Napoleon ordered Decrès to construct a prototype of a flat-bottomed boat, Andrew Roberts explains (in Napoleon: A Life), which could carry one cannon and one hundred men across the English Channel, and to contact Cambacérès, Lebrun, and Talleyrand to find individuals who would privately sponsor the building of these transports, which would be named after them.:
The flat bottoms of many of the boats, whose maximum draft was 6 feet fully loaded, meant that they could be run up on a beach, but although most were ready by the spring of 1804 they tended to ship water and sailed very badly unless the wind was dead astern, and south-eastern winds are rare in the English Channel. The pinnaces also needed to be rowed if they were not going dead ahead, which over 22 miles of sea would have been exhausting for the troops. Although a night attack was intended, a full eight hours of darkness came only in the autumn and winter, when the weather was too bad to risk a crossing in flat-bottomed boats. The Channel made up for its narrowness by its notorious unpredictability; there were sound logistical reasons why England hadn’t been successfully invaded since the fifteenth century (when she had been by land from Wales). By the early nineteenth she had the largest, best-trained and best-led navy in the world.
Napoleon was undeterred. On July 30, 1804 he told General Brune, we ‘only await a favourable wind in order to plant the imperial eagle on the Tower of London. Time and fate alone know what will happen.’
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All the leading French admirals – Ganteaume, Eustache Bruix, Laurent Truguet, Pierre de Villeneuve, as well as Decrès – opposed the English expedition as far as they reasonably could, chastened by the two Channel squadrons of over thirty British ships-of-the-line on permanent station.
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Napoleon and his senior advisors recognized that it would be impossible to send large numbers of men over on a single tide, and a surprise crossing in fog was also deemed too dangerous.
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The best strategy that could be devised on any of these occasions was the ruse of luring the Royal Navy away from the English south coast for long enough to cross the Channel. Yet the idea that the Admiralty Board in London could be induced to leave the narrows of the Channel under-guarded for even one tide was always utterly fanciful.
Napoleon wrote to Ganteaume on November 23, 1803 about the flotilla of 300 armed longboats (chaloupes cannonières), 500 gunboats (bateaux cannoniers) and 500 barges he hoped soon to have ready. ‘Do you think it will take us to the shores of Albion? It can carry 100,000 men. Eight hours of night in our favour would decide the fate of the universe.’
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Yet even if Napoleon had succeeded in getting ashore in Britain, Nelson’s return would have cut him off from resupply and reinforcement, and 100,000 men was not a large enough force with which to conquer 17 million waiting Britons, many of them under (admittedly makeshift) arms. Britain had undertaken intense preparations to repel an invasion from 1803 onwards: southern towns were garrisoned, fire beacons prepared; provisions stockpiled in depots located at places such as Fulham, Brentford and Staines, and every landing place was itemized from Cornwall to Scotland. Seventy-three small ‘Martello’ beacon towers were built along the south coast between 1805 and 1808, defensive breastworks were dug around south London, and some 600,000 men (between 11 and 14 per cent of the adult male population) were enlisted in the British army and Royal Navy by the end of 1804, with a further 85,000 in the militia.