All other things being equal, the system of transport that is able to afford the greatest average speed will be certain to command the lion’s share of business. There are, however, both natural and economical limits to speed, alike on water and on land. The natural limit up to the present time may be put at 50 miles an hour for railway travelling, 20 knots per hour for sea transport, and four or five miles an hour for canal navigation. The economical limits are, however, very different. A goods train cannot be worked economically at a greater speed than 20 to 25 miles an hour, and many railway companies decline to work their mineral traffic at a higher speed than 15 miles. At sea, the ordinary rate for a cargo-carrying steamer will vary from 10 to 14 knots, but seldom exceeds the latter figure. On an artificial waterway it is not possible, even in the absence of locks or other obstructions, to maintain a higher rate of speed than 4 or 5 miles without doing serious injury to the banks.
A very excellent paper on the rate of speed which it is possible or usual to attain in canal navigation, under varying conditions of towage, locks, depth, and other elements that influence the question, was submitted to the Institution of Civil Engineers some years ago by the late Mr. Conder, who devoted much attention to the subject.[285]
On the Belgian canals, where human labour is employed for towage, the rate of speed does not exceed 1 to 1⅓ mile per hour, against 2⅔ miles on the same canals with steam towage. On the Grand Junction Canal the speed varies from 3 to 3½ miles, and on the Rotterdam Canal it is 5 miles per hour. The limiting speed on the Suez Canal is about 5¾ miles per hour, but there is a loss of speed on that waterway, due to the trapezoidal form of section, which is estimated at about half a mile per hour. The average retardation of speed on English canals, due to locks, has been calculated at between 1·75 and 1·95 minute per mile.
The greatest difficulty that lies in the way of extending canal navigation is the uneven character of the country that has usually to be traversed, and the consequent necessity of overcoming elevations and depressions by locks, lifts, inclines, or other costly mechanical devices. In crossing England, between the Thames and the Severn, a height of 358 feet has to be overcome on the 204 miles of the Wilts and Berks route; a height of 474 feet on the 180 miles of the Kennett and Avon route; and a height of 392 feet on the 206 miles of the Thames and Severn Canal route. The average difference of level on these routes, counting ascent and descent, is 4·14 feet per mile, or a little more than one-fourth of the ruling gradient laid down by Mr. Robert Stephenson for the London and Birmingham Railway. Canal lifts would overcome these differences better than locks, but then they are much more costly, and perhaps not, on the whole, so convenient. Tunnelling or cutting, as in the case of a railway, is in a large number of cases out of the question. There is, therefore, only the alternative of making locks, which involve tedious delays, and add largely to the cost of transport.
In the year 1825, the same year that saw the opening of the first passenger railway, Charles Maclaren undertook to prove that for all velocities above 4 miles an hour, a railway was much more economical than a canal. At 6 miles an hour he calculated that nearly three times as much power would be required to move an equal mass on a canal, while at 20 miles an hour he computed that twenty-four times as much power would be required. At 8 miles per hour the same writer estimated that the resistance in water increased so much that two horses on a road would do as much as one on a canal, although at 2 miles an hour the same amount of horse power that is required to drag one ton on a good road would drag 30 tons on a canal.
It is not a little amusing, in the light of our present experience, to find this author gravely stating that “the tenor of the evidence given before the Parliamentary Committee (on steam navigation) renders it extremely doubtful whether any vessel could be constructed that would bear an engine (with fuel) capable of impelling her at the rate of 12 miles an hour without the help of wind or tide;” while as for railway speed, he asserted that, “in speaking of 20 miles an hour it is not meant that this velocity will be found practicable at first, or even that it should be attempted.”
Canal engineers have found that where they can concentrate the rise of level on a canal by the use of lifts, or inclined planes, they can usually obtain a considerable increase of speed. Thus, on the river Weaver, a height of 51 feet is cleared by the Anderton lift in about eight minutes. On the incline of the Morris Canal, again, a height of 51 feet is overcome in three and a half minutes; while on the Forth and Clyde Canal the Blackhill incline enables a height of 96 feet to be overcome in ten minutes. This averages about three times the speed that could be attained in overcoming the same rise or fall by means of locks.
We have already seen it computed that there are in Great Britain one lock to every 1·37 mile of canal.[286] Mr. Conder has calculated that there is, at this rate, “an average rise or fall for the system, as far as it is represented by the time returned, of 5·84 feet per mile.” On the more uneven sections a running speed of 5 knots, or 5·76 statute miles per hour, will be reduced on an ordinary English canal by the delays caused by the locks, to a speed of 4·9 miles per hour. In other words, the rate of speed should be nearly double the speed of prompt canal service at the present time. Between Gloucester and Birmingham the merchandise sent by river and canal is delivered as quickly as that despatched by railway.[287]
Speed on canals is regulated by the effect of breaking waves on their banks. In narrow canals or rivers, such a wave first appears at from 3 to 3½ miles per hour, and it has been found that at 4 miles per hour it exercises an injurious effect on the banks of the canal. When the speed is increased to 5 miles an hour, the effect becomes much more marked, the waves breaking over the towing-path, and rendering navigation destructive.
Mr. Conder appeared to think that a speed of 5 miles an hour, or 8·37 feet per second, which is the limit of speed fixed for the Suez Canal, may be taken as the normal speed to be sought on the canals of England; and he adds that, “on the determination of the normal speed, and of the tonnage of the boats to be accommodated, will depend not only the steam-power required, but the sections of the canals and of the dimensions of the locks.”[288]