Fig. 185.—Kingpost Attachment
Fig. 186.—Detail of Twin Hooks
Fig. 187.—Cross Member Joint
Fig. 188.—End View of Screw
Attention may now be given to the propellers. As twin screws are used, they may be made of fairly long pitch, since their torques will be opposite and consequently balanced. [Fig. 189] gives a perspective view of the propeller block marked out ready for carving, and [Fig. 188] shows the screw in end elevation. The screw is first carved as a true helix, and then shaped up to the form shown in the sectional view ([Fig. 182]). American whitewood should be used for the blocks, or, failing this, poplar would do. Circular tin discs are pinned to each side of the propeller boss, to which the shaft is soldered. Great care is essential to ensure that both propellers are of the same weight, and that each is poised; that is, assumes an angle of 180° when balanced on a shaft. They may be finished with a coat of gold size and one of varnish. Each gear is driven by six strands of ¼-in. strip elastic, well lubricated with soft soap and glycerine.
Fig. 189.—Propeller Block before Carving