Two things seem to stand out from the mass of testimony taken upon this issue:
First. When a ship moves into shoal water, particularly where the volume of water is restricted upon one side as by the bank of a channel or a canal, forces are set up which exercise a disturbing influence upon the behaviour of the vessel. It has been estimated that, in the case of a 10,000-ton vessel moving at 15 knots, 900 tons of water are displaced forward and replaced aft in each second, and it must be that if the flow of water thus set up is markedly unequal upon different portions of the hull, the disturbing influence will be of very considerable extent.
Second.The effect upon the vessel's course of the forces so set up cannot be accurately predicted or appraised.
The trouble is, of course, the lack of scientific data. Such knowledge as there is is drawn principally from observation of the behaviour of vessels under various conditions, which usually contain many unascertained facts.
In each case, the contour of the channel, the slope of the bank, the depth of the water under the keel and its distribution over the shoal, the lines of the vessel, the nature of her propeller action, and the angle of her approach, are all elements which affect the result. Experimentation with actual vessels is practically out of the question. Experiments have been made with ship models in tanks, but those conducted by the government under Admiral Taylor's supervision were primarily for the purpose of developing improved designs for the vessels themselves. Experiments with models have also been conducted for the purpose of observing the suction produced by ships passing or overtaking one another, but very little has been done to determine the effect of banks and shoals with tank bottoms reproducing actual channel conditions.
Lacking sufficient data, general laws are yet to be evolved and the subject remains in a theoretical state. With so many necessary factors to be ascertained in each case, the application of the generally accepted laws of hydrodynamics is largely a matter of speculation. For example, in applying Bernouilli's theorem, Admiral Taylor apparently discounts the retarding effect of friction upon the current on the restricted side of the vessel, and it also appears that he considers the propeller action of small moment. His opinion is undoubtedly entitled to great weight, but, without the aid of extensive experiment, no one can positively say whether these factors are of great importance or not, and I should not attempt to do so.
It seems to me that all that is definitely known is that shoal water will cause erratic action on the part of a vessel, and, so far as it affects the science of navigation, the only general rule that can be laid down is that it should be foreseen and avoided; but I doubt whether the way in which the vessel will act in any given case is predictable.
The fact that scientists and experienced navigators of such high standing, wide experience, and unquestionable sincerity have differed so diametrically upon both the theory and the application of it to the situation in hand, as have the witnesses in this case, confirms the foregoing.
Without any very great feeling of certitude, I should be willing to accept the testimony of Admiral Robison and Captain Sheridan to the effect that when vessels approach the bank of a channel it usually happens that they sheer off toward the deep water side. Their testimony was based upon actual experience, and I cannot doubt that the instances which they have cited were actual occurrences. Even Admiral Taylor who, for the libelant, testified that, under the conditions as they existed in this case, if any sheer had been felt it would have been to starboard, agreed that, had the ship been inclined outward as she approached the area of constriction, the sheer, if any, would have been to port, though he asserted that in neither case would the sheer have been uncontrollable or even appreciable.
But even so, there remains the question of how much value can be given to opinions drawn largely from personal experience of what has occurred upon other occasions, necessarily observed under widely varying conditions; diverse, complicated, and partially unknown factors governing in each instance.
Moreover, assuming that the shoal water on the Bessemer's starboard side was what caused the Bessemer to sheer, it by no means follows that such sheer would have sent her, entirely out of control, across the channel, a distance of some 600 feet from the channel bank, or about 1,500 feet diagonally.Admiral Robison testified that the force which, in his opinion, produced an uncontrollable sheer was "material for about a half a ship's length from the shoal itself," and that "if it is only 200 feet, where the force begins, where the force * * * could take command of the situation." Again:
"Q.Do you mean that at that point (200 feet away from the shoal) the ship ought to respond to the rudder and come back? A. Yes, sir."
But the evidence is that the Bessemer at no time before the collision did come back. Very significant is the record of the gyro compass, which shows that, from the beginning of the sheer to the point of collision, her head was continuously swinging to port. If the collision point is correctly marked on the compass record the ship really had not begun to answer her helm, since there is no trace of a shift to starboard; merely a slowing down of the swing to port. Of course, the collision point may not have been accurately marked on the gyro record, but if it is, then it certainly follows from Admiral Robison's testimony that, whatever the condition was which kept the Bessemer's head continuously swinging to port five or six hundred feet from the bank, it could not have been the external forces set up in the shoal water, but must have been something else; and a wheel turned the wrong way is pretty clearly indicated.
Decree for the libelants, with costs.
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