Whatever description of wheel be used, there is a point on its radii which describes a circle, known as the rolling circle, equal in its de veloped length to tho distance actually traversed by the vessel during one stroke of the piston, and which serves to measure the ratio of the useful effect to the power exercised. It is Important that the floats should be placed beyond this lino, and in sea-going vessels exposed to cross, or rolling seas, the inner edge of the floats should even be kept at some considerable distance beyond it.. There is still a remarkable discrepancy in the proportions assigned by different constructors to the various details of paddle-wheels, but the moat generally received ones seem to be as follows. The diameter of the wheels is made equal to about from 4 to 44 times the length of the stroke, in ordinary con densing engines of vessels designed for deep-sea sailing ; the breadth of the wheel is made from i to of the diameter; the width of the floats is made about of the diameter of the wheel; and their dis tance apart is made about 3 feet to 3 feet 6 inches. Some constructors make the surface of their floats dependent upon the area of the mid ship section ; and its proportion then varies between and of the latter, the best proportion seeming to be about of the said section.
The beat velocity to be given to the outer edges of the floats appears to be of that of the horizontal velocity of the vesseL Wheels of large diameter may be used on rivers without inconvenience; but in sea going vessels a practical limit is placed to the increase of this dimen sion by the effect of the rolling of the ship, and the wheels are rarely made more than # greater in diameter than the depth of the hold ; but it must be repeated that at present all these rules are little better than empirical ones.
The screw has been occasionally used as a propeller for many years, but it was not until 1836, as above stated, that its use became suffi ciently understood to allow of its being applied commercially. In that year Mr. Smith attracted attention seriously to the subject; hut Cap tain Ericsson may fairly be considered to be entitled to the merit of having been the first to overcome the practical difficulties connected with the application of this description of propeller. At the present day it certainly seems that for all ordinary deep sea goods traffic, and, the experience of the Dutch also shows, for steam transport on canals and narrow rivers, this description of mechanism will ultimately super sede every other one ur for the same purpose. For vessels of war the sheltered position of the screw, and of the engines, give that description of propeller an incomparable advantage over the exposed engines and wheels of paddle.boats.
The screw-propeller ordinarily used maybe described as consisting of two or more arms (though at the present day it is very rarely that more than three arms are used), forming parts of screws of large diameter arranged upon a central shaft. This shaft is set in motion, either by means of gearing, or by direct-acting engines working upon the crank of the shaft ; so that the screw, being set in revolution, advances in the water in the same manner that a wood screw would advance in that material. The width of the screw is usually made equal to about A of the convolution of the thread in small screws, and to nearly of that dimension in larger ones, whilst the pitch is found to be of the most advantageous proportion when it bears to the diameter the ratios of 1.2 to 1 in large screws, and of 1'4 to 1 in small ones. All these dimensions, however, must vary with the peculiar form of the ship, and its resistance to motion in the water; and in Bourne's ' Catechism of the Steam Engine,' or, better still, in his 'Treatise on the Screw-Propeller; the several conditions affecting the proportions of the screw are carefully discussed. The screw is usually placed in the dead wood of the ship, immediately before the rudder ; and its efficiency materially depends on the depth of its immersion; as also upon the fineness of the stein of the vessel, perhaps almost as much as upon the entry, or upon the midship section. The velocity
of revolution of the screw seems to range between 50 and 70 revolu Gone per minute, according to the shape of the vessel and to the dimensions of tho screw itself. The size of the vessel to which the screw-propeller is applied seems to have a more favourable influence upon the useful effect produced, than is the case when the paddle wheel is used ; and for this reason also it is preferred for vessels of heavy burden and deep immersion.
In a well proportioned vessel, provided with a screw-propeller of the best form and mechanical conditions, there is very little slip or loss of the velocity attained by the screw itself ; and it may be con sidered to range between 10 and 30 per cent. There is, however, a greater loss of the power developed by the engine in screw steamers than in paddle boats, in consequence of the friction and of the passive resistances to be overcome. This fact renders the use of the screw propeller inadvisable on board vessels which trust mainly to their steam power; because it renders necessary the transport of a larger quantity of coal. Under ordinary circumstances it seems, however, that in cases where the paddle-wheels are kept at their normal immersion, and they are so proportioned as not to give an unnecessary amount of slip, they possess a slight mechanical advantage over the screw, for the purposes of propulsion. When the vessel is brought too deep in the water, or when the action of the sea disturbs the working conditions of the wheels, or when a side wind depresses one wheel whilst it leaves the other with an insufficient immersion, the advantage is inifavour of the screw-propeller. Perhaps the screw interferes more with the steering properties of a ship than the pa-ddle-wheels do because when it ceases to be in motion its blades form a species of fan which intercepts the stream of water which would otherwise strike the rudder. The greater facility with which the screw-propeller can be adapted to the combination of steam and sail power more than compensates for this occasional inconvenience, which after all ceases to be felt when the ship is in the hands of en able seaman. The screw is least advantageous against head winds.
The majority of the screws used in the merchant service are made of cast iron, and they are cast solid. In the Royal Navy theyare often made of brass; and on some occasions they are even forged of wrought iron. A very great number of modifications of their forms has been proposed, both for the purpose of obviating the loss of power occasioned by the slip, and of preventing the interference with the action of the rudder. Feathering screws, movable screws, converging blades, screws with a pitch increasing at their extremities, corrugated screws, Griffith's screw, with a circular boss upon the shaft, Beattie's screw, placed behind the rudder, lifting screws, and other contrivances, literally too numerous to mention, testify at least to the earnest character of the efforts made to remedy the inconveniences, real or imaginary, of a mechanical agent of such importance. Hitherto the results attained by these modi fications have been far from satisfactory, and for ordinary traffic at least the screw is made upon nearly the same principles as it was in the earlier essays. It may perhaps be fair here to state that much of the merit of overcoming the difficulties attending the application of the screw-propeller was due to the late Mr. Brunel, on the occasion of the construction of the Great Britain; and he, in conjunction with Mr. Scott Russell, subsequently tried an experiment on a larger scale, in the application of both paddle and screw propulsion in the Great Eastern—with what commercial success remains yet to be seen.
(The best treatises in English upon the subject of steam propulsion in its different forms are Tredgold on the Steam-Engine and its Supplements• Hann and Genner on the Steam-Engine; Bourne, on the Screw-Propeller, and his Treatise on the Steam-Eigine.)