The Suffren class was an interwar treaty cruiser built by France for the French Navy. The design was based on the preceding and traded speed for protection while retaining the same armament. The first ship, Suffren, was completed to this design. The following ships, Colbert, Foch, Dupleix, were completed to a modified design with heavier secondary armament and rearranged topside. The ships entered service from 1930 to 1933, with Suffren being the sole survivor of the Second World War.
Prior to the London Naval Treaty, the French Navy classified cruisers as armoured (croiseur cuirasse) or light (croiseurs legers); afterwards cruisers were divided between first class (croiseur de 1ere classe) and second class (croiseur de 2e classe). The Suffren was initially classified as a light cruiser, and then as a first class cruiser on 1 July 1931.
Design and description
Hull and protection
The design of the Suffren was based on the while maintaining the standard displacement of 10.000 tons. The hull maintained the high freeboard with the raised forecastle of all post-war cruisers with excellent sea keeping qualities. The hull design also kept the fine lines for the speed requirement complemented by the clipper bow with a gentle sheer and the marked flare keeping the forecastle dry even in heavy weather. The framing in a French hull was numbered from aft to stem, frame numbering was based on the distance from the aft perpendicular in meters. The longitudinal framing was used for cruisers throughout the interwar period. The hull was between the perpendiculars with an overall length of and a beam of . At her normal displacement of 11,769 tonnes the draft of the ship would be .
The protection was increased over the previous Duquesne. The armour protection in the hull for the Duquesne was only 370 tons whereas Suffern would have 670 tons. The upper deck was thick amidships with bulkheads running to within of the sides enclosing the funnel uptakes and ventilation trunking between the main and first deck. The magazines were enclosed in an armoured box with sides and crowns, forward and aft bulkheads. The turrets and conning tower had plating. The roofs of both the turrets and range finders was of cast nickel. The steering gear was housed inside an armoured box with sides and an top. A Belt of made of non-cemented 60 kg steel was installed to a height of and below the waterline abeam of the machinery spaces in Suffren and Colbert only. The space between the machinery bulkhead and the inside of the hull was approximately . For Suffren and Colbert this space was filled with 500 tons of coal for the cruising boilers and therefore did not count against the standard displacement.
Reinterpreting the rules of the Washington Naval Treaty, the STNC decided to change and improve the level of protection on the next cruiser of the series, Foch. The interpretation was for Washington standard displacement to mean a peacetime load versus what would be required for a full wartime load. With the concept of shells striking the sides above the shallow armoured belt being able to penetrate to the machinery spaces, it was decided to do away with the belt and replace it with an armoured box around the machinery. The sides of this armoured box was with the deck above of the armoured box was . The bulkheads of the box varied in height from to . The protection on the sides of the magazines was increased to with all other protection remaining the same. The last cruiser of the program, Dupleix would be protected along the same lines as Foch.but with a slight increase. All longitudinal bulkheads were increased from to on the magazines and machinery spaces. The main deck over the machinery spaces was increased from 18 mm to . The upper deck however, was decreased from to . The magazine crowns were also increased to .