MARINE ENGINES.
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view; in the end view they are bent backwards. This form pre-sents a number of blades of small area, and it gives equal satisfac-tion in other arrangements with two or more blades of greatersurface in each; but it is obvious that it is not nearly so strong asthe ordinary mode of construction, and therefore more liable to get
damaged.
We now notice a form of pro-peller which has been exten-sively used both in the royalnavy and the mercantile marine.The boss is of a ball shape,and may be considered damageproof; the blades have a curvedshape on the sides, and theiro-reatest breadth is at the centre
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Fig. 365.—Boss for a Six-bladed FeatheringPropeller.
a, Boss, b, Blade. c, Key. d, Wedgepieces. e e, Plates.
Fig. 366.—Four-bladed Propeller cast onthe Boss.
a, Boss, b, Blade, c, Taper on shaft.D, Nut with split pin.
or nearly so; the points and that portion cast on the boss aremade narrow; they are bent forward at the points, by whicha better grip of the water is obtained. This form of propellerhas generally either three or four blades, all cast along with theboss. The success which has attended it has produced various