Chapter X: The ‘boy’s Own’ Model Launch Engine

BY H. F. HOBDEN.

I propose in this chapter to give a few practical hints showing how to build a perfect model of an inverted-cylinder direct-action engine with link-motion reversing gear, like the sketch below, which represents a type in daily use on the river and sea. Such a model, having a fixed cylinder, has not the friction of other types, and therefore it gives more power, size for size, than an oscillating engine, and does not get so easily out of order.

You must of course have a lathe, which I will therefore suppose you to possess; but should there not be a slide-rest to it, you must get the cylinder bored by a professional turner, for which he will charge about two shillings, according to the size of your castings.

Let me first briefly explain the action of the steam in the engine by a diagram (Fig. 1, p. 139). The cylinder A is bolted into the standard, B; the ports or steam-passages are shown at C; and the slide-valve that allows the steam to pass alternately to each side of the piston is marked D, in its case F. G G are the stuffing-boxes, which have to be packed with lamp-cotton greased to make them steam-tight, H is the piston, with its rod finishing in a cross-head J, which is cut with a groove to slide up and down the standards to guide it and prevent the piston-rod being bent out of shape. K shows the connecting-rod, attached at its lower end to the crank L. M is one of the eccentrics working the slide-valve. N is the main shaft, resting on the plummer blocks O O, having a heavy fly-wheel at P and the coupler at Q. R is the top cylinder plate, drilled to screw in the grease-cock, of which I will presently give a drawing on an enlarged scale, S is the bed-plate, T the steam supply, and X the exhaust.

You will observe that the steam is coming in at the top of the cylinder, through the top port, as shown by the arrow, pressing the piston down and allowing the waste steam that has already raised the piston to escape through the lower port, and so into the exhaust. By that time the slide-valve is raised (by the eccentric) sufficiently to cut the steam off from the top port, which by that means is in its turn put in communication with the exhaust, and allows the steam to pass out of the top part of the cylinder, whilst it admits it to the lower portion, and so on alternately.

And now to the practical work. After having the cylinder bored, as already mentioned, get a piece of oak or other hard wood 1-1/2 inch square and about 6 inches long. Turn one end of it in the lathe, so that it fits the inside of cylinder, and drive it on. Then put it in the lathe again, and turn the flanges A (Fig. 2) down, and be very careful that they are quite true and square.

The top and bottom cylinder-covers, with the stuffing-box, come next. Screw a piece of hard wood on the end of your lathe mandrel, turn it down to about a quarter of an inch less in diameter than the flanges of your cylinder, make a small hole for the stuffing-box to be driven in, as in Fig. 3. You can now turn the edge and side--that next the cylinder. The projecting part A is to be the exact size of the diameter of cylinder. When this is done, take it out and place it in another chuck, and drill and turn the stuffing-box out, and screw it to receive the gland (Fig. 4).

Now chuck the top cover and turn it down to size. The piston is a casting, and has to be turned in the lathe to fit the cylinder, and a groove run round it to hold the greased cotton to make it steam-tight. Whilst in the lathe drill a hole in the centre, and tap it to receive the piston-rod, which you can make out of steel wire. Then pass one end through the stuffing-box on cylinder-cover and screw it on the cross-head J (Fig. 1), having first filed it up quite square and true and finished it off with emery. Now take the standards B (Fig. 1), and finish them up with a file in the same way, and be careful that the insides forming the guides for cross-heads are quite true. We can now make the lagging for cylinder. Get a piece of mahogany the length of the outside circumference of cylinder and the width of the distance between flanges of same. Then plane it down to about an eighth of an inch and score it with a penknife every eighth of an inch down its width; it will then bend round the cylinder, and you can fasten it on by a couple of brass bands, screwing the ends down near the slide-valve case.

We will next tackle the steam-ports in the cylinder B (Fig. 2). They are simply two holes drilled side by side until they reach the openings C C (Fig. 2) in the casting; they must not be drilled any farther.

Now place the ends on cylinder and drill through them so as to screw them on to the flanges. The slide-valve case is a casting with separate lid (Fig. 5), and has to be faced up with a file, and four holes drilled through the lid and corners to screw on to the cylinder face. The boss on lid must now be drilled and tapped for steam-pipe to be screwed in.

The slide-valve itself is like Fig. 6, has a hollow cast in its face, and a small projection on the back (B), which you must make a narrow groove in with a saw, and file the end of the valve-rod down to fit it, as shown at C, Fig. 6.

The face of the cylinder and also of the slide-valve must now be made to work steam-tight by rubbing on a perfectly flat stone until true, and then putting some emery and oil on a board and working them up until they are quite true.

The eccentrics may now receive attention. They will require to be chucked twice, and the true centre marked. Do not drill it out yet, as the hole for the crank-shaft must not be in the centre, but half the travel of the slide valve from the centre. For instance, if the valve travelled one inch you would have to drill hole for shaft half an inch out of true centre of eccentric.

The straps (Fig. 7) have to be turned quite true to the size of the groove on eccentrics, then taken out of lathe and cut through line A B with a fine saw, and screwed together at C C. A hole has now to be drilled at D and tapped for the eccentric rods to be screwed into, one of which will have to be bent like Fig. 8, so as to allow it to work on to the quadrant. It is the neatest way to key the eccentrics on to the shaft with a small steel wedge.

The quadrant (Fig. 9) is of brass, and will have to be finished up with a file and emery, and the holes A B B drilled through. The shaft ought to be turned up in the lathe as well as the fly-wheel and coupler, with a slight groove sunk in where the plummer blocks support it, so as to take the thrust.

The reversing quadrant with the lever attached I have shown at Fig. 10. It is best cut out of brass. The notches are cut with a small file after the two pieces have been brazed together with a small piece an eighth of an inch thick between either end. It is then screwed on to the slide-valve case.

The lever is drilled at A, B, and C with small holes, and can be made of flat steel wire; A is for a pin to work into a joint or hinge on bed-plate. B is attached to the hole A (Fig. 9) by a small length of brass rod, so as to work easily. Cut with a slot at each end and then drill like Fig. 11.

The small spring D (Fig. 10) is to keep the ratchet down in place, and is best made from a watch-spring, and the handle F is turned out of some brass wire.

The different-size drills you will require can be easily made from various steel knitting-needles warmed, filed up to shape, and then tempered to a light-straw colour.

We now come to the grease or oil cocks, which I have mentioned before. They can be bought ready finished at most model shops, but for those who like to make everything for themselves, this is the way to proceed. Fig. 12 is a section showing interior oil chamber that allows the cylinder to be oiled without stopping the engine by turning off cock A and opening cock B, then filling with oil; then shutting B and opening A allows the oil to descend into the cylinder and lubricate the surface.

Now for the method. Chuck a piece of brass wire about a quarter of an inch in diameter in the lathe, and turn up to external shape; then turn out cup C and drill through from end to end with fine drill; then enlarge chamber D with small bent graver, and take out of lathe and drill through at right angles to previous hole at A and B with larger drill; then put plugs of brass wire in and fit them with emery and oil; rivet over one end, and the other turn up into a handle. Then turn them in straight line with the oil-cup, and drill through with the small drill again. Tap the end E, and screw into cylinder cover, when it is finished.

To keep the boiler full of water as the fire empties it by driving it off in steam, the usual thing is to use a force-pump worked by an eccentric on shaft; but, as the friction is excessive, it takes a great deal of power away from a model. It is best, therefore, to work it by a hand lever, and the pump may be screwed on to the side of boat, the suction A (Fig. 13) being led through the boat’s side and riveted over, and the supply B brazed into lower part of boiler. C is the lever, and D the plunger, which must be quite true, and turned up in the lathe; likewise the valves E and F and the stuffing-box tapped and drilled. It is best to work it up from a casting, and the outside smooth down with an old file. The projection G will then have to be drilled and the lever pivoted through, having first cut a slot at H to allow the lever to rise and fall.

I will now describe a method of making an injector, or machine for filling the boiler with water by the power of the steam alone, and not in connection with the engine.

The injector was an accidental discovery by a Mr. Gifford, and has now become a universal favourite on board both large and small craft, as it works splendidly without affecting the engine. So you can run the boiler up with water whilst the engine is at rest in harbour or otherwise. And another great advantage over pumps is that the steam, being mixed with the water, raises it in temperature to nearly boiling-point, and so is a great saving in fuel.

Fig. 14 is a section of the instrument as fit for model work, and if you will follow these instructions carefully it will act well.

It consists of three parts--the cone A, the cone B, and the casing C. The steam is admitted at D, and the water at E, the waste water overflows at F, and the hot steam and water is projected with great force into the boiler through the pipe H, which should be led to the bottom of boiler well below low-water mark, and it is quite imperative that the steam-pipe should come from top of boiler as so to get plenty of dry steam, and must not be tapped on to any other pipe.

The injector can be fastened to side of boat by brass band and screws, and the water-supply pipe brought through the side and riveted, as in the case with the pump. The injector will lift water several inches, but it always works better if the water can flow into it freely.

Now we will set to work at it. Take a piece of brass rod and chuck it in the lathe and turn two cones the shape of A and B (Fig. 15). Take them off the lathe and drill A through as far as practicable, and finish with a small rhymer, having first made a small hole right through not larger than a knitting-needle; then tap the port C with an internal screw to take the steam-pipe, and turn a screw on the outside at D.

Now, with the rhymer bore out the conical hollow at E in B, and tap it outside at F and inside at G, in the same manner as the former cone; then drill a small hole right through from end to end, and a smaller one at right angles to the other right through at H. This communicates with the overflow, and takes off the water not carried into the boiler.

Next take a piece of brass tubing five-eighths of an inch in diameter, and turn a screw at each end inside (Fig. 16). The screws turned on the outside of the cones must be the correct size to fit these; then drill a hole at A, and screw in a small tube for water-supply with tap; then drill another at B for the waste water to escape by. Finally, screw in the cone A (Fig. 15) and attach it to the boiler by a pipe, and the nearer the boiler the better, as if the steam condenses before reaching the injector it will stop working. The steam-pipe must of course have a tap to cut off steam when not required.

We must now screw in the lower cone B (Fig. 15) until there is an annular space between the two cones not exceeding a sixteenth of an inch. Then screw in the small pipe at C (Fig. 15), and attach the other end into the boiler below the water-line, where it must have a stop-valve to prevent the water returning.

To start the injector, turn on the water-tap until it runs out of the overflow freely. Then turn on the steam full power, and the overflow will cease, or nearly so. Should it still drip at the overflow, reduce the water supply by the tap accordingly.

It requires carefulness and patience to make an injector, but when done, and working properly, there are few boys with a mechanical turn of mind who would not think themselves well repaid in watching and controlling its mimic action. They would then have an engine fit to show to their most critical friends, and one they might well be proud of; and I shall be content if I have helped in any way to contribute to their happiness.