are 1/32 inch apart, then the screw will move
1/32 inch every time it revolves.

If a propeller acts in the same way as a screw, then it too must have a
pitch. The pitch, or the distance that a propeller will advance in one
revolution, is measured in inches. A propeller with a pitch of ten
inches should move ten inches through the water at each revolution.
However, there is a certain amount of "slip," and a propeller does not
actually advance the distance that it should theoretically. The pitch of
a propeller is really the distance it would advance in one revolution
if it were revolving in an unyielding or solid substance.

To make a simple propeller, first cut out of thin sheet brass three
blades as shown at _A_, Fig. 37. Sheet brass with a thickness of 1/32
inch is very suitable for this purpose. Next, a block, as shown at _B_,
is carefully carved out so that the propeller can be hammered down into
the depression. The same block is used for the three blades, so that
each will have the same curvature. The block should be cut from oak,
since this wood will not split or lose its shape when the forming is
done.

The hub is made next. This is shown at _C_, Fig. 37. The hub, of brass,
is made according to the stream-line method. It is filed to shape from a
piece of round brass stock. A hole runs lengthwise in the brass, as
shown, and a set-screw is used to hold the hub of the propeller-shaft.
The method of cutting the slots in the hub is shown at _D_, Fig. 37. The
hub is clamped between two boards placed in the vise, and a hacksaw is
used to cut a slot in the hub. The hub is then turned around one third
of a revolution, and another slot cut, using the same saw-marks in the
boards, so that the angle of the second slot will be the same as the
first one. The third slot is cut in the same manner. The three blades
that were cut out are now fastened in these slots and held there by
solder. This completes the propeller and it is now ready to be fastened
upon the propeller-shaft.

Let us consider the general method of putting the propeller-shaft in
place. The young boat-builder will readily understand that it would be
very impractical merely to bore a hole in the hull of the boat to put
the propeller-shaft through. In this way water would surely leak into
the hull and the boat would sink in a short time. Some method must be
evolved to keep the water out of the hull, and yet allow the
propeller-shaft to revolve freely.

The p