MARINE ENGINES.

479

the ship if desired, bearing in mind that the power required variesas the cube of the speed. The question of relation between powerand speed has received recently much attention through progressivespeed trials carried out by Mr. William Denny of Dumbarton, who,as to the usual formulae or rules, says:—“The common fault is thesupposition that the power required to produce certain speeds variesas the cubes of these speeds. This is a very fallacious theory, andone which experience does not bear out.” See page 513 for an in-vestigation of this question.

In the moulding shop a nearly uniform plan of constructing themoulds is adopted for all the forms of screw propeller. In theearly screws the boss was round, tapering from the front to the backto suit the taper turned on the screw shaft, the thickness of theboss being greater at the front, with a reduced thickness at the endof the shaft. The blades are formed or struck up by means of aboard, revolving round a central spindle, placed vertically in thecentre of the mould for the boss. An inclined plane is formed ofwrought-iron plating to suit the pitch required, and this plate isbent round to the true circumference of the outer diameter of theblades; in this way the mould is formed of loam by moving roundthe board on the edge of the wrought-iron plating, care being takenthat the board always moves upwards on the spindle in a levelmanner; the bottom edge of the board then scrapes the loam, whichis built up on a large cast-iron plate, and the true flat form of thescrew’s disc is described. It is then necessary to divide the outercircumference of the blades, which is generally one-sixth of thetotal circumference, or the radius of the blade from the centre tothe point, the sides or edges of the blade radiating from thosepoints on the circumference to the centre. The thickness of theblades at the point and root is then laid on, the corners of the bladeat the circumferential line are rounded off, and the mould finallyfinished for casting. With this form of screw propeller, a greatportion of the central part of the blades strikes the water nearlyflat, merely disturbing the water, and throwing it off at right anglesto the path of the vessel, causing much Vibration without producingany useful effect. It was also found that the greatest effect wasproduced by cutting down the blades at the circumferential line, andforming the edges curved, having the greatest breadth nearer thecentre, instead of on the extreme circumferential line of configuration.

This effect of Vibration in the earlier screws was got rid of in a