It should not be lost sight of that in performing the first half of the operation of tacking—that is, bringing a vessel head to wind—the assistance given by the sails to the vessel in turning does not retard or deaden the vessel's way like the action of the rudder. By letting fly the head sheets at the right moment, the centre of effort of the other sails would be thrown so far aft, that a very long arm would be formed by the couple C E s (see Fig. 11). It is also quite possible to tack a vessel by working her sails without putting the helm down at all, especially if she carries a fair amount of weather helm; upon letting go the tiller and jib sheet the vessel would fly head to wind, and, the fore sheet being kept a-weather, her head would soon pity off ; it might, however, be found necessary to ease the main sheet before she would gather way, as the foresail being aback would necessarily tend to put stern way on her. At the meeting of the Institution of Naval Architects in 1871 the late Sir E. Belcher, speaking on the handiness of sailing ships, stated that when he was in charge of the Samarang he gave a written order to the officers that the helm was never to be put down in tacking. The helm was let go, and the vessel came head to wind, the head yards were braced aback, and thus the Samarang was always got round. Possibly this was a smart thing to do, but there can be no doubt of the value of the rudder for quick staying, if used judiciously.
The rudder action of vessels propelled by screws is attended by some anomalies which cannot always be accurately pre-determined. In the first place, the action of a propeller on a ship whose helm is amidships tends to turn her; that is, a right-banded screw will turn the ship's head to starboard, and a left-handed one to port; and from experiments made with the Great Britain in 1845 it was known that a propeller, if suddenly reversed, would tend to turn a ship against her helm until her way was stopped. In spite of this knowledge, much attention does not appear to have been given to the subject until three or four years ago, when several accidents were clearly traceable to the fact that the vessels under port or starboard helm, whilst their screws were being reversed to deaden their way, turned the opposite way to that expected. Professor Osborne Reynolds gave the subject much attention, and at last, in 1877, succeeded in getting a committee appointed, consisting of himself, Mr. James R. Napier, Sir William Thomson, and Mr. W. Fronde, to inquire into the matter. The committee made experiments with the Duke of Argyll's steam yacht Columba, the Earl of Glasgow's steam yacht Valetta, and several large steamers in the merchant navy.
The result of the experiments bore out all that was previously known or suspected of the action of the screw on the steering. With the steamer
going nine or ten knots ahead, the engines were suddenly stopped and reversed ; simultaneously with the reversal of the engines, the helm was put hard a-port, the vessel, of course, still forging ahead. According to what is usual, with the helm hard a-port the ship's head should have turned to starboard ; but, instead of doing this, her head went off to port twenty eight degrees, or about two and a half compass points, before the vessel's way was stopped. Next, the ship was turned full speed ahead again, and when she had full way on the engines were stopped and reversed as before, but the helm was put hard a-starboard. Between the moment of reversing the engines and that of the ship's way being stopped, the ship turned forty degrees to starboard, or nearly four points, or nearly one-eighth of a circle. The experiment was tried over and over again, and the invariable result was that, during the interval of reversing the engines and the stoppage of the ship, the rudder acted just contrary to the way expected.
A fact hitherto not generally known was brought to light, that the effect of the screw is largely governed by its immersion, and that its influence is greatest on the steering when it is near the surface "churning " the water. When deeply immersed it has little or no effect of itself in turning a ship one way or the other, and never under the most favourable circumstances—that is, with the screw near the surface, and a low speed— could the ship be got round in a circle less than double the radius of the one she would describe under the influence of her rudder with the engines turning ahead.
Also it was clearly proved that an experiment is required for every individual vessel, and for varying speeds ; thus, one steam yacht, with helm hard one way or the other, may keep a straight or nearly straight course whilst her engines are reversed and before she stops, and another may turn in direct opposition to the way she should turn under the ordinary influence of a rudder. It is thus necessary that a simple experiment should be made for every screw steam yacht, so that her master might be acquainted with her behaviour as to steering whilst her engines are being reversed. It can, perhaps, be said that a vessel turning through a arc of twenty or forty degrees cannot be of much consequence when her way is being stopped; and it might not be if it was clearly understood which way she was going to turn. It might, however, be of the utmost consequence, if the vessel whilst losing her way turned thirty degrees to port, instead of thirty degrees to starboard. This would practically be a difference of sixty degrees, or two-thirds of a quadrant, and would be quite sufficient to bring about a disaster.