320 DETAILS OP SCREW ENGINES.

leading corners are curved with a larger radiithan those trailing, and thus the vibration isduly lessened. The shaft passes through theboss, and the latter is secured by keys laterallyand longitudinally, also by a nut at theextremity. The stern tube is shown, fittedwith lignum-vitae strips, to prevent heating ;but in most instances gun-metal bearings areused with examples of ordinary size. Suchwas the practice of Messrs. Watt for the pro-peller shafts of theengines shown by Plate 23;and, excepting the lignum-vitse strips, Fig. 193is a truthful illustration of one of the propellersadopted. Messrs. Dudgeon also prefer a simi-lar design for the propellers of their engines,and many other firms follow the same practice.The actual dimensions of the example illus-trated are : diameter of screw 7 feet 6 inches,pitch of screw 11 feet 6 inches, and length ofblade on line of keel 1 foot 10 inches; thediameter of the shaft being 7<]- inches, and thelength of the boss of the propeller, 2 feet

5 inches.

Messrs. Maudslay, when adopting thecommon two-bladed propeller, secure eachblade to the boss by studs and nuts, ratherthan cast the boss and blades entire, even withscrews 11 feet in diameter. As a conclusionto the remarks on common screws, it maybe added that the Fig. 193 is also a truthfulillustration of a two-bladed screw by an imagi-nary opposite location for two of the blades,and the omission of the third.

The correct pitch of the helix of the blade atits extremity is a matter of the highest import-ance, and no one versed in the subject hasillustrated his ideas in practice better thanMr. Griffiths, the inventor of the propeller

bearing his name. As experimental evidenceis of certain value, the inventor preferred toinsert the blades of the screw in the “ boss,”regulate the angle or pitch by suitablemeans, and thus be enabled to determine thegreatest effect attainable. He also curvedthe blades forward, to grasp the water onentering it, and thereby gained a proportionateadvantage.

Now, setting aside the form of the blade,the means of regulating the pitch of the screwhas been duly considered by Messrs. Maudslayalso, by the adoption of gearing, levers, <fcc.,with the advantage of using the contrivancewhen the propeller was revolving, if desirable.

The Griffiths propeller has been adoptedfor some time by many firms, and each makerhas endeavoured to simplify the connection ofthe root of the blades with the boss, whichis the main attainment. The arrangement of• the connection, however, greatly depends onthe fact whether “ the propeller is to be liftedfrom, or fixed on, the shaft,” which will beunderstood from the following descriptionand illustrations.

The first example to be alluded to is illus-trated by Fig. 194—page 321—being of recentconstruction by Messrs. Ravenhill for H.M.S.“Lord Clyde,” the engines of which are illus-trated by Plate 33. The boss is hollow, witha tube at the centre, through which the shaftpasses. The connection is made secure byside keys, the position of each being shown idthe elevations. The roots of the blades areflanged and secured to the boss by studs andnuts, one of which is shown in the sectionalelevation, and the total number for one bladein the plan. To prevent the nuts becomiDg