Steam-ships

by R. A. Fletcher

A difficulty which paddle-vessels have to contend with is that of securing a proper immersion of the floats. For a vessel in smooth water the immersion of the top edge is usually calculated at about one-eighth of the breadth of the float; but for a vessel intended for general sea service, an immersion of not less than half the breadth of the float is allowed, that is to say, the float at its moment of deepest immersion has a height of water above it equal to half its diameter. If the float goes much deeper the efficiency of the wheel becomes impaired. This is a point which has to be taken into consideration in designing paddle-boats, so that the maximum power shall be available when the vessel is fully laden, and shall not be much lessened when the vessel is running light. The earliest steamers suffered greatly in this respect as their designers had not discovered the right size of wheels or floats to suit the hulls. A loaded vessel consequently went very slowly owing to the great depth to which her floats were immersed. To overcome this difficulty an ingenious system of what can best be called reefing was invented. Affixed to the axle of the wheel was a rod with an arrangement of cogs at the end, and these fitted into a series of teeth in rods affixed to the floats, so that it was a simple matter to expand or contract the effective diameter of the wheel by altering the position of the floats as required. The same result has sometimes been obtained by a system of levers, but the toothed wheel business was the older. It was tried on a few of the earlier boats on the Clyde, not always, however, with success.

A peculiarity of some of the larger paddle-wheels in use in America is that they are not only of much greater size than those in use in Great Britain in proportion to the size of the boat, but they have a proportionately less immersion and the wheel is constructed in a very different fashion. The floats, instead of being of one piece, as here, are constructed of three narrow fixed strips, two of which are on the same radius but have a space between them equal to the breadth of the third strip, which is placed a few inches behind the vacant space. It is contended that this method disturbs the water less than the broad float and increases the propelling efficiency. Probably the most notable instance is the great wheel of the Sprague.

Referring now to the construction of the engines of the earliest boats, Symington’s Charlotte Dundas used a horizontal direct-acting engine, and the general arrangement of her machinery would be considered creditable even at the present day.[79] The engine of the Savannah was of the inclined direct-acting type. The type of engine which Newcomen invented has been retained for many years, but the oscillating or walking beam which is such a conspicuous feature of nearly all the American river craft has been placed by engineers in this country below the crank axle instead of above. The type of engine with the beam below the crank axle is known as the side lever. It is a type peculiarly suitable to paddle-wheels, and this being the only method of propulsion adopted on this side of the Atlantic for many years, there was little change for a considerable period in the shape of the engines, which therefore attained to a high stage of perfection until the limit of their profitable employment was reached. When larger engines became necessary, in consequence of the rapidly increasing size of vessels, the great weight of the side-lever engines proved a serious drawback.

[79] Sennet and Oram’s “The Marine Steam-Engine,” 1898.

Engineers were not long in devising a more compact form of machinery, and direct-acting engines were introduced, these involving the abandonment of the use of the heavy side levers. As the side-lever engines were made larger it became customary to use two beams, one on each side, and a rod from one end of each of these connected with a cross-piece at the top of the piston-rod. The other ends of the double beam were united by a cross-piece which carried from its centre the rod or lever which worked the crank of the paddle-shaft. Where it became necessary to use two engines in one vessel, they were so arranged that while one rod and crank were at their period of least activity, the other pair were exerting their greatest effort. The system of condensation of steam, which it would take too much space to describe in detail, is also a matter of great importance in determining the power of the engine, but the principle upon which the condensation is effected is well known, and the various methods of condensation can easily be ascertained from the numerous handbooks on engineering.

Maudslay’s Oscillating Engine.

Another early form of marine engine was that in which the side levers were arranged as levers of the third order, the fulcrum being at one end and the steam cylinder placed between it and the connecting-rod. The peculiar motion thereby given to the machinery caused this type to be known as the grasshopper engine, from a fancied resemblance to the long legs of a grasshopper. The direct-acting engines were much more compact, more powerful, and lighter than the old side levers. The necessity of providing a connecting-rod of sufficient length was met by Messrs. Maudslay by the provision of two cylinders. The cross-head was not unlike the letter T, the foot of which passed down between the cylinders, and the lower end of this was fitted with a journal from which the connecting-rod extended to the crank in the axle. A still further improvement was made when the oscillating engines were invented, which form an even more compact and simple type. Messrs. Maudslay fitted a pair of oscillating engines in 1828 into the paddle-steamer Endeavour, and subsequently into several ships. This form of engine was improved upon by Mr. John Penn, the famous engineer at Blackwall, and the perfection which he gave it has not been surpassed.

The great feature of this method is that the trunnions are hollow, and the steam is admitted to and exhausted from the cylinders through them. The connecting-rod is dispensed with and the upper end of the piston-rod acts directly on the crank pin. This type of engine is the most economical for space and weight that has yet been provided for paddle-wheel engines, the majority of which of late years have been made on this system.

Its adaptability for certain classes of work has given the paddle-wheel a long lease of life. Paddles are peculiarly suitable for certain conditions, such as smooth waters and shallow rivers, where speed and light draught combined with considerable carrying power are essential. The Indian rivers, for instance, early demanded suitable steamers, and the paddle-steamers Lord W. Bentinck, Thames, Megna, and Jumna were built of iron in 1832 for the East India Company for the navigation of the Ganges. They were designed and constructed by Maudslay, Sons, and Field, and fitted with oscillating cylinder engines of 30 nominal horse-power. They were flat-bottomed and were shipped to India in pieces. They were 120 feet in length, 22 feet beam, and had a draught of 2 feet. Their tonnage was 275, builders’ measurement.