The third class are the steamboats proper, varying in price from five shillings upwards. In these the steam-power is applied as in ordinary engines. The cheapest have one single-action oscillating cylinder, and the better sorts two double-action cylinders.
As the two first-mentioned classes are, after all, only imitations, I do not think it worth while to describe them; and of the third class I have chosen the largest to describe, as I think that if it is worth while making a model at all, it is worth while to make a good one, and the small engines take almost as much time to make, and quite as much care to fit, as the large ones, and unless they are well fitted the loss of power by friction and waste of steam is very great.
The engine here described is a model of a real screw-engine, with a pair of double-action oscillating cylinders, having reversing gear and boiler complete, ready to be put into the boat. It will be capable of driving a boat from four to five feet long, provided it is well hollowed out and that the engine is made and fitted with care, to reduce friction and waste of steam as much as possible.
In this section the exact dimensions of the several parts are given when possible, but, owing to small differences in the size of the cylinders, I am only able to approximate in some cases—in which cases, however, I have used the word ‘about,’ at the same time explaining how to obtain the exact measurements.
In all engines the most important parts are the cylinders, which must be well fitted. Boys who have a turning-lathe and the requisite practice in metal-turning can buy rough castings of all the parts of the engine for a few shillings and finish them up themselves. But as only a few of my readers may be so favoured, I will suppose that the cylinders are purchased ready for use. For these cylinders there is a great range in the prices quoted by different firms, the prices varying for the No. 4 cylinder from eight shillings at one firm to twelve shillings and sixpence at another. Messrs. Theobald and Co., of 20, Church Street, Kensington, quote the lowest prices to me, and have further consented to supply the No. 4 double-action oscillating cylinders for this engine at seven shillings each to any one mentioning this section.
The dimensions of these cylinders are three quarters of an inch in the diameter of the bore, and an inch and a half in the length of stroke—i.e., an inch and a half difference in the length the piston-rod projects from the top of the cylinder when in and out to its fullest.
Get a pair of these cylinders which have the steam-blocks, pivot-pillars, and screw-crossheads complete. Ask for the No. 4 double-action oscillating cylinders. When buying them see that the piston-rods work true, and not to one side; see also that the small indentations on the opposite sides to the steam ports are correctly drilled, so that when the cylinders are swung between the blocks and pivots they work true. To test this, place the block on its back on the table, and put the cylinder on it, with the pivot in the proper hole for it. Now turn the cylinder round on the block and place a pin in the indentation, and if it is truly drilled the pin will not move; but if not, the point of the pin will describe a small circle. You can find out by this pin the exact spot where the pivot-hole ought to be drilled.
Fig. 1
We will now set to work at the construction of the engine, and the first thing to be done is to make the top plate ([Fig. 1]). For this get a small brass plate four inches long and three inches wide, and an eighth of an inch thick, with a projecting piece an inch and a quarter square at one end of it, as in the [figure]. Get two of these plates, as the second will be required for the bed-plate, but will not have the square projecting piece. Take the first of these plates and square it up, so that each corner is a right angle. Now proceed to mark it as in [Fig. 1]. Divide the large part lengthwise into two equal parts by the line C D, and crosswise, also into two equal parts by the line A B, these two lines intersecting in the point O. From this point mark off, each way along the lines O C and O D, the following distances. O to x a quarter of an inch, and x to y an inch and a quarter; and through these four points draw the lines e-g, f-h, k-m, and l-n, making them two inches long each, and projecting one inch on each side of the line C D. Join the points e-f, g-h, k-l, and m-n.