COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS.

592. Q.—­What species of screw do you consider the best?

A.—­In cases in which a large diameter of screw can be employed, the ordinary screw or helix with two blades seems to be as effective as any other, and it is the most easily constructed.  If, however, the screw is restricted in diameter, or if the vessel is required to tow, or will have to encounter habitually strong head winds, it will be preferable to employ a screw with an increasing pitch, and also of such other configuration that it will recover from the water some portion of the power that has been expended in slip.

593. Q.—­How can this be done?

A.—­There are screws which are intended to accomplish, this object already in actual use.  When there is much slip a centrifugal velocity is given to the water, and the screw, indeed, if the engine be set on when the vessel is at rest, acts very much as a centrifugal fan would do if placed in the same situation.  The water projected outward by the centrifugal force escapes in the line of least resistance, which is to the surface; and if there be a high column of water over the screw, or, in other words, if the screw is deeply immersed, then the centrifugal action is resisted to a greater extent, and there will be less slip produced.  The easiest expedient, therefore, for obviating loss by slip is to sink the screw deeply in the water; but as there are obvious limits to the application of this remedy, the next best device is to recover and render available for propulsion some part of the power which has been expended in giving motion to the water.  One device for doing this consists in placing the screw well forward in the dead wood, so that it shall be overhung by the stern of the ship.  The water forced upward by the centrifugal action of the screw will, by impinging on the overhanging stern, press the vessel forward in the water, just in the same way as is done by the wind when acting on an oblique sail.  I believe, the two revolving vanes without any twist or obliquity on them at all, would propel a vessel if set well forward in the dead wood or beneath the bottom, merely by the ascent of the water up the inclined plane of the vessel’s run; and, at all events, a screw so placed would, in my judgment, aid materially in propelling the vessel when her progress was resisted by head winds.

594. Q.—­But you said there are some kinds of screws which profess to accomplish this?

[Illustration:  Fig. 49.  THE EARL OF DUNDONALD’S PROPELLER.]