About 80 per cent of the 11.5 metre (38ft) vessel had survived for 1,800 years - buried under 5ft of mud near Newport, Gwent.
Study of the boat will shed new light on ancient Celtic shipbuilding technology. For although third-century AD Britain was ruled by the Romans, boats were often constructed according to native Celtic tradition.
Ancient Celtic boats and ships were built around a frame. Shipwrights constructed the wooden skeleton of a vessel first, and then attached the hull planking to that frame as a modern shipbuilder would.
This Celtic technology is believed to have been lost before being reinvented in the Mediterranean in about AD1000. It reappeared in north-west Europe only in the late 15th century.
The Welsh vessel was discovered in a silted-up former tributary of the river Severn, and was almost certainly used for carrying cargo across the estuary between South Wales and Somerset.
It would have had a normal speed of three to five knots and almost certainly had a flax or leather sail - either of square design, or more probably a more sophisticated 'fore and aft' one. Built of oak, it was capable of carrying up to three tons of cargo. The vessel must have had a long life, for there is evidence of heavy wear and tear. The compressed plant fibres used to plug leaks have survived.
The flat bottom of the boat was built of four large planks attached to a frame of slender skeleton timbers. At its broadest point, the vessel was probably about 2.5 metres wide; and with a depth of just under a metre, it must have sat low in the water when laden.
The archaeologists, directed by Nigel Nayling of the Glamorgan-Gwent Archaeological Trust, found the boat lying next to the remains of a stone and timber Roman bridge or jetty. The discovery occurred during the construction of a new Tesco distribution depot at Magor near Newport. The company has invested pounds 50,000 in excavating the vessel, which it has donated to Newport Museum. Five Romano-British leather shoes, pottery, two bronze Roman coins, and horse, sheep and cattle bones were found near by. Several years of conservation work will be carried out on the boat before it goes on public display. Newport Museum is seeking funds to ensure its preservation.
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