The crank-shaft _B_ revolves in two bearings, _D D_. The flywheel is held to the crank-shaft by means of a set-screw _S_.
Most small engines with a bore under one inch will operate nicely on from 20 to 30 pounds of steam, and this pressure can easily be generated in the boiler that was described in the chapter on model-boat power plants.
AS many of the readers probably know, a dry-dock is used in assisting disabled vessels. Some dry-docks are permanent, while others are built so that they can be floated or towed to a disabled vessel that is not able to get to a land dry-dock. The land dry-dock operates as follows. It is first filled with water, and the disabled boat is towed in by tugs. After the tugs leave, the gates are closed, and the water in the dry-dock is pumped out, leaving the boat high and dry. Large props are put in place to prevent the boat from tipping.
The dry-dock here described is a model that is towed to a disabled vessel. It is then sunk until it passes under the boat. The sinking is brought about by filling the dry-dock with water. After it has sunk to the proper depth it is passed under the boat to be repaired, the water is pumped out, and the dry-dock rises, lifting the disabled boat with it. Repairs can then be made very easily.
The model here described does not possess all the fittings and additions of the original. However, it is able to rise or sink as required, carrying the machinery necessary to bring about these functions.
A general view of the completed model is shown in Fig. 120. The first part to construct is the framework for the hull. Four pieces of wood will be required for this, and they should be cut to the shape and size shown in Fig. 121. To make this it is best to cut the two side parts first, as indicated by the dotted lines. This done, the bottom piece can be clamped on from behind by means of pieces of lath. These are for the two end pieces. The other two pieces are made in the same way, except that they contain holes for the water to pass through, as shown at _B_. The wood for these frames, or ribs, should be not less than 1/4 inch thick in order to accommodate the pieces used in the construction of the remainder of the hull.