479. Q.—­Are there several lengths of screw shaft?

A.—­There are.

480. Q.—­How then are these secured to one another?

A.—­The best mode of securing the several lengths of shaft together is by forging the shafts with flanges at the ends, which are connected together by bolts, say six strong bolts in each, accurately fitted to the holes.

[Illustration:  Fig 44.  End of the Screw Shaft of Correo, showing the mode of receiving the Thrust.  A, discs; B, tightening wedge.]

481. Q.—­How is the thrust of the shaft usually received?

A.—­In some cases it is received on a number of metal discs set in a box containing oil; and should one of these discs stick fast from friction, the others will be free to revolve.  This arrangement, which is represented in fig. 44, is used pretty extensively and answers the purpose perfectly.  It is of course necessary that the box in which the discs A are set, shall be strong enough to withstand the thrust which the screw occasions.  Another arrangement still more generally used, is that represented in figs. 55 and 56, p. 331.  It is a good practice to make the thrust plummer block with a very long sole in the direction of the shaft, so as to obviate any risk of canting or springing forward when the strain is applied, as such a circumstance, if occurring even to a slight extent, would be very likely to cause the bearing to heat.

482. Q.—­Are there not arrangements existing in some vessels for enabling the screw to be lifted out of the water while the vessel is at sea?

A.—­There are; but such arrangements are not usual in merchant vessels.  In one form of apparatus the screw is set on a short shaft in the middle of a sliding frame, which can be raised or lowered in grooves like a window and the screw shaft within the ship can be protruded or withdrawn by appropriate mechanism, so as to engage or leave free this short shaft as may be required.  When the screw has to be lifted, the screw shaft is drawn into the vessel, leaving the short shaft free to be raised up by the sliding frame, and the frame is raised by long screws turned round by a winch purchase on deck.  A chain or rope, however, is better for the purpose of raising this frame, than long screws; but the frame should in such case be provided with pall catches like those of a windlass, which, if the rope should break, will prevent the screw from falling.

DETAILS OF THE PADDLES AND PADDLE SHAFT.

483. Q.—­What are the most important details of the construction of paddle wheels?