The fleet in which Henry V transported 12,000 soldiers and up to 20,000 horses across the Channel to a famous victory at Agincourt 600 years ago was considerably less spectacular than early historians claimed: not a stunning assembly of 1,500 ships, but less than half that, a fleet of mainly English ships with foreign vessels hired or commandeered to join the expedition.

Craig Lambert, a historian at the University of Southampton, has gone back to original sources, including English exchequer rolls in the National Archives at Kew, to work out how the king got his army across to France. The fleet sailed from Southampton on 11 August 1415, more than two months before the victory over a larger French army on 25 October – St Crispin’s Day – immortalised by Shakespeare.

He concluded that the proud boast by the anonymous St Albans chronicler of the “Deeds of Henry the Fifth”, who claimed the king sailed with 1,500 ships, was wildly incorrect. The chronicle was written by a witness, possibly a chaplain who sailed with the fleet and saw the battle.

Lambert said: “Later historians have seized on this figure because it was based on an eyewitness account, but a fleet of 1,500 ships would have been ridiculous. And – I’m going to get shot down for this – Henry is always praised as the great naval technician, but he was not an innovator in any way, he was following precisely the example of his predecessors, who knew exactly how to raise a fleet to transport an army.”

Shakespeare has the young king, enraged by the taunting gift of tennis balls from the Dauphin of France, vow: “I will rise there with so full a glory / That I will dazzle all the eyes of France / Yea, strike the dauphin blind to look on us”, and conjures up a magnificent and terrifying vision of the fleet as “a city on the inconstant billows dancing / For so appears this fleet majestical”.

Lambert believes that Henry eventually assembled about 650 ships, with about 50 more put to sea to protect the fleet. He spent more than £7,000 from February to May hiring ships from Holland and Zealand, a vast sum but one that may have brought in no more than 245 ships, fewer than Henry had hoped. In March, his admiral, the Earl of Dorset, was told to commandeer all English ships over 20 tons and get them to Southampton by May. In April, further orders were issued to seize all ships including foreign vessels from the mouth of the Thames to Newcastle, and from the ports along the south coast to Bristol. In July, ships in London over 20 tons were also seized.

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Detailed records including ship names and crew numbers which survive from earlier campaigns are missing for Agincourt, but Lambert went back to the authorised payments for ship hire and crew wages to calculate the size of the fleet. Foreign ships were hired at three shillings, four pence a ton for every three months, plus the crew’s wages, while English ships were requisitioned – despite repeated bitter protests by ship owners – at two shillings a ton plus wages.

However, Henry did not need an enormous fleet. Lambert, who will present his findings at a major international conference at the university next weekend to mark the 600th anniversary of the battle, points out that, almost a century earlier, Edward III needed fewer than 750 ships to get a larger army of 14,000 men to France.

He excuses the anonymous chronicler of the Gesta Henrici Quinti – the Deeds of Henry the Fifth – for overestimating the size of the fleet in his excitement at what he was seeing.

Lambert said: “If the fleet was spread out over a wide area we can understand why the author of the Gesta, keen to convey the sheer scale of what lay before his eyes, overestimated the size of the fleet. As he stood on the deck of a ship, he would have witnessed the fleet grown in size by the hour as each section gradually coalesced to form one large armada.”

When the fleet sailed in August 1415 – a town on the water rather than Shakespeare’s city – Lambert said it would still have been a magnificent spectacle. Henry himself was on board his ship, Trinity Royal, and five other royal ships joined the fleet. He added: “We know the king’s ships were freshly painted and decorated with depictions of animals and the royal coat of arms, and these brightly coloured ships would have sparkled in the summer’s morning as they sailed towards Harfleur and into the annals of history.”