IMPROVED SCREW PROPELLER.
While the last few years have seen great advances made in the designs of steamships and of their engines, little or nothing has been done in the way of improving the screw propeller. As a general rule it would appear to be taken for granted that no radical improvement could be made in the form of the propeller, although various metals have been introduced in its manufacture with the view of increasing its efficiency. For sea-going steamers, however, the shape remains the same, the variation chiefly relating to the number of blades employed. A striking departure from ordinary practice, however, has of late been made by Mr. B. Dickinson, who has invented a screw propeller which, on practical trial, has given an efficiency far in advance of the ordinary screw. This new propeller we illustrate here in Figs. C and D, while Fig. A shows an ordinary propeller. The Dickinson propeller illustrated has six blades, giving a surface of 30 square feet; it is right handed, and has pitch of 15 ft. and a diameter of 10 ft. 6 in. The ordinary screw propeller shown at Fig. A is right handed and two bladed, with a pitch at the boss of 13 ft. 6 in. and at the tip of 15 ft. It has a diameter of 10 ft. 9 in. and 32 square ft. of surface. The projected area looking forward is 22 square ft. and the projected area looking athwartship 22.84 square feet. The most graphic way of illustrating the principle of Mr. Dickinson's propeller is to take a two bladed propeller of the ordinary type as shown at Fig. A in the annexed cuts, and divide into three sections as in Fig. B, then move section No. 1 to the line position on the shaft of No. 3, and No. 3 to that of No. 1, No. 2 remaining stationary. The effect of this interchange will be that (having regard to the circle of rotation) No. 3, the rearmost section, will rotate in advance of No. 2, and No. 2 in advance of No. 1 (see Fig. C). By this arrangement the water operated on escapes freely astern from every blade—that from No. 1 passing in the wake of No. 2, while that from Nos. 2 and 1 passes in the wake of No. 3. Fig. D represents the blades with a wider spread as practically used. The advantages claimed by Mr. Dickinson for his propeller, and which are sufficiently important to be given in detail, are:
1. That the blades of each section, when the vessel is in motion, necessarily cut solid, undisturbed water, each blade operating upon precisely the same quantity of water as an individual broad blade would do, though, of course, it parts with it in one-third of the time.
2. That each sectional blade exerts the equivalent efficiency of the first or entering third portion of the breadth of an ordinary propeller blade, and that consequently the combined sections have greater effective power. It is now regarded by experts as an ascertained fact that the after or trailing portion of the broad blade is relatively non-effective as compared with the forward or entering portion.
3. When three blades are fitted, the spent water from No. 2 being delivered immediately in the wake of No. 3, and that from No. 1 in the wake of No. 2, has the effect of destroying or reducing to a minimum the back draught of sections Nos. 2 and 3, No. 1 alone being subject to this drawback. This is of greater importance than might at first thought appear, as in cases where there are three or four blades revolving in one plane, the water is drawn after the retreating blade, lessening the resistance to the face of the advancing one.
4. That by the subdivision of the blades, as arranged spirally, the water passing through within the radius of the propeller has its resisting capacity more thoroughly worked out than is possible with any propeller whose blades are all on the same plane. This view is confirmed by the visibly increased rotation of the water in the wake of the vessel.
5. That by broadening the blades or increasing the number of sections, the diameter of the propeller may be proportionately diminished without the sacrifice of engine power. This is often desirable with vessels of light draught, the complete immersion of the screw being at all times necessary to avoid waste of power.
6. The propeller being made and fitted on the shaft in sections, all that is necessary in case of accident is to replace the broken section. This in many cases could be done afloat.