It has been found that wire-rope tugs cannot work in less than 3 feet of water, or only with difficulty, whereas chain tugs can work in one-half of that depth. As regards steering facility, they are much alike. The delay caused by fractures is an important item in the comparison. Repairs to chains usually occupy considerably less time than repairs to wire ropes. Chain tugs in any depth under 3 feet, and in sharp curves, are said to be preferable to rope tugs; in moderately strong currents, and in larger curves, they are about equal; but in canals, and in large deep rivers, rope tugs are the best, and both are superior, in ordinary circumstances, to paddle tugs.
In canal tunnels, as in the 4-mile section between Mons and Paris, where steam cannot be used on account of the smoke, chain tugs, worked by a horse capstan, tow a barge through in one-third the time, and at one-fourth the cost, of the former system, when men were employed for towing.
Where strong rapids are met with, special appliances called “grapins” are sometimes employed. This consists of an iron wheel of about 20 feet in diameter and 17½ tons weight, furnished with projections or picks, fixed in a well-hole at midships, and worked by a chain attached to the paddle-shaft. On ascending a river the “grapin” is lowered till the picks grip the bed, on which the wheel slowly turns, and the paddles, working at the same time, in this way tow barges over the strongest rapids. Busquet’s tug, which is used in France, works on a chain, though it is similar to a wire-rope tug. The Baxter steamboat, used on the Erie canal, was the outcome of a competition invited by the State of New York for a prize of 20,000l. for the steamer which best fulfilled the following, viz. a mean speed of 3 miles per hour with a load of 200 tons, small cost, and no wash or swell. This steamboat is 100 feet long, 17½ feet wide, and about 9 feet deep, with a flat bottom and vertical sides, and, including engines and coal, weighs 52 tons. It carries a load of 200 tons, with a draught of 6 feet of water, and has an average speed of about 4 miles, but can work up to 7½ miles an hour.
On the Saar coal canal Jacquel’s steam-tug system is in use, where the screw is within the body of the vessel, and surrounded by a cylinder, and is fed with water by two large channels leading from the sides of the vessel to the front of the screw.[270]
The tugs of the Rhine are large, very tapering vessels; some of them have engines of from 600 to 700 horse-power, and they are provided with all the latest improvement for economising fuel. Vessels with two screws are preferred, as combining adequate power with small draught; nevertheless, when the river is very low, paddle-wheel tugs of the old type have to be resorted to. Towing by aid of a submerged cable was started some years ago, but it has since been abandoned, except in the most difficult part of the river between St. Goar and Bingen, where it has proved serviceable, especially when the water is low. A serious disadvantage of this system is that in descending the river the tug has to let go the cable, and act simply as a tug, for which it is not well suited.
Improvements have been introduced in the vessels as well as in the tugs. Narrow iron vessels have been substituted for the broad wooden barges in order to reduce the tractive force. Some of these vessels are 1000 tons register; but vessels from 400 to 500 tons are the most common. On the Rhine, vessels forming one convoy are not connected together in trains, as in France, but each is provided with its tug, which is a great advantage where the navigation is difficult.
Human labour is still employed for towage on some of the Dutch, Belgian, and German canals. Boats of from 15 to 26 tons are towed by men at a speed of 1 to 1⅓ miles per hour. Dr. Mitzen, a German authority, allows for this system of transport a duty of 11 miles a day, including all stoppages. Steam-tug boats on the Belgian canals are restricted to a speed of 2⅔ miles per hour, and on the wider rivers to 4½ miles per hour. On the canal joining the Tiege to the Vistula, steam-tugs draw trains of barges 410 feet long, the speed being restricted to three miles per hour. The steam-tugs put by Mr. Beardmore on the river Lea towed from 50 to 60 tons, at from two to two and a half miles per hour, in the cuts, three to three and a half miles per hour in the larger sections, and five miles per hour in the Thames. On the Grand Junction Canal the speed of a steamer towing one vessel is put from three to three and a half miles per hour. On the Rotterdam Canal, four boats, of 130 tons each, are towed by a screw steamer.
Several attempts have been made on the Leeds and Liverpool Canal to introduce steam towage, and in the year 1879 the company tried a screw steamer with compound condensing engines, to tow six 40-ton barges on a river or deep canal.
It was very quickly discovered that the vessel was next to useless on a shallow canal—the section of that particular waterway only averages from 40 feet to 50 feet in width at the surface, with flat sloping sides under water, tapering down to a mid-channel or gutter with an average depth of only 4½ feet—inasmuch as with that depth (in mid-channel only) a screw propeller of sufficient diameter could not be used to utilise the power of the engines without a very great amount of “slip” and churning of the water instead of doing useful work. It was also found that when the least obstruction took place by meeting other barges near bridges or sharp curves, causing the slowing up or stoppage entirely of the tug, the barges in tow would, so to speak, insist on running pell-mell into one another, for the simple reason that they could not apply a brake, and besides they used to get zig-zagged across the canal in every direction, which often caused a delay of fifteen or twenty minutes before all could be marshalled and got under weigh again.
Another attempt has since been made, which utilised the power of the engines with more success. Two narrow boats of about five feet beam were braced side by side under one deck, with a longitudinal space of about three feet between each, and in this space was one paddle-wheel with a long-stroke horizontal engine on deck over each boat (two engines) driving a crank on each end of the paddle shaft, set at right-angles, and across the deck stood a locomotive boiler, each boat carrying its own proportion of the weight of the boiler. The funnel had to be placed at an angle of 45 degrees, so as to get under the very low bridges. This steamer towed fairly well five barges of coal, but caused a great waste in the canal, to the injury of the banks, and was subject to the steering difficulties whenever any obstruction took place, which in this canal are frequent, owing to its very tortuous character.