ropeller-shaft is arranged within a brass tube, as shown at Fig.
38. The brass tube should be about 1/8 inch larger in diameter than the
propeller-shaft. A little brass bushing must also be arranged at each
end, as shown. When the propeller-shaft is mounted in place in the
tube, there will be a space between it and the tube. Before the
propeller-shaft is put in place it is well smeared with vaseline, and
when it is placed in the tube the space between the shaft and the tube
will be completely filled with it. This will prevent water from
entering. Owing to the fact that vaseline is a soft, greasy substance,
it will not prevent the rotation of the propeller-shaft. The brass tube
is placed through a hole bored in the hull of the boat. The hole should
be a trifle smaller than the diameter of the brass tube, so that the
tube can be forced into the hole.

[Illustration: FIG. 39]

[Illustration: FIG. 40]

[Illustration: FIG. 42]

One of the simplest methods of propelling a boat is by means of rubber
bands. Such a boat is shown in Fig. 39. This is a small wooden hull
fitted with a two-blade propeller. The propeller is shown at Fig. 40. It
is cut in a single piece and held to the propeller-shaft merely by a
drop of solder since there will not be much strain upon it owing to the
low power of the rubber-band motor. The opposite end of the
propeller-shaft is bent into a hook, and the rubber bands run from this
to another hook placed at the bow of the boat. The rubber bands may be
similar to those employed by model airplane builders. The motor, of
course, must be wound up by turning the propeller around until the bands
become twisted into little knots, as shown at Fig. 39. Boats driven by
rubber bands cannot be very large unless a great number of rubber bands
are used. Even then the power is short-lived. However, building a few
small boats driven by rubber-band motors will do much to teach the
young boat-builder some valuable lessons in boat construction.

Probably the best method of propelling model boats is the electric
method. By building a boat large enough to accommodate two dry batteries
or a small storage battery and a little power motor, a very reliable
method of propulsion is made possible. The boat must have sufficient
displacement to accommodate the weight of the dry-cells and storage
battery. A boat two feet long, with a beam of 4-1/2 inches, is large
enough to accommodate one dry-cell and a small motor, providing t