480

MODERN STEAM PRACTICE.

measure by making the boss spherical, and filling up a great por-tion of the central part of the blades with the ball, from which thewater glides off, while the blades cut their passage through thewater in a more direct line, instead of striking* it at right angles to the vessel’s path. This| // \ is the greatest improvement that has as yet

been introduced in the formation of the screwpropeller, and it is now universally adopted,in some cases with slight but non-essentialvariations.

Great difference of opinion exists as re-gards the number and configuration of theblades for the screw propeller. When thevessel is pitching or rather plunging in aheavy sea, a two-bladed screw cannot haveso much hold of the water as one with threeor more blades; but the two-bladed screwpossesses a great advantage in being moreeasily lifted for repair or inspection; and inaddition the “well,” as it is termed, that is cut down from the deckdoes not require to be so large. For war vessels so fitted, two-bladed screws are adopted; but for the mercantile marine threeand even six blades are beneficially used.

The common propeller is arranged for two, three, or four blades;

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Fig. 360.—Form of Blade verygenerally adopted.

A, Boss, b, Blade, c, Bladeshowing thickness of metal.

A, Boss.

Fig. 361.—Common Screw Propeller.b, Blade, c, Blade showing thickness of metal.

they are generally cast along with the boss, of an elongated orspherical shape. The side view of the blade is quite parallel andmore or less rounded at the point, and the thickness graduallytapers from the centre to the point. Some of the common forms