Propelled on the surface by two diesel engines producing a combined 6,000 horsepower (4,474 kW), they had a maximum speed of 18.6 knots (34.4 km/h; 21.4 mph).

When submerged, their two electric motors produced a combined 2,250 horsepower (1,678 kW) and allowed them to reach 10 knots (19 km/h; 12 mph).

Also called “deep-cruising submarines”, their range on the surface was 10,000 nautical miles (19,000 km; 12,000 mi) at 10 knots (19 km/h; 12 mph).

[3][9] On 8 August 1940, the French Navy put a reorganization into effect which placed Poncelet and Persée in the 6th Submarine Division and transferred them to Dakar in Senegal.

[3] On 2 September 1940, Poncelet got underway from Dakar with the aviso Bougainville and the banana boat Cap des Palmes, which was loaded with troops and supplies.

[12] British forces provided cover for the landings, and at 06:30 Alpha Time on 7 November the Royal Navy heavy cruiser HMS Devonshire[11] — flagship of the British task force commander, Admiral John Cunningham[13] — launched a Supermarine Walrus biplane flying boat to search for Poncelet.

[11] It returned at 07:45 Alpha Time and reported that Poncelet was anchored off Port-Gentil, 8 nautical miles (15 km; 9.2 mi) and bearing 138 degrees from Cape Lopez.

[11] Poncelet had put about a quarter[3] or a third[11] (according to different sources) of her crew ashore to reinforce the Vichy French garrison at Port-Gentil,[11] but she received orders to attack the transports carrying the Free French invasion force[12] off Libreville[3] with the crew she had on board, and she set out toward the Baie des Baleiniers.

[11] At 17:00 Alpha Time, Milford reported herself at 00°11′S 008°57′E / 0.183°S 8.950°E / -0.183; 8.950 and that Poncelet was 6.5 nautical miles (12.0 km; 7.5 mi) distant, making 16 knots (30 km/h; 18 mph) and steering a course of 39 degrees.

[11][14] Poncelet attempted to fire a second torpedo, but it got stuck in its tube, gave off toxic smoke into the interior of the submarine, and created a leak.

[11] The British light cruiser HMS Delhi received orders at 18:01 Alpha Time to close with Poncelet and put a prize crew aboard her.

[11] Although Delhi received orders to assist in the rescue, Milford brought aboard all 54 survivors — three officers and 51 enlisted men.

[11] Milford transferred Poncelet′s survivors to Delhi at 07:45 Alpha Time on 9 November 1940 while a Walrus from Devonshire flew over the scene to provide antisubmarine cover.

[18][19] Chemnitz — the merchant ship Poncelet captured in September 1939 — was renamed Saint-Bertrand in honor of de Saussine after his death.