Encyclopædia Britannica, Ninth Edition/Pontoon

by James Thomas Walker

PONTOON. Pontoons are vessels employed to sup port the roadway of floating bridges. They may be either open or closed, heavy and only movable when floated, or light enough to be taken out of the water and transported overland, as when required to form part of the equipment of an army in the field.

From time immemorial floating bridges of vessels bearing a roadway of beams and planks have been employed to facilitate the passage of rivers and arms of the sea. Xerxes crossed the Hellespont on a double bridge, one line supported on three hundred and sixty, the other on three hundred and fourteen vessels, anchored head and stern with their keels in the direction of the current. Darius threw similar bridges across the Bosphorus and the Danube in his war against the Scythians, and the Greeks employed a bridge of boats to cross the river Tigris in their retreat from Persia. Floating bridges have been repeatedly constructed over rivers in Europe and Asia, not merely temporarily for the passage of an army, but permanently for the requirements of the country ; and to this day many of the great fivers in India are crossed, on the lines of the principal roads, by floating bridges, which are for the most part supported on boats such as are employed for ordinary traffic on the river.

But light vessels which can be taken out of the water and lifted on to carriages are required for transport with an army in the field. Alexander the Great occasionally carried with his army vessels divided into portions, which were put together on reaching the banks of a river, as in crossing the Hydaspes ; he is even said to have carried his army over the Oxus by means of rafts made of the hide tents of the soldiers stuffed with straw, when he found that all the river boats had been burnt. Cyrus crossed the Euphrates on stuffed skins. In the 4th century the emperor Julian crossed the Tigris, Euphrates, and other rivers by bridges of boats made of skins stretched over osier frames. In the 17th century the Germans employed timber frames covered with leather as pontoons, and the Dutch similar frames covered with tin ; and the practice of carrying about skins to be inflated and employed for the passage of troops across a river, which was adopted by both Greeks and Eomans, still exists in the East, and has been introduced into America in a modified form, india-rubber being substituted for skins.

Pontoons have been made of a variety of forms and of almost every conceivable description of material available for the purpose of combining the two essential qualities of transportability over land and power of support in water. As these qualities are not only distinct but conflicting, one of them has been frequently sacrificed to the other. Thus history records many instances of bridges having failed because incapable of supporting all the weight they were called on to bear, or of resisting the force of the current opposed to them ; it also records instances of important strategical operations being frustrated because the bridge equipment could not be brought up in time to the spot where it was wanted. Numerous expedients for lightening the equipment have been suggested, in America more particularly ; but the proposers have not always remembered that if a military bridge is intended to be carried with an army it is also intended to carry the army, with its columns of infantry and cavalry, its numerous waggons, and its ponderous artillery, and it ought to do so with certainty and safety, even though a demoralized rabble should rush upon it in throngs.

Pontoons have been made of two forms, open as an undecked boat, or closed as a decked canoe or cylinder. The advantage claimed for the closed pontoon is that it cannot be submerged by the river, but only by having to bear a greater load than its buoyancy admits of ; the disadvantages are that it is difficult to make and keep water-tight, it requires special saddles for the support of the baulks which carry the roadway, and it cannot be conveniently used as a row-boat. During the Peninsular War the English employed open bateaus, as did and still do all the other European nations ; but the experience gained in that war induced the English to abandon the open bateau ; for if large it was very difficult to transport across country, and if small it was only suited for tranquil streams, being liable to fill and sink should the river rise suddenly or become disturbed by the wind. Thus closed pontoons came to be introduced into the British army. General Colleton devised the first substitute for the open bateau, a buoy pontoon, cylindrical with conical ends and made of wooden staves like a cask. Then General Pasley introduced demi-pontoons, like decked canoes with pointed bows and square sterns, a pair, attached sternwise, forming a single “pier” of support for the roadway ; they were constructed of light timber frames covered with sheet copper and were decked with wood; each demi-pontoon was divided internally into separate compartments by partitions which were made as water-tight as possible, and also supplied with the means of pumping out water; when transported overland with an army, a pair of demi-pontoons and the superstructure of one bay formed the load for a single carriage weighing 3110 ℔ when loaded. The Pasley was superseded by the Blanshard pontoon, a tin coated cylinder with hemispherical ends, for which great mobility was claimed, two pontoons and two bays superstructure being carried on one waggon, giving a weight of about 5000 ℔, which was intended to be drawn by four horses. The Blanshard pontoon was long adopted for the British army, but it is now being discarded; experiments made with it in peace time showed that it would probably break down under the strain of actual warfare, and efforts were constantly made to improve on it; when immersed to a greater depth than the semi-diameter it became very unstable and lively under a passing load, a defect which Serjeant-Major Forbes proposed to remedy by giving it a triangular instead of a circular section, thus increasing the stability by presenting a continually increasing area of bearing surface up to the level of total immersion; but the angles of these pontoons were found so liable to injury as to counterbalance any advantages over the cylinders. After many years experience of the closed pontoon the English engineers came to the conclusion that it was desirable to return to the form of the open bateau to which the engineers of all the Continental armies had meanwhile constantly adhered. Captain Fowke, R.E., invented a folding open bateau, made of water-proof canvas attached to sliding ribs, so that for transport it can be collapsed like the bellows of an accordion and for use it can be extended by a pair of stretchers; it is very mobile, but it is also deficient in power of support, for whereas the buoy ancy due to the outline form out of the water is 13,600 ℔ the actual buoyancy in the water is only 8640 ℔, because of the cavities in the canvas between the ribs which are formed by the pressure of the water outside; moreover, the surface irregularities cause the pressure exerted by a current upon a bridge formed of these collapsible pontoons to be about three times as much as upon one of equal power formed with Blanshard’s or Pasley’s pontoons; there is thus great risk of the bridge being carried away by a strong current.

The following table shows the powers of various pontoons at present or recently in use by different nations. The “working power of support” has been calculated in most instances by deducting from the “available buoyancy” one-fourth for open and one-tenth for closed vessels:—

The Powers of Various Pontoon Bridges.

Pontoon.	Length.	Displace-  ment of Pontoon.	Actual Buoyancy of Pontoon.	Weight of pontoon and one bay of Super- structure.	Available Buoyancy.	Working Power of  Support.	Central Interval in Bridge.	Power per lineal foot  of Roadway.	Greatest ordinary Load per foot lineal.	Width of Roadway.	Greatest possible load at 100 ℔ per  foot superficial  of roadway.
	Ft.	Cub. Ft.	℔	℔	℔	℔	Ft.	℔	℔	Ft.	℔
Gribeauval: open bateau, oak	36·3	593	45,044	8,044	37,000	27,750	22·8	1,215	840	15·6	35,568
Austrian: open, wooden, 1799	27·0	354	22,123	3,332	18,791	14,093	16·6	849	560	11.4	18,924
Aust.-Birago: open, wooden; two pieces	28·0	303	18,907	3,249	15,658	11,744	21·7	542	560	9·3	20,181
„   „   „three „	39·4	445	27,791	3,884	23,907	17,930	21·7	827	560	9·3	20,181
„   „   iron; two pieces	28·0	353	22,090	3,698	18,392	13,794	21·7	636	560	9·3	20,181
„   „  „   three „	39·4	530	33,135	4,501	28,634	21,476	21·7	991	560	9·3	20,181
French: open, wooden; reserve	30·9	325	20,286	3,608	16,678	12,509	19·7	635	560	10·5	20,685
„   „  „ advanced guard	19·7	156	9,734	1,506	8,228	6,171	16·4	376	560	9·3	15,252
„   „  „ general	30·9	321	20,065	3,153	16,912	12,684	19·7	644	560	9·8	19,306
Prussian: open, wooden; open order	23·7	164	10,226	2,393	7,833	5,875	15·3	384	560	9·9	15,147
„   „  „   close order	23·7	164	10,226	2,213	8,013	6,010	11·2	535	560	9·9	11,088
„   „  iron; open order	24·7	214	13,385	2,209	11,176	8,382	15·3	561	560	9·9	15,147
„   „  „  close order	24·7	214	13,385	2,029	11,356	8,517	11·2	759	560	9·9	11,088
Italian: open wooden; one piece.	19·6	283	17,660	3,582	14,078	10,559	26·3	402	560	9·8	25,774
„  „   „    two pieces	39·2	565	35,320	4,572	30,748	23,061	26·3	878	560	9·8	25,774
„   modified; one piece	24·6	325	20,290	3401	16,889	12,669	23·0	551	560	9·8	22,540
„   „    two pieces	49·2	649	40,580	4,489	36,091	27,068	23·0	1,178	560	9·8	22,540
Russian ${\Big \{}$ open, canvas on wooden framework; ${\Big \}}$ open order close order	21·0	209	13,042	2,355	10,687	8,015	16·6	493	560	10·4	17,264
	21·0	209	13,042	2,083	10,959	8,219	11·7	705	560	10·4	12,168
Belgian: open, iron; one piece	24·8	297	18,584	3,336	15,248	11,436	19·7	580	560	9·5	18,715
„   „  „    two pieces	49·2	595	37,168	4,548	32,620	24,465	19·7	1,244	560	9·5	18,715
American ${\Big \{}$ india-rubber,  three; cylinders connected; ${\Big \}}$ open order close order	20·0	130	8,125	1,980	6,145	5,530	18·0	307	580	11·0	19,800
	20·0	130	8,125	1,824	6,301	5,761	14·7	393	560	11·0	18,370
English Pontoons.
Peninsular  equipment ${\Big \{}$ open, tin; reserve   „  „   advanced guard	18·9	209	13,092	2,374	10,718	8,039	16·8	477	560	10·0	16,800
	15·1	120	7,520	1,654	5,866	4,400	14·0	314	560	9·0	12,600
Pasley: closed demi-canoe; copper	25·0	141	8,781	2,103	6,678	6,010	12·5	481	560	10·0	12,500
Blanshard: cylinder, tin; open order	22·5	109	6,785	1,600	5,185	4,667	12·5	373	560	10·0	12,500
„   „  „   close order	22·5	109	6,785	1,408	5,377	4,839	8·3	581	560	10·0	8,300
„   „  „   light pattern	15·5	26	1,640	340	1,300	1,170	5·3	220	280	7·0	3,710
Fowke: open, collapsible, canvas; open order	22·0	134	8,460	1,246	7,214	5,411	10·0	541	560	10·0	10,000
Forbes: closed, spherangular, tin; open order	24·2	128	7,977	1,689	6,288	5,659	11·0	514	560	10·0	11,000
Blood: open, wooden; general	21·6	280	17,500	2,300	15,200	13,350	15·0	890	560	10·0	15,000

In the English and French equipment the pontoons were originally made of two sizes, the smaller and lighter for the “advanced guard,” the larger and heavier for the “reserve”; in both equipments the same size pontoon is now adopted for general requirements, the superstructure being strengthened when necessary for very heavy weights. The Austrian and Italian pontoons are made in three pieces, two with bows and a middle piece without; not less than two pieces are ordinarily employed, and the third is introduced when great supporting power is required, but in all cases a constant interval is maintained between the pontoons. On the other hand in the Prussian, Russian, Dutch, and American and in the English Blanshard equipments greater supporting power is obtained not by increasing the number of supports but by diminishing the central interval between the pontoons. Within certain limits it does not matter whether the buoyancy is made up of a large number of small or a small number of large vessels, so long as the water-way is not unduly contracted and the obstruction offered to a swift current dangerously increased; but it is to be remembered that pontoon bridges have failed as frequently from being washed away as from insufficient buoyancy.

On comparing the “available buoyancy” with the “greatest possible load at 100 ft per foot superficial of roadway” for each of the bridge equipments in the preceding table, it will be seen that very few of the bridges are really capable of carrying the maximum load they may be called on to bear. Strictly speaking the roadway superficies should in all instances be proportioned to the buoyancy of the pontoon, or, as the central interval between the pontoons cannot be reduced below certain limits, the width of the roadway should be proportioned to the buoyancy; in other words the “chesses” or planks which form the roadway should be made of a shorter length for a bridge which is designed for light traffic than for one which is designed for heavy traffic. The employment of chesses of different lengths for the pontoon equipment of an army would, however, be very inconvenient and troublesome, and this has led to the adoption of a constant breadth of roadway, on the understanding that the traffic will always be controlled by the officer in charge of the bridge.

The latest form of pontoon for the English army is one with which the name of Colonel Blood, R.E., is mainly associated. Its powers are given in the lowest line of the preceding table. It is an open bateau with decked ends and sides partly decked where the rowlock blocks are fixed. It consists of six sets of framed ribs connected by a deep kelson, two side streaks, and three bottom streaks. The sides and bottom are of thin yellow pine with canvas secured to both surfaces by india-rubber solution, and coated outside with marine glue. The central interval between the pontoons in forming a bridge is invariably maintained at 15 feet; for the support of the roadway five baulks are ordinarily employed, but nine for the passage of siege artillery and the heaviest loads; they fit on to saddles resting on central saddle beams. The pontoons are not immersed to within 1 foot of the tops of their “coamings” when carrying ordinary loads, as of infantry in marching order “in fours” crowded at a check, or the 16-pounder gun, which weighs 4800 ℔; nor are they immersed to within 6 inches when carrying extraor dinary loads, such as disorganized infantry, or the 64-pounder gun weighing 11,100 ℔. In designing this pontoon the chief points attended to were (1) improvement in power of support, (2) simplification in bridge construction, (3) reduction of weight in transport, and (4) adaptation for use singly as boats for ferrying purposes. One pontoon with the superstructure for a single bay constitutes a load for one waggon, with a total weight behind horses of about 4500 ℔.

For the British army in India the standard pontoon for many years was the Pasley ; it was seldom used, however, for boats could almost always be procured on the spot in sufficient numbers wherever a floating bridge had to be constructed. Of late years an equipment has been prepared for the Indian army of demi-pon toons, similar to the Blood pontoon cut in half, and therefore more mobile ; each has a bow and a square stern, and they are joined at the sterns when required to form a “pier” ; they are fitted with movable covers and can therefore be used in much rougher water than pontoons of the home pattern, and their power of support and breadth of roadway are the same.

For the British army there is a light form of the Blanshard pontoon suitable for infantry un crowded, guns unlimbered, and cavalry in single file. The Berthon collapsible boat, for infantry in single file, is also employed ; when open it is 9 feet long and 4 feet wide; it weighs 109 ℔ with a pair of oars and a removable thwart or seat (to enable it to be used as a boat), and can be slung on to a bamboo and carried by two men ; the superstructure for one bay weighs 97 ℔, and is also carried by two men ; the width of roadway is 18 inches; twelve boats are required to bridge a stream 100 feet in width.

The india-rubber pontoon does not appear to have been generally employed even in America, where it was invented. The engineer officers with the army of the Potomac, after full experience of the india-rubber pontoon and countless other inventions of American genius, adopted the French equipment, which they found “most excellent, useful, and reliable for all military purposes.” The Russians in crossing the Danube in their war with Turkey in 1878 employed the Austrian equipment.

Authorities. Colonel Lovell, R.E., Prof. Papers, Royal Engineers, vol. xii., 1S63 ; Brig.-Gen. Cullum, U.S.A. Engineers, System of Military Bridges in use by the United States Army, 1863; Gen. Barnard, U.S.A., Report on Army of Potomac, 1863 ; Lord Wolseley, Pocket-Book for Field Service, 1882 ; Military Bridges, Chatham, 1879.  (J. T. W.)