At this juncture the author would caution the more inexperienced young mechanics not to build double-acting engines. The valve mechanism is somewhat intricate and very difficult to regulate. The construction is also much more complicated, and this also holds true of the designing.

On the other hand, single-acting engines, while not so powerful for a given size, will do very nicely in driving model boats, and will deliver sufficient power for all ordinary purposes.

[Ill.u.s.tration: FIG. 119]

Your attention is directed to Fig. 119. This shows a design for a model single-cylinder, single-acting steam-engine. The reader should carefully study each drawing before continuing to digest the following matter. The cylinder _L_ can be made from a piece of tubing. This can be either bra.s.s or copper. Aluminum should not be used, owing to the fact that it is difficult to solder and difficult to work with. The piston is made so that it will fit nicely into the cylinder and move up and down without binding. It will be seen that a groove, _M_, is cut around the piston near the top. String soaked in oil is placed in this groove. This is called packing, and the presence of this packing prevents steam leakage between the piston and the cylinder walls and thereby materially increases the efficiency of the engine.

In this case the connecting-rod _R_ is made in a circular piece. It is attached to the piston by a pin, _F_. The connecting-rod must be free to revolve upon this pin. The engine shown has a stroke of 7/8 inch.

Therefore, the crank-pin _K_ on the crank-disk _N_ must be placed 1/2 of 7/8 or 7/16 inch from the center of the disk _N_, so that when this disk makes one revolution, the piston will move 7/8 inch in the cycle. Thus it will be seen that the distance of the crank-pin _K_ from the center of the crank disk _N_ will depend entirely upon the stroke of the engine. It may be well to mention here that the worker should always start designing his engine by first determining the bore and stroke.

Everything depends upon these two factors. It is also well to mention here that the piston should never travel completely to the top of the cylinder--a small s.p.a.ce must always be left for the steam to expand.

One eighth of an inch is plenty of s.p.a.ce to leave.

It will be noticed that the valve mechanisms on the particular engine shown bear no resemblance to the _D_ valve previously described. The holes _G_ which are bored around the cylinder are the exhaust ports. It will be seen that when the piston is at the end of its downward stroke it uncovers these exhaust ports and permits the steam to escape. The momentum of the flywheel _A_ pushes the piston upward, closing these holes. As these holes are closed the valve _H_ uncovers the entrance _I_ and permits steam to enter from the boiler through _J_. By the time the piston has reached the upward limit of its stroke a considerable steam pressure has developed on top of the cylinder, and this again forces the piston downward. Thus the same cycle of movement is gone through repeatedly.

The valve on this little engine is extremely simple. It consists of a circular piece of bra.s.s drilled out, as shown. A hole (_I_ and _J_) is drilled transversely through this. The little cylinder shown in the insert at _O_ slides in the larger hole, and when it is at its upper limit it cuts off the steam. At the proper intervals the valve is pulled down by the eccentric _C_. It will be seen that the moving parts, i.e., the valve and the piston, must be properly timed. That is, the eccentric _C_ must be mounted on the crank-shaft _B_ so that the valve will close and open at proper intervals. When the engine is made, the eccentric can be shifted about by means of a set-screw, _Q_, until the engine operates satisfactorily. This set-screw is used to hold the eccentric to the crank-shaft. The word eccentric merely means "off center." Thus the eccentric in this case is formed by a little disk of bra.s.s with the hole drilled off center. The distances these holes are placed off center will depend entirely upon the motion of the valve. It will be seen that the valve is connected to the eccentric by means of the valve-rod _E_. The valve-rod, in turn, is held to a circular strap which is placed around the eccentric. A groove should be cut in the surface of the eccentric, so that this strap will not slip off. If the strap is not put on too tightly and the eccentric is free to revolve within it, the valve will be forced up and down as the eccentric revolves.

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.

CHAPTER XI

A MODEL FLOATING DRY-DOCK

AS many of the readers probably know, a dry-dock is used in a.s.sisting 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 pa.s.ses 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 pa.s.sed 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.

[Ill.u.s.tration: FIG. 120]

[Ill.u.s.tration: FIG. 121]

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 pa.s.s 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.

When the builder has made the four ribs, he should proceed to construct the lower deck, which consists of a single piece of wood nicely planed and finished, measuring 14-1/2 inches long by 8 inches wide and 1/8 inch thick. This piece must be nailed to the bottom of each of the ribs, one at each end, and the other two containing the holes at equal distances apart. Tiny nails, similar to those used on cigar-boxes, will be found very suitable for this work. Some old cigar-boxes may be broken apart to obtain the nails for this purpose. Before nailing on the board it should be marked out to present ordinary deck-boards. The reader is referred back to Chapter 9 which describes this process, using a straight-edge and knife.

When this board is nailed in place, the next requirement will be two pieces for the sides the bottom edges, of which must rest on the top of the deck-board. These boards are the same length as the deck. They should reach to the top of the ribs, and be fastened in the same way as the bottom deck. It is good practice, when doing this, to place a little white lead on the bottom edge before finally driving the nails in place.

This will tend to produce a water-tight joint. This done, three pieces of wood 5/8 inch square must be screwed in place, flush with the bottom ends of the ribs, to form a flat keel. They should be firmly fixed since a lead keel is afterward screwed on the bottom of the boat. Attention should now be directed to fitting the two middle decks. These are placed 4 inches from the top and are 4 inches wide. In this s.p.a.ce the engine and pumps are placed. Therefore, the top deck is made in the form of a lid, and the outside plate made to draw out. In this way the mechanism below the deck can be made very accessible.

The framework of the dry-dock is now completed, and the builder can proceed to fix on the side plates. These are made from sheet tin with a width of 14-1/2 inches. The length must be sufficient to reach from the top of one side, around the bottom of the hull, to the top of the other side. Having cut the tin to the required size, one side is put in place with small nails, s.p.a.cing them an equal distance apart.

Before securing the opposite side, the builder must first arrange the inlet-valve. This particular member is constructed as follows. First, obtain an old gas-pipe union which measures about 5/8 inch in diameter and 3/4 inch long. With a hacksaw this is cut off in a sloping direction with an angle to correspond with the slope in the bottom of the dry-dock. When this is done, a lid must be fitted to the top by means of a long rod, as clearly shown in Fig. 122. On the under side of this lid a small piece of sheet rubber should be glued, so that when the lid is screwed down the valve will be made water-tight. The valve must now be soldered to the inside of the hull. It is placed in such a position that it will rest just under the center of one of the upper decks when the controlling rod is upright.

[Ill.u.s.tration: FIG. 122]

The top end of the rod must contain a thread for about 1 inch, and a round plate made to screw on. This plate should be about 3/4 inch in diameter, and contain three small holes around the edge. These holes are used in fastening the plate to the deck. The top of the rod is fitted with a small crank-handle, which is used in turning the rod in either direction. In this way the valve can be either opened or closed. At the bottom of the rod a small swivel should be provided, as indicated in Fig. 122.

The plate or sheet of tin on this side of the hull can now be permanently fixed in place. When this is done a light hammer should be used around the edges to turn the tin into the wood.

With the plates secured in place, the builder must next fix a flat wood keel along the bottom of the dry-dock. This should be screwed to the inside keel, screws pa.s.sing through the tin plate. A lead keel is then screwed to the wooden keel, and when this is done the dry-dock can be launched. If the foregoing instructions have been carried out carefully the dry-dock should ride lightly on the water.

As a trial the inlet-valve is now unscrewed and water is permitted to enter the hull. When the water rushes in, the hull will begin to sink.

The water should be allowed to enter until the hull sinks to within an inch of the lower or inside deck. The valve should then be closed. The exact position of the water should now be found, and a line drawn all around the hull, which can afterward be painted in.

The engine and boilers must now be constructed and placed on the dry-dock, so that the water that was permitted to enter may be pumped out. As a temporary arrangement, a thin rubber tubing is inserted through a hole in the lower deck and allowed to hang outside the water-level. The siphon can then be formed by simply drawing the water up by suction with the lips. A continuous flow will result, emptying the hull within a short time.

[Ill.u.s.tration: FIG. 123]

Attention is now directed to the construction of the boiler and pumps.

The boiler, which is rectangular in shape, is made of thin sheet copper, and measures 4 inches long by 3 inches wide by 2 inches deep. A hole is made in the top, and a bra.s.s or copper tube 6 inches long and about 3/4 inch in diameter is soldered in position, as depicted in Fig. 123. This tube acts as a chimney on the dry-dock, but it is really used for filling the boiler, and the top is supplied with a tightly fitting cork.

The ends of the boiler also act as supports, and they are made 4 inches long. The bottom edge is turned up for about 1/4 inch to enable the boiler to be screwed firmly to the lower deck. The boiler occupies a position at one end of the hull, and should fit easily in between decks.

A small spirit-lamp is used to furnish heat, and no description need be given of this particular part of the equipment. Before the boiler is firmly fixed in place a small hole should be made near the top at one end. The feed steam-pipe is inserted in this, and soldered in place.

Two small oscillating cylinders, similar to those made for the engine on the _Nancy Lee_ (Chapter 6), should be made. They should not be more than 3/4 inch in length, with a 3/8-inch bore. If the builder has any old model steam-engines in the shop, he may take the cylinders from them instead of constructing new ones for the dry-dock.

The engine is set up as shown in Fig. 124. The first job is to make the frame or standards, and this is in one piece. Two pieces of bra.s.s (_A_), measuring 5-1/2 inches long by 1/2 inch wide and 1/16 inch in thickness, are cut. Next the builder should mark off 1-1/2 inches from either end, and carefully bend at right angles, after which holes are drilled to accommodate the crank-axle _B_. Two holes must also be made for screws to enable the machine to be screwed to the deck.

[Ill.u.s.tration: FIG. 124]

[Ill.u.s.tration: FIG. 125]

The flywheel should be 1-1/2 inches in diameter, while the bent crank has a throw of 3/16 inch. The steam-cylinder is fixed on the outside of one of the uprights, and the steam-pipe must, of course, be fitted from the inside.

The pump-cylinder is composed of a small piece of bra.s.s tube 1 inch long and 3/8 inch in diameter. The plunger is 1/2 inch long, and the diameter is just sufficient to enable it to work freely up and down inside the bra.s.s tube. One end is shaped as shown in Fig. 125. This contains a saw cut that enables the pump-rod to be placed between and connected with a pin. The bottom end of the cylinder is now fitted with a bra.s.s disk in which a hole is made and a 3/32-inch tube soldered in place. The inside surface of this piece of bra.s.s should be countersunk, and the piece is then soldered into the end of the cylinder. Before the plunger is inserted a small lead shot is dropped in, which should be larger than the hole at the bottom of the cylinder, thereby covering it. A hole is drilled in at the side of the cylinder, and a small bent pipe fixed in it. At the top of this pipe a short piece of 3/8-inch bra.s.s tube is fixed in place, as indicated. This piece of tubing is closed at both ends. The bottom end is treated like that of the pump-barrel and supplied with a large shot. An outlet-pipe is soldered into the side of the delivery-valve chamber and leads to the side of the hull.

The pump _E_ is fixed at the bottom midway between the engine uprights as indicated in Fig. 124. The suction-pipe pa.s.ses through a hole and down through the deck nearly to the bottom of the hull. After the engine and boiler are connected, a trial can be made. If the foregoing instructions have been carried out, the engine will run at a good speed and a continuous flow of water will be pumped out of the hull. All parts of the engine and pump should be carefully oiled and water should be poured into the pump in order to prime it before its start.

It is understood that two complete boilers and pump units are made for the model, and one is mounted on each side. If the builder desires to increase the capacity of the pumps and install a more powerful boiler and engine, only one pump will be necessary. Otherwise the water will not be pumped from the hull very rapidly.

When the builder has finished the pump units, he should turn his attention to the remainder of the fittings. Two small cranes are made, and one is placed at each side of the hull. They are made of tin. The cab of each crane measures 2-1/2 inches high by 2 inches long by 1-3/4 inches wide. A small roof is fitted on, and a piece of wood fitted to the bottom to serve as a floor. The jib measures 6 inches long by 3/4 inch at the base, and tapers to 1/2 inch. It has 1/4 inch turned down at each side, thus adding considerable strength. The jib is fitted to the cab by means of a wire pa.s.sed through the sides, and two guy-ropes are arranged as shown. A small piece is now cut out at the top, and a pulley wheel fixed in position by means of a pin pa.s.sed through the sides.

[Ill.u.s.tration: FIG. 126]

The winding-drum can be made of either tin or wood. The axle pa.s.ses through both sides of the cab, the crank being attached to the outside.

Fig. 126 shows the completed crane, which is held to the deck by means of a small bolt and nut. A washer should be placed between the bottom of the crane and the deck, to allow the crane to turn freely with little friction.

A hand-rail, made of fine bra.s.s wire, is placed around the deck.

Dummy port-holes are fixed to the sides of the dry-dock for the purpose of lighting up the interior of the engine-room. These are furnished from top rings taken from gas-mantles. Anchor-chains are fixed at each end of the dry-dock. The whole dry-dock is painted with two coats of gray paint and the water-line painted in bright red.