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 propeller-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.
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.