A float before it was a boat
A pontoon is a buoyant unit, and the word has been applied to several different things. In military history it is a boat or float used to support a temporary bridge. In harbours and docks it can be a floating platform used as a landing stage or work surface. In modern leisure use it is a type of flat, wide boat built on two or more floats. All three uses rely on the same principle, which is that a broadly shaped float supports a load on the surface of the water. The word itself comes through French from the Latin word ponto, which referred to a flat-bottomed boat.
Bridges of boats in antiquity
The idea of bridging a river or strait by lining up boats is very old. The best-known ancient example is the pair of bridges built across the Hellespont for the Persian king Xerxes at the start of his invasion of Greece in 480 BC, as described by the Greek historian Herodotus. Ships and other vessels were moored side by side and linked with cables, with planks laid across them. The details come from Herodotus and are not confirmed by other sources, so figures for the number of ships are best treated with caution. Another account of the Persian king Darius using a boat bridge across the Bosporus on an earlier campaign is also from Herodotus. Xerxes’ invasion later met a major naval defeat at the Battle of Salamis, which is described separately.
Armies elsewhere used similar methods. Roman military engineers are known to have built bridges of boats on rivers, and the Roman navy fleets of the Rhine and Danube supplied both vessels and expertise. Boat bridges also served civilians in towns on large rivers where a permanent bridge was difficult or costly to build. Some remained in service for centuries, replaced only when fixed bridges became practical.
Standardised military pontoons
In the early modern period, armies began to carry pontoon equipment as a regular part of their trains. Specialist troops were trained to assemble the bridge quickly, and the boats were designed to be carried on wagons. Different armies used wooden boats, copper or tin-covered boats, and later canvas-covered frames, depending on weight and cost.
Napoleonic armies relied on specialist pontoniers for river crossings, and during the American Civil War, pontoon bridges were used in several campaigns, and the crossing of the Rappahannock at Fredericksburg in December 1862 is a well-known example, because the construction was contested by enemy fire.
In the Second World War, military engineers made wide use of prefabricated floating bridges, including assault boats and heavy pontoon sections, to cross rivers such as the Rhine in 1945. Advances in materials, including steel and later aluminium, and the use of inflatable floats, made the equipment lighter. The inflatable principle is connected to the history of rubber boats, which also appeared in military service.
Floating platforms and docks
Not every pontoon was a bridge. Floating stages have been used for centuries where tides or river levels rise and fall, because a platform that rises with the water keeps its height relative to the boat. Landing stages in ports like Liverpool, which had a large floating stage in the 19th century, allowed passengers to board ships at any state of the tide.
Pontoons also support work in yards and harbours. Floating cranes, workshops and landing stages all rely on large hulls that do not move through the water under their own power. The principle also appears in floating dry docks, in which a pontoon with tanks can be sunk, a ship floated in, and the pontoon raised again, which links this subject to the story of dry dock construction. Seaplanes later used pontoons in the form of floats for landing on water, a different use of the same word.
The pontoon boat
The leisure pontoon boat is a 20th-century development, with its roots in North America. Usually it consists of a flat deck supported by two or three long tubes, with seats and sometimes a small cabin or canopy, powered by an outboard motor. Because the floats are wide apart, the boat is stable, and the deck offers much space for its size, which appeals to families and anglers on lakes and slow rivers. Accounts of who built the first one vary, and it is safer to say simply that such boats appeared in the middle of the 20th century and grew popular afterwards.
Many are designed for calm water rather than open sea, and their wide flat decks are not suited to heavy waves. In hull form, they have more in common with the old military float or the raft than with a conventional displacement boat, which makes them a late descendant of a very old idea.
One idea, three roles
The long history of the pontoon shows a simple principle used repeatedly: a float supports a platform, and platforms can be joined to span water. Whether the platform carries an army, a gangway or a picnic, the engineering underneath it is the same.