 we have a pipe of variable diameter through which water is flowing in the direction of the arrows. At the points of maximum diameter, P, P, we have the full pressure of the head, while through the contracted section, V, we have increased velocity but diminished pressure. Bearing the simile of this pipe and the varying pressures due to changes of diameter in mind, we can easily trace the action of the forces at play between two vessels where one is overhauling and overlapping the other. In Fig. 2, the smaller ship is within a short distance of the bigger leading one and abaft the beam of the latter. Both vessels being in motion, the effect of the stream-line forces is a counterpart of current sweeping stern-ward. The hulls of the two ships form for us the adjacent walls of the pipe, while the relative nearness of the bottom on one hand and the water's surface on the other box in, so to speak, the stream-lines. At the point V we have a restricted passageway for the current and, hence, reduced pressure and increased velocity of flow. This action may possibly be augmented by the propeller wake of the big ship. At point P we have full pressure acting against the starboard bow while on the port bow at V there is a loss of pressure. Naturally the bow swings to the left and the stern in the opposite direction, drawing the smaller overtaking boat toward the larger one. The accounts of the movements of the cruiser “Hawke” just prior to her collision with the “Olympic” seem to agree that the warship was crowded toward the liner and brought approximately into the position indicated by Fig. 2. The “Olympic” is a vessel of 45,000 tons displacement, while the “Hawke” is about one-sixth as large and of much lighter draft. It is easy to understand how the cruiser would be drawn toward the liner under these circumstances, and even though the commander of the “Hawke' ' did throw his helm over, the hydrostatic energy at work would be more than the latter's rudder could offset within the time available—especially if the warship were making considerable speed. In Fig. 4 we see what followed, and this bears out what Naval Constructor Taylor found to be the case with his models. Fig. 3 shows us how the forces are distributed when the two vessels are directly abeam. The space between the two boats is a counterpart of the condition indicated by the pipe in Fig. 1. Assuming P to have the same force value in each instance, while V is an indication of reduced pressure, it is not liard to understand how the resultant is a movement of the smaller vessel bodily to port and toward the bigger overweighted by P, P, P. It is not necessary to confuse this problem with the crests and hollows of neighboring waves formed by the ships in motion. In fact, the vessels need not be moving at speeds high enough to disturb the water to this extent in order to arouse the suction needful to draw the vessels '• toward one another. It is quite likely that the dispar-~^_S^> ity in size between the “Olympic” ~^jp and the “Hawke” may have contributed ^^22> to the disaster, because there was also a P considerable difference in size between the “Prinzess Irene” and the “Parima.” In the experiments made at the Washington Model Basin, the models were of identical displacements. It is fortunate that the collision between the warship and the liner occasioned no loss of life; ause. and if it were due to suction—even admitting imprudence on the part of the naval skipper— it is but another object lesson on a large scale of the care that ought to be taken under circumstances of this sort. We must not forget the im. mense size of the “Olympic” and the majority of the ships now engaged in transatlantic passenger and freight service, nor must we overlook the steady development of speed in these big vessels. Undoubtedly, both of these factors have an important bearing upon this question of suction, and these craft should be given a wider berth when in motion. The mishap is fruitful of valuable suggestions, and, at the same time, it brings us to a realization of the stupendous energy latent in the water when the balance or equilibrium of the stream-lines is once disturbed.
This article was originally published with the title "The “Hawke”-“Olympic” Collision" in Scientific American 105, 18, 389 (October 1911)