Along either bank of the Estuary, as well as in and around the port of St Nazaire itself, were some seventy pieces of ordnance, varying in calibre from 20mm quick-firing cannon up to the 240mm railway guns of Battery Batz, west of La Baule. To defend the approaches of the estuary were heavy batteries of 280 Naval Artillery Battalion, close by the Gun Battery and Naval Radar Station on Chémoulin Point. The defence of the port itself, were waiting three battalions of the 22nd Naval Flak Brigade at St Marc.

Ranging in calibre through 75, 150, 170 and 240mm, the coastal guns were arranged in battery positions, mainly along the northern shore of the estuary, close to which lay the deep-water channel which any ship of substance must use in order to reach the port. These fixed emplacements began at the estuary mouth, and ran eastwards as far as the Villès-Martin – Le Pointeau narrows, at which point the sea-space was a mere 2.25 sea miles, and beyond which lay the province of Mecke’s Flak Brigade.

Approaching the estuary mouth in their attack formation of two long parallel columns extending over almost 2,000 metres of sea, with the gunboat in the van, and Campbeltown steaming between the leading troop-carrying MLs, the ‘Chariot’ force would find itself entering into a perfect trap - from which it would be difficult to escape from. On their starboard beam and guarding the southern extremity of the estuary shore would be the 75mm guns of Battery St Gildas; while to port and guarding the north, would be the railway guns just inland from the Pointe de Penchâteau. Fine on their starboard bow, as they approached across the shallows, would be the guns of Battery Le Pointeau, backed by the 150mm searchlight ‘Yellow 3’; fine to port would be the cluster of batteries comprising the 150mm guns of Battery Chémoulin and the 75mm and 170mm cannon of the cliff-top position close by the Pointe de l’Eve. Backing the cliff-top emplacements the 150cm searchlight ‘Blue 2’.

It was to divert the attention of these defences that the diversionary air raid had been proposed, for without it the ‘Charioteers’ would be forced to rely on luck, their low silhouettes, their unexpected line of approach and such devices that might confuse the enemy into mistaking them for a friendly force. With or without the bombers, this passage of the outer portion of the estuary would be fraught with danger, including that from mines, patrol vessels and possibly even Schmidt’s destroyers.

Every sea-mile gained towards the target without the alarm being raised would be a triumph. Assuming the ‘Charioteers’ made it to the narrows, they would then be passing into the restricted throat of the estuary, less than two sea-miles from their target, but with the full weight of the three flak battalions ranged close by them on either hand. The lighter, dual-purpose weapons of 703, 705 and 809 Battalions, made up of 20 and 40 mm with a sprinkling of 37s, would be able to switch quickly from air to surface targets.

COHQ’s original concept of approaching by stealth had been constructed around the premise that their crews must be far too busy firing skywards to worry about the seaward approaches to the town. Running past 703 Battalion and backed by large searchlight ‘Blue 1’, the ‘Charioteers’ would come within easy range of the defences both of the outer harbour and of the Pointe de Mindin on their starboard beam and where the searchlights and 20mm cannon of the 809 Battalion would be situated. With the range short, the ‘Charioteers’ would also be able to reply - but would also be at their most vulnerable. The air raid plan had been designed to reach its crescendo during this period.

If the diversion should succeed, then the force might just reach the dockyard intact, at which point Campbeltown would race for her caisson, the columns of MLs would break to port and make for their own landing points.