haft should be turned so as to
afford a very long bearing which will prevent rapid wear.

478. _Q._--How is the hole formed in the deadwood of the ship in which the
screw works?

_A._--A great frame of malleable iron, the size of the hole, is first set
up, and the plating of the ship is brought to the edge of this hole, and is
riveted through the frame. It is important to secure this frame very firmly
to the rest of the ship, with which view it is advisable to form a great
palm, like the palm of a vice, on its inner superior corner, which,
projecting into the ship, may be secured by breast-hook plates to the
sides, whereby the strain which the screw causes will be distributed over
the stern, instead of being concentrated on the rivets of the frame.

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 fr