CHAPTER IV
STEAM AND ELECTRIC PROPULSION
BOATS are propelled by two different systems. Some inland-water boats still employ side paddle-wheels, while ocean-going vessels use the more modern propeller or screw.
The paddle-wheel really acts as a continuous oar. Such a wheel is shown in [Fig. 33] . As the wheel goes around the paddle dips into the water and pushes the boat forward. If the direction of the boat is to be reversed, the rotation of the paddle-wheels is reversed.
Before passing onto the screw, it may be well to explain just how a paddle-wheel causes a boat to move. When a man gets into a rowboat, he generally pushes himself off by placing his oar against the dock or shore and pushing on it. That is just what the paddle does in the water. It dips into the water and pushes against it. It must be remembered, however, that water is unlike a solid substance and it "gives." When a man places his oar against the bank and pushes it, the bank does not move, and all of the man's energy is used in starting the boat. Water, however, does not remain stationary when the paddles push against it, and therefore all of the power it not utilized in moving the boat—part is used in moving the water.
The paddle-wheel is not so efficient in moving a boat as the more modern propeller—or screw, as it is more often called. The screw receives its name from the ordinary metal screw, because its theory of operation is exactly the same. A wood screw, when turned, forces itself into wood. A propeller, when turned, forces itself (and thereby the boat) through the water. A small propeller is illustrated in [Fig. 34]. This is an ordinary three-blade propeller. (The writer prefers the word propeller instead of screw.)
From the drawing, it will be seen that the propeller-blades are mounted at an angle. This angle of the blades causes them to force water back as they cut through it when the propeller is revolving. This forcing of the water back tends to produce a forward motion of the propeller, and in this way the boat on which the propeller is mounted moves through the water. The propeller is caused to revolve by a steam-engine, steam-turbine, or gasolene-engine, as shown in [Fig. 35] . Longer boats have more than one propeller. A boat that has two propellers is called a twin-screw boat. A boat driven with four propellers is called a quadruple-screw boat.
When a machine screw is turned around just once, it moves forward a certain distance, as a glance at [Fig. 36] will show. The distance the screw moves forward will depend entirely upon the distance between the threads. The distance between the threads is called the pitch of the thread. If the threads 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 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.
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 41/2 inches, is large enough to accommodate one dry-cell and a small motor, providing the fittings of the boat are not too heavy.
A suitable power motor for small boats, which will run with either one or two dry-cells, is shown in [Fig. 41]. The connections for the motor are given clearly in [Fig. 42], and a suitable switch to control the motor is shown at [Fig. 43].
Owing to its greater power, the storage battery is to be preferred. Dry-cells are extremely heavy and occupy considerable space. They are also costly, since they do not last long and cannot be worked too hard unless they polarize.
A very suitable method of mounting an electric motor is illustrated in [Figs. 44] and [45]. It will be noticed that the motor is inverted. A small pinion or gear is mounted upon the armature-shaft of the motor. A larger gear (about three times the diameter of the small one) is placed upon the propeller-shaft. This gives a speed reduction of three to one. It will be seen that the propeller-tube is strapped within a strip of brass to a small cross-piece nailed to the bottom board of the hull. The hull is of the built-up type, and the other three boards that go to make it up are not shown. When the three boards are glued in place, a brass strip is run across the top board and the base of the motor is screwed to this. This holds the motor rigidly in place so that it will not move when the power is turned on. The brass strip used should have sufficient thickness to hold the motor rigid. It will also be seen that the motor is tipped slightly so that it will come in line with the propeller-shaft.
It is not always possible to obtain small gears. For this reason the model boat builder may find it necessary to use a different method of fastening the propeller-shaft to the motor. A very good method of doing this is shown in [Fig. 46]. Here a coiled wire spring is used. This is wound to shape on a rod, and a drop of solder holds it to the propeller and motor shafts. In the method of propulsion shown in [Fig. 44] the armature-shaft of the motor must be perfectly in line with the propeller-shaft, or the gears will bind and unsatisfactory operation of the motor will result. With the little spring the motor will not have to be mounted exactly in line with the shaft, and it will also be possible to mount the motor standing up. Of course, if the motor is mounted in this way it will be necessary to make the propeller-shaft longer, as is shown in [Fig. 47].
Still another method of driving the propeller is illustrated in [Fig. 48]. This method is so simple that the author feels explanation to be unnecessary.
Clockwork can often be employed for propulsion purposes, but this method is not very satisfactory. It is also very difficult to obtain suitable clockworks to install in a boat. Oftentimes it will be possible to salvage the works of an old alarm-clock, providing the main-spring is intact. It is a very easy matter to mount the clock-spring and connect it to the propeller. Any one of the aforementioned methods can be employed.
Steam propulsion has its advantages; but, on the other hand, the writer is not inclined to recommend it as strongly as the electric method for reliability. Of course, steam is a more powerful agency in the propulsion of small boats and thereby greater speed is attainable by its use.
Here is a very simple small power plant suitable for driving boats up to 31/2 feet in length. The boiler is shown in [Figs. 49] and [50]. The method of assembling the boiler is pictured clearly in [Fig. 49]. A brass or copper tube about 21/2 inches in diameter is used. Two end pieces are cut to shape and forced into the boiler ends. A hole is drilled in the center of these pieces before they are put in place. After the end pieces are forced in place solder is carefully flowed around their edges. The brass rod is then threaded at each end and placed concentrically within the boiler, as shown in [Fig. 49]. A nut is placed on each end of this rod and tightened. The nut is then soldered in place. This brass rod, called a stay-rod, prevents the end of the boiler from blowing out when the steam pressure has reached its maximum value. Three holes are drilled in the brass tube, as shown. One is to accommodate the steam feed-pipe that goes to the engine; another is for the safety-valve, and still another for the filling plug. The safety-valve and filling plug are both shown in [Fig. 51]. The little spring on the safety-valve is adjustable, so that the valve can be regulated in order to prevent it from blowing off at pressures lower than that at which the engine operates.
A suitable firebox for the boiler is shown clearly in [Fig. 52]. This is cut to shape from stovepipe iron and held together with small rivets. Holes should be punched or drilled in the side of the firebox to give the burner a sufficient supply of air. The burner is illustrated clearly in [Fig. 52]. The fuel-tank can be made from an ordinary tin can with the cover soldered on, and a hole made for a cork by means of which it is filled with denatured alcohol. A little pipe runs from the fuel-tank to the burner. It is advisable, if possible, to place a small valve in this pipe to cut off the fuel supply when necessary. The only other method of putting the burner out would be to stand it on its end. The burner consists of a rectangular tin box with a top cut out as illustrated. A piece of brass or copper gauze is placed in the top. Asbestos wool is used to fill the can, and the alcohol is drawn into the wool by capillary attraction, where it burns with a steady hot flame at the surface of the copper gauze. In the corner of the can near the feed-pipe another small piece of copper gauze is soldered as shown. This covers up the feed-pipe entrance so that the asbestos will not plug up the pipe.
The engine to be used in connection with the boiler just described is shown in [Fig. 53]. This is a very simple engine of the oscillation type, and there should be little trouble in making it. A more mechanical drawing of the engine is shown in [Fig. 54]. The details of the engine are shown in [Fig. 55].
The cylinder of the engine should be made first. This is made from a piece of brass tubing with an internal diameter of 3/4 inch. Two end pieces, or a cylinder-end cover and cylinder head, must be cut to fit inside the cylinder. These should be cut to shape from 1/16 inch brass, and a hole drilled in the cylinder head 1/8 inch in diameter to accommodate the piston-rod. The cylinder head is then soldered in place. The cylinder-end cover should be left until the piston-rod and piston are made.
The piston head is cut to shape from a piece of 3/16-inch sheet brass, or it can be cut from a piece of 3/4-inch round brass with a hacksaw. The piston-rod is soldered into a hole in the piston-head. A small square piece of brass is placed on the opposite end of the piston-rod to act as a bearing. This little piece is cut and drilled as shown in the drawing. Before it is soldered in place on the piston-rod the cylinder-end cover should be placed on the rod. Both the piston and the cylinder-end cover can then be placed inside the cylinder, and the piston-end cover is soldered in place. Before final assembling the piston should be made to fit nicely into the cylinder. This can be brought about by applying emery cloth to the piston-head until it slips nicely into the cylinder with little or no play. Thus a steam-tight fit is made, and this contributes greatly to the efficiency and power of the engine.
The cylinder blocks are shown in [Fig. 55]. These are cut and brought to shape with a hacksaw and file. With a half-round file one side of one of the blocks is filed slightly concave, so that it will fit on the outside of the cylinder. Two 1/8-inch holes are drilled in this piece as shown in the drawing. The hole at the top is the steam entrance and exhaust for the engine; that is, when the cylinder is at one side steam enters this hole, and when the crank throws the cylinder over to the other side steam leaves through the same hole after having expanded in the cylinder. This cylinder block is soldered to the piston as shown in [Fig. 56]. The pivot upon which the cylinder swings is then put in place in the hole at the bottom of the block. Solder is flowed around the pivot to hold it securely in place.
The second cylinder block is now finished according to the drawing. This has two holes 1/8 inch in diameter bored in it. One of these holes is the steam inlet and the other the exhaust. When the cylinder is at one side of its stroke the hole that was bored in the top of the steam block which was soldered on the cylinder is in line with the inlet hole in the block under consideration. Steam then enters the cylinder and forces the piston down. This turns the crank around, and the crank in turn pulls the piston over to the opposite side, so that the hole in the first piston block of the cylinder now comes in line with the exhaust hole on the second cylinder block. The steam in the cylinder escapes and the same operation is repeated over again. Of course, it must be understood that this steam admission and exhaust takes place very rapidly. The hole in the second cylinder block, which goes over the pivot, must be made a trifle more than 1/8 inch in diameter, so that it will slide freely over the pivot.
The engine is mounted on a very simple frame, which is a piece of 1/16-inch brass cut and bent as illustrated. After it is cut and bent to shape the second cylinder block is soldered in place. The cylinder can then be mounted. It will be seen that the pivot goes through both the second cylinder block and the engine standard. A small spring is placed over the protruding end of the pivot and a nut put in place. By turning this nut the pressure on the face of the two cylinder blocks can be adjusted, and the model engineer must always remember that the pressure on these springs must be greater than the steam pressure in the feed-pipe. Otherwise the steam pressure will force the cylinder-block faces apart and steam leakage will result. On the other hand, the pressure of the spring should not be too great, since that would interfere with the free movement of the engine cylinder.
Nothing now remains to be made except the crank and the flywheel. The crank revolves in a small brass bearing which is soldered in place on the engine standard. It will be seen that the sheet brass that makes up the engine standard is not thick enough to offer a good bearing for the crank. The crank is bent to shape from a piece of 1/8-inch brass rod, and the author advises the builder to heat the brass rod red-hot while the bending is done. This will prevent it from fracturing, and will also permit a sharp bend to be made.
The flywheel is a circular piece of brass 1 inch in diameter. Its center is drilled out and it is soldered to the crank as illustrated in [Fig. 54]. Two other holes 1/8 inch in diameter are drilled in the flywheel as illustrated, and two small brass pins are cut out from 1/8-inch brass rod and forced into these holes and then soldered. These provide a method of driving the propeller-shaft that is shown very clearly at [Fig. 57].
The steam feed-pipe that runs from the boiler to the engine can be of small copper tubing. It may be necessary to mount the engine on a small block, as shown in [Fig. 53]. After the steam in the boiler has reached a sufficient pressure the engine crank should be given a couple of twists in order to start it. Before operating the engine a little lubricating oil should be run into the cylinder through the inlet or exhaust ports. The cylinder should always be kept well lubricated. The contacting faces of the cylinder blocks should also be kept lubricated.
Caution. Always keep water in the boiler. Never permit it to run dry, as this would cause a boiler explosion. When the engine is started and cannot be made to run, take the burner from under the boiler so that steam will cease to be generated. With the safety-valve the model boat builder need have little fear of an explosion. Nevertheless the foregoing directions should be carefully adhered to.