The mechanism by which the emersion of the screw is effected is as follows:—There are two stern-posts; between these, and connecting them with each other and with the keel, is a massive metallic frame, in which rests another frame, or châssis, in which the screw is suspended; near the water-line, the deck and wales are extended to the after stern-post, and through an opening or trunk in this overhanging stern the frame suspending the screw is raised by worms, working in a rack secured to the frame, and operated from the deck, as shown in the accompanying drawing,—or by a tackle, as is now most common. In the British ship Agamemnon, of ninety guns, the propeller is raised by a hydrostatic pump,—a neat arrangement, but liable to get out of order. When it is desirable to raise the propeller, the blades are first placed in a vertical position, and the operation of lifting is performed in a few minutes.

The relative advantages of the propeller fitted to lift, and that which is permanently fixed, have long been the subject of much discussion.

For merchant steamers, having an established route to perform, on which the aid of steam is in constant demand, it is generally conceded that the position of the screw should be permanent. The construction of the ship is then less costly, while greater strength is preserved; and as these vessels are out of port but for short intervals, should repairs be needed, they have access to the docks. But for men-of-war the case is widely different. Having frequently to keep the sea for long periods, much under canvas, and often far distant from a dock-yard, they should be provided with the means of lifting the screw to repair or to clear it, or to be relieved from the impediment it offers to sailing and to evolution, and also from the injurious "shake" occasioned by a dragging propeller.

[Illustration: MODE OF LIFTING SCREW.]

On the other hand, the construction of a trunk or well impairs the solidity of the stern, renders it much more vulnerable, and weakens its defences, while it opposes to speed the very considerable resistance of the after stern-post.[*] Nevertheless, no modern ship of the British navy is without the means of raising her propeller, and the best opinion of commanders and engineers of that service, of longest experience in screw-ships, goes to establish the conviction, that, for men-of-war, the advantages of being able to lift the propeller far more than outweigh the objections urged against lifting. In this connection we mention the fact, that all screw-ships "by the wind" have a strong tendency to gripe. Would not this be obviated by having a gate or slide to fill out the dead-wood when the screw is lifted?

[Footnote *: Might not a metallic stern-post, combining strength, lightness, and little resistance, be introduced?]

The best illustration of the effects of a dragging propeller was afforded on the departure of a Russian squadron from Cronstadt, bound to the Amoor, in 1857-'58, consisting of three sloops of war bark-rigged, and three three-masted schooners, under the flag of Commodore Kouznetsoff. The vessels of each class were built from the same moulds, and at the time of the experiment were of the same draft and displacement. On clearing the land, signal was made to lift screws and make sail. Soon after, all the squadron reported the execution of the order, except the Voyerada sloop, which had the misfortune to break a key in the couplings, and therefore could not lift her screw. Every effort was tried to get out the key, and meanwhile a very instructive example was presented to the squadron of the effect of a dragging propeller on the speed of the vessel. The circumstances were as follows:—The wind, a gentle breeze, right aft; the Voyerada carrying all sail but the main course; the other two sloops holding way with her with their topsails on the cap, and the schooners with their peaks dropped. Under these conditions, the Voyerada, having her screw-blades fixed horizontally, could scarcely keep her position, running two and a half and three knots. The Voyerada next succeeded in getting her screw vertical, when, without any change in the wind, the speed increased to four and a half knots. The other sloops then mastheaded their topsails, and the schooners peaked their gaffs. At length the Voyerada succeeded in lifting her screw, when immediately all the sloops under the same canvas continued their course, making six to six and a half knots. A better example of the obstruction offered by a dragging propeller could not have been afforded.[1]

The "shake," to which reference has been made, is the tremulous or vibratory motion communicated to the after-body of the ship, and particularly to the stern, by the revolution of the propeller, often opening the seams, and in old ships sometimes starting the butts and causing dangerous leaks. This movement arises from two causes,—one inherent in the screw, the other due to its position in the deadwood. The first cause is the difference in the propelling efficiency of the upper and lower blades when in any other position than horizontal. The centre of pressure of the lower blade, being at a greater depth below the surface than the centre of pressure of the upper blade, acts upon a medium of greater resistance to displacement, and the differential of the pressures of the two blades produces inevitably a vibratory motion in the stern of the vessel. This effect is greatly increased when the clearance given to the screw in the dead-wood is too small; for the reduction of the hydrostatic pressure at the stern-post, and the increase of it at the rudder-post, on each passage of the blades, must be followed by concussion. Therefore, if the "well," or distance between the posts, be made sufficiently long in proportion to the screw, the "shake" due to the latter cause can be almost entirely obviated.

In 1851, the British Admiralty selected three auxiliary screw-ships, of different classes and qualities, for an experimental cruise, namely:—

[Footnote 1: Russian Nautical Magazine, No. XLI., December, 1857.]