It also follows that if the power wasted in vibrating the ship was,




Fig. 42. — Oval Blades of Equal Surface for Different Diameters.
as it could be, applied to propulsion, the coal consumption or speed

would thereby be improved.
The following facts were at the time vouched for by reliable

witnesses : —

knots. knots.
The " Louise " (German Navy), Griffiths screw, 1337 ; Hirsch, 14*07

S.S. " Herder " (Mercantile),

S.S. "L'Isere," „ „

S.S. " Louisane," „

S.S. "Pereire,"

S.S. "Conrad,"

Consumption of coal per 100 miles.
S.S. "L'lsere," Griffiths, 4*55 tons ; Hirsch, 4*03 tons.
S.S. "Conrad," „ 1118 „ „ 10*28 „

S.S. " Louisane," „ 23*32 „ „ 2077 „

The ordinary mercantile cast iron screw for cargo steamers is
made as shown in fig. 41, with the boss and blades cast in one piece.
The centre line of the blade is sometimes as shown, and sometimes the



11-48


'1


12-38


7*985


n


8-99


10*05


>i


11-10


14-459


))


15-459


9*595


n


10-65



138



MARINE PROPELLERS.
blades are " thrown aft " by the other means described in Chapter VIII.
Sometimes the screw is made with the face square with the shaft,
with only a slight curvature near the tip.
Fig. 42 shows the contour of the blades of four screws differing




Fig. 43. — Modern Bronze Naval Screw.
largely in diameter but having the same area of acting blade surface,
the blade shapes in each case being elliptical. The surface taken for
purposes of comparison is 36 square feet, and the diameters range
from 9 feet to 12 feet. The pitch of each is 15 feet, and the calcula-
tion is based on the revolutions being also the same, viz. 100 per



VARIOUS FORMS OF SCREW PROPELLER.
