MARITIME CONVEYANCE. 



Although she has disappointed all the expecta- 

 tions of her promoters, and proved a gigantic 

 failure as far as the original shareholders are con- 

 cerned, we cannot conclude this notice of steam- 

 navigation without mentioning the Great Eastern, 

 the largest vessel ever constructed. She was in- 

 tended to carry coals enough for a journey to 

 Australia and back, to attain a speed of 20 miles 

 an hour (16 knots), and, in fact, to be the com- 

 mencement of a new era in the history of ocean- 

 steaming. She was built at Millwall by Mr Scott 

 Russell, from the designs of Mr Brunei." Her prin- 

 cipal dimensions are : length between perpendicu- 

 lars, 680 feet ; length over all, 692 feet ; and breadth, 

 83 feet. She is propelled both by paddles and screw. 

 She failed altogether commercially at the outset ; 

 and an attempt to run her between New York and 

 France during the great Paris Exhibition failed 

 also. She was for many years successfully 

 engaged in laying submarine telegraph cables, 

 but latterly she was sent to Gibraltar to take up 

 her position there as a coal hulk. 



PROPULSION. 



We now come to the consideration of the means 

 of propulsion used in ships. Of the ancient 

 method of propulsion by oars, it is unnecessary 

 here to speak ; and the sails used when the wind 

 is the propelling power have already been de- 

 scribed. It only remains, therefore, to speak of 

 propulsion by steam. Two methods only of using 

 the steam-engine to propel vessels have ever come 

 into practical use namely, the paddle-wheel and 

 the screw-propeller. The former, being the older, 

 claims our first attention. A paddle-wheel was 

 used in the little boat of Mr Miller's which we 

 have already spoken of; and wheels were also used 

 in the Comet and in all the earlier steam-boats. 

 Paddle-wheels are placed about the centre of the 

 ship's length, on an axis or shaft running athwart- 

 ships. The paddles or floats are made of some 

 hard wood, and are placed radially on the wheel, 

 and fastened securely to wrought-iron framework. 

 The diameter of the wheel and the height of the 

 shaft are so arranged that at the ordinary draught 

 of the ship the upper edge of the lowest paddle- 

 board is a little below the surface of the water, 

 while at the lightest draught the board is just 

 immersed. To obviate some of the disadvantages 

 of fixed paddle floats, they have been made mov- 

 able in what are called feathering paddle-wheels. 

 These were very largely used at one time, in spite 

 of their complexity, but are now almost entirely 

 superseded by the screw-propeller. The engines 

 used in paddle-steamers were almost invariably 

 either oscillating or side-lever engines. Both 

 kinds will be found described in the article STEAM- 

 ENGINE, and it is therefore unnecessary to de- 

 scribe them here. The former had the merit of 

 simplicity, but was very ill adapted to be used 

 with the high pressures of steam now considered 

 necessary for economy ; nor could the compound 

 system be easily applied to it. The latter type 

 was exceedingly complicated, heavy, and expen- 

 sive, and indeed was to a great extent superseded 

 even before the paddle-wheel itself gave place to 

 the screw. Paddle-wheels as a means of propul- 

 sion possessed many grave objections, and in ships 

 of war they would have been almost useless : as 

 they could not have been rendered invulnerable, or 



even fairly protected, they would simply have 

 afforded a good .target to the enemy, and when 

 iisabled, would have been a great hindrance to 

 the motion of the ship under sail. In rough weather 

 too, when the vessel was rolling much, one wheel 

 would frequently be lifted entirely out of the water, 

 while the other was deeply immersed in it The 

 strain thrown on the shaft in this way was enor- 

 mous, and the danger of breakage very great. Fig. 8 



r\ 



Fig. 8. 



is an illustration of a screw-propeller of the 

 ordinary form. It will be seen that it consists 

 of small portions of four spirals, or screw threads, 

 wound round the shaft A. At first, a whole spiral, 

 or even a double spiral, was used, but experiments 

 proved this to be unnecessary, and even injurious. 

 Each of the arms, B, is called a blade. The action 

 of the propeller in the water may be popularly 

 compared (although this is not scientifically 

 correct) to the action of an ordinary screw-nail 

 in a piece of wood the water remaining, as it 

 were, stationary, while the screw continually bores 

 its way through it. The number of blades given 

 to a screw varies. When it is only used as an 

 auxiliary to sails, and is taken out of the water 

 (for the sake of avoiding useless resistance) when- 

 ever it is not used, it is made with two blades 

 only.* A two-bladed screw, however, causes a 

 great deal of jar and vibration in the stern of the 

 ship, besides being not so efficient a propeller as 

 one with a larger number of blades. It is, there- 

 fore, never used except where rendered necessary 

 by such a reason as is given above. Some 

 engineers make three-bladed screws, but there 

 seems no reason why three should be chosen in 

 preference to four, a number which possesses 

 several advantages. The screw is always placed 

 in an opening made in the stern of the ship just 

 forward of the rudder and stern-post It revolves 

 on a shaft which lies along the ship horizontally, 

 or nearly so. The diameter of the propeller must 

 be such as to allow its upper edge to be fully 

 immersed at the ordinary draught of the ship. 

 Screw-propulsion is, therefore, not applicable to 

 large steamers which have to work in very shallow 

 water ; while paddle-wheels of almost any size can 

 be used with a draught of only two or three feet, 

 as only such a small portion of their diameter 

 tias to be immersed. In many cases of light- 



The blades of auxiliary screws are often made so as to ' feather ' 

 or turn round till they lie nearly in the plane of the keel, for the 

 same purpose that the whole screw is sometimes made to bft out 

 of the water namely, to prevent their acting as hindrances to the 

 vessel's motion when she is under sail only. 



