There are no greater errors entertained by the public on any subject connected with steam navigation than concerning the Screw Propeller. It is generally supposed that it is a more economical and effective application of power than the side-wheel, which is a mistake: it is generally supposed that, with the same amount of power and all other conditions equal, the propeller will not run as rapidly as the side-wheel, which is true of steaming in a sea-way or against a head-wind, but a mistake as regards smooth water: it is generally supposed that the engines weigh less, take up less room, and cost less, which is all a mistake. The best authors on this subject and the most eminent builders generally agree, that in England and Scotland, where the propeller has attained its greatest perfection, the difference between the side-wheel and the propeller as an application of power is very slight and hardly appreciable; or that the same number of tons of coal will drive two ships of the same size at the same speed in smooth water; but that the side-wheel has greatly the advantage in a head-sea or during rough weather generally. Many persons who do not understand the subject, have theorized in just the contrary direction. They say that in rough weather the screw has the advantage, because it is alway in the water, etc. Experience shows just the reverse; and theory will bear the practice out. If, in the side-wheel one wheel is part of the time out, the other has, at any rate, the whole force of the engines, and the floats sink to and take hold on a denser, heavier, and less easily yielding stratum of water; so that the progress is nearly the same. The back current or opposing wave can not materially affect it, because the float is at the extreme end of the arm where the travel is greatest, and is always more rapid than the wave. It is not so with the screw. The blade which meets the wave is not placed at the end of a long arm where the travel is very rapid and the motion more sudden than that of the wave. This blade extends all the way along from its extreme end, where the motion is rapid, to the centre, or the shaft, where there is no motion; and all intermediate parts of this blade move so slowly, that the wave of greater rapidity counteracts it, and checks its progress. The side-wheel applies its power at the extreme periphery, where the travel is greatest, while the screw applies it all along between the point of extreme rapidity, and the stationary point in the shaft. There is, moreover, much power lost as the oblique blades of the screw rise and fall in a vertical line while the vessel is heaving.
In the new edition (1855) of "Bourne on the Propeller," he says in the preface:
"Large vessels, we know, are both physically and commercially more advantageous than small vessels, provided only they can be filled with cargo; but in some cases in which small paddle vessels have been superseded by large screw vessels, the superior result due to an increased size of hull has been imputed to a superior efficiency of the propeller. No fact, however, is more conclusively established than this, that the efficiency of paddles and of the screw as propelling instruments is very nearly the same; and in cases in which geared engines are employed to drive a screw vessel, the machinery will take up about the same amount of room as if paddles had been used, and the result will be much the same as if paddles had been adopted. When direct acting engines, however, are employed, the machinery will occupy a much less space in screw vessels than is possible in paddle vessels, and the use of direct acting engines in screw propellers is necessary, therefore, for the realization of the full measure of advantage, which screw propulsion is able to afford."
Atherton says of the propeller in his "Marine Engine Construction and Classification," page 45:
"Its operation has been critically compared with that of the paddle-wheel, under various conditions of engine power, and experience has shown that, under circumstances which admit of the screw propeller being favorably applied, it is equal to the paddle-wheel as an effective means of applying engine power to the propulsion of the vessel." Again:
I recently addressed to Mr. Atherton the following question: "Taking two ships of the same size, displacement, and power, or coal, the one a side-wheel, the other screw: What will be their relative speed and carrying capacity in smooth water? What in a sea-way, or in regular transatlantic navigation?" He replied under address, "Woolwich Royal Dock Yard, 14 Sept., 1857:
"It is my opinion, based on experiment, that a well-applied screw is quite equal to the paddle-wheel for giving out the power by which it is itself driven, that is, in smooth water. I can not say from observation or experience what is the comparative operation at sea."
I addressed the same inquiry to Mr. Robert Murray, of Southampton, who has written an able work, entitled, "The Marine Engine," and who is considered excellent authority, and have from him the following reply, dated Southampton, 19 Sept., 1857:
"With regard to the relative efficiency of the paddle-wheel and screw for full-powered mail steamers, I am disposed to prefer the paddle-wheel for transatlantic steaming, in which the vessel has to contend with so much rough weather and heavy sea, and the screw for the Mediterranean and the Pacific routes.
"For auxiliary steamers of any kind the screw has manifestly the advantage.