TRANSACTIONS OP THE SECTIONS. 241 



The paddle acts at the surface of the water and pushes it in the direction of the 

 keel, when working ahead. Thus the current produced by the resistance of the water 

 is useless to the rudder, because it acts only on the upper part, where it presents no 

 flat surface. 



The screw acts on the water by a twisted surface, which, instead of pushing back 

 the water in the direction of the keel gives it a whirling motion and projects it abaft 

 in the shape of a cone, producing a current in the same way that the paddle-wheels 

 do; but being below the surfaee of the water, and the propeller being just ahead of 

 the rudder, the latter receives the impulse of this artificial current which acts before 

 the ship has moved, because the inertia makes her resist, for a few minutes, the 

 impulse of the propeller. 



Hence a principle is deducible, viz. that the paddle-wheel ship cannot steer without 

 moving, and that, on the other hand, screw ships steer before moving, and that even 

 long after the propeller is at work, if any object offers resistance to its translating 

 action. 



Another difference arises from the action of the screw, because its blades are 

 oblique to the length of the ship, and all of them are pushing the stern not only ahead 

 or astern, but also sideways, so that if the water were equally resistant close to the 

 surface and below it, the equilibrium of both vertical blades would make the screw 

 act equally throughout its path. But this is not the case : the water being more 

 resistant as the depth increases, the lower blade finds more difficulty in moving than 

 the upper one ; and the stern being acted on sideways by this difference in the resist- 

 ance, the ship will not move straight ahead; and if the rudder does not balance this 

 effect, she will always deviate to the same side when going astern. This effect will 

 naturally be more or less energetic according to the immersion of the screw and the 

 relative pitch ; for if the screw shaft were at the level of the sea, and the pitch in- 

 finite — that is, should the blade be in the place of the axis, the stern will only be de- 

 viated and not propelled ; consequently, in the actual state of things, the side action 

 of the screw on the stern is a mixture of the propelling and of the lateral effect : this 

 cannot be avoided, and is only lessened by a deeper immersion, or reduction of pitch ; 

 and the direction is according to the side of the thread ; so that a right-handed thread 

 deviates the ship to larboard when going ahead, and to starboard when going astern ; 

 it is the reverse for a left-handed thread. 



From this it would appear, at first sight, that the paddle acts much better in making 

 a ship steer well than the screw, and that the disturbances of the screw on the true 

 shipway present obstacles to the management of the ship. But it is not so ; and these 

 properties of the screw can be used in such a way as to make various manoeuvres, 

 impossible with paddles. 



Thus if a ship is required to turn short at the moment before leaving her anchorage, 

 the paddle vessel will want ropes, or at least sails, if the direction of the wind per- 

 mits, and her propeller will be used only to resist the wind or to act in the direction 

 of the keel. The screw, however, enables her to turn round on the same place when 

 in a calm ; for if the ship has a little more cable out than the depth of water, so that 

 the anchor will still offer a small resistance, and she moves her screw slowly, the 

 anchor holding on, prevents the ship from going ahead, whilst at the same time the 

 screw throws water on the rudder and makes it steer the ship as though she were 

 under way : this is well known ; and many vessels are handled in this way to give 

 them the proper direction without moving ahead ; and when at the proper point of 

 the compass, they weigh anchor and go ahead. 



If she is not at anchor, a screw ship can also turn herself by her own inertia : thus, 

 if the screw backs, the ship will begin to turn her head to starboard, and when she 

 has gone about half her length, reverse the engines, and work them quicker with the 

 helm a-port — the ship will go ahead but turn on the same side ; so by repeating 

 several times the same reversing operation, the turn of the horizon will be made 

 much more quickly than would at first be supposed, and the space required to turn 

 in may be lessened at pleasure by shortening each period of the operation. 



If there is any breeze the sails can be employed to accelerate the evolution, either 



by their oblique action, as with the gib or the mizen sail, or by being used only to 



resist the impulse of the propeller, in order to give it a more energetic oblique action. 



So with the wind ahead, and the main-top sail bearing on the mast, a stronger current 



1859. 16 



