Who designed the ‘unsinkable’ Floating Batteries and for what purpose?


The success of the sortie raised the garrison’s morale and considerably lowered those of the French and Spanish forces. However, news of the British surrender at Yorktown, in America and of Minorca in the Mediterranean, gave the French and Spaniards fresh hope.
A competition was started to see who could devise the surest way of capturing Gibraltar. A French engineer, Jean Claude D’Arcon, came up with the best idea. His plan was to build specially designed floating batteries that he claimed would be indestructible. These floating batteries would fire at a given point on the sea wall until it caused a breach in the seaward defences – the batteries would concentrate their fire on the curtain wall between Montagu and King’s Bastion to cause a breach - and then specially constructed barges, fitted with landing ramps to transport assault troops would rush in through the breach and into the city.
Ten ships of the line were stripped down to their hulls and reconstructed at Algeciras according to D’Arcon’s design. The side facing the fortress was reinforced with an extra hull consisting of cork, tow and sand. Irrigation pumps would be used to keep the sand wet at all times and would prevent the hull catching fire from red-hot shot being fired against it. He then pitched the roofs and reinforced it with hides also kept wet by pumps in case the British lobbed mortars at the ships. He believed that in this way most bombs would roll off before exploding and even those some might explode, the thick layers of hide would contain the blast. The floating batteries would then be towed into
position to commence the attack. The four large hulks had 18-21 cannon and the six smaller ones had between 6-11 cannon each.

A cross section of one of D’Arcon’s floating batteries, after a diagram by G. Palao.

Eliott, Boyd and Green were well aware of the enemy’s
intention and began to prepare countermeasures. Three ideas which had been successfully developed during the course of the siege were put into practice.

  • Red-hot cannon balls would be fired on a very large scale using specially constructed grates to heat up the shot.
  • Warrant Officer Ince of the Artificer Company proposed to enfilade the new enemy batteries by firing from the new vantage points as seen from the tunnels being mined by his men.
  • Lt. Koehler’s new invention, the depressing carriage, would be used with devastating effect from Willis’s Batteries as well as the new galleries along the North Face of the Rock.

The Duc de Crillon, the French commander in charge, had set August 25th 1782 as the date for the attack, but
preparations were far from over. The batteries, which had been destroyed during the sortie along the isthmus, were not fully repaired and under constant attack by British gunners. D’Arcon meanwhile complained that his floating batteries were still not fully equipped.
He claimed that:

  • The irrigation pumps were not working properly.
  • The anchors and hauling cables had still not arrived.
  • The crews were not fully trained.

De Crillon, however, decided not to wait any more and so D’Arcon was forced to put his floating batteries to the test even though they were not ready.