Dunkerque-class battleship

The Dunkerque-class battleship was a type of warship constructed for the French Navy in the 1930s.

The Dunkerques were designed to counter the German pocket battleships. Their main armament was two quadruple 330 mm turrets forward, with a 225 mm thick armored belt. They were smaller, with a 26,500- to 27,300-ton standard displacement and a smaller main artillery caliber, than the battleships authorized by the Washington Naval Treaty, but their speed was 7 knots higher than all the battleships built from 1920 to 1937. When they were commissioned, only the last existing battlecruisers of the British Royal Navy and the Imperial Japanese Navy were their equals.

Given their characteristics, they were alternatively classified as fast battleships, small battleships, battlecruisers, and even as "ships of the line" (Fr. navires de ligne).

Two ships, FRENCH BATTLESHIP Dunkerque and FRENCH BATTLESHIP Strasbourg, were completed. Together they formed the 1ère Division de Ligne ("1st Division of the Line"), and saw service during the early years of the Second World War. While they never encountered the German pocket battleships they were designed to counter, they suffered the British attack of Mers-el-Kebir, and stayed under the Vichy authorities control until they were scuttled at Toulon in November 1942.

Background
In 1922, the Washington Naval Treaty imposed a ten-year moratorium on the construction of new battleships. But France and Italy were allowed to replace two old battleships after 1927, for a total of 70,000 tons.

In the late 1920s, the most powerful battleships had been designed before the Washington Treaty, and were armed with four double turrets of 15 in or 16 in diameter guns. The s, built between 1922 and 1927 with three triple 16-inch turrets forward, were based on the 1921 G3 battlecruiser concept. The top speed was, for most battleships of the time, 21 - 24 kn, although the s had 26.5 kn top speeds, and a few under Western flags, the fast battleships or battlecruisers, had top speeds exceeding 30 kn.

But neither France nor Italy intended to build battleships that were similar to the most recent American, British, or Japanese battleships, which were very heavily armed and armored. They wished only to modernize their aging battleships, refurbishing the propulsion machinery, and upgrading the main artillery, despite the Treaty of Washington authorizing them to undertake much more radical modernizations than could the other treaty powers. In the same way, both France and Italy reserved the right to employ their replacement capital ship tonnage allocation (70000 LT) as they saw fit, subject to Treaty limits – not only were two battleships of 35000 LT possible, but also three of 23000 LT or four of 17500 LT.

Early development concepts


In 1925, the first of the Italian s was laid down, and launched the following year – these ships could threaten French sea lanes between metropolitan France and colonial holdings in North Africa. In response, Vice Admiral Henri Salaun of the French Admiralty began discussing counters in 1926 and 1927. A 17,500-ton design was considered to allow four ships under the treaty limit. They would have two quadruple 305 mm (12-inch) turrets forward. The use of quadruple-gunned turrets had been planned for the cancelled and s, while the all-forward configuration was inspired by the British s. Speed would have been 34 to 35 knots, with armor to resist 203 mm shells. However, these ships were not ordered, as they could not withstand the old Italian battleships.

The 17,500-ton design was followed by studies for a 37,000 ton battlecruiser in 1927 to 1928. A trial displacement of 37,000 tons was equivalent to a standard displacement of 32,000 to 33,000 tons, which was close to the treaty limit for battleships. At least two draft designs were made, showing an enlarged with a 254 m long hull, 30.5 m beam, and two raked funnels. The main armament fire control director was atop a tripod foremast. Armament included three quadruple 305 mm turrets (two fore and one aft), eight single 90 mm Mle 1926 HA guns, twelve 37 mm Mle 1925 single AA mountings, and triple torpedo tubes. The armor belt varied from 220 to 280 mm in thickness. A hangar was behind the bridge structure, with two catapults abeam the second funnel. Power would have been from two groups of boilers and turbines, as on the, although with 12 (rather than 8) Guyot-du-Temple boilers, giving a total of 180,000 hp and a 33 knot top speed.

In 1928, an alternate 37,000-ton design emerged as a fast battleship, which had a heavier armament of three twin 406 mm turrets and four quadruple 130 mm turrets. The hull was slightly shorter at 235 m, but featured a slightly wider (31 m) beam. The armor was thicker, but engine power was reduced by one-third, allowing a top speed of only 27 knots.

The 37,000 ton battleships also did not go forward. The French Navy did not have a dock large enough to build a 35,000 ton hull that was longer than 250 m. For comparison, the 247 m long SS Île-de-France had been built at the civilian Penhoët shipbuilding yard at Saint-Nazaire. Building the required naval infrastructure would have cost the same as the two battleships, and interfered with existing building programs. Furthermore, more stringent naval restrictions were then being discussed. The Disarmament Subcommittee of the League of Nations had, by then, also turned their attention to naval limitations, and with the British pushing for a limit of 25,000 tons with 305 mm guns for battleships, the French did not want to jeopardize the negotiations.

In 1929 Vice Admiral Violette, then Chief of Staff of the French Navy, ordered a study for a "protected cruiser" from the Service Technique des Constructions Navales, the department responsible for naval construction. Their design had a displacement of 23,690 tons with three 305 mm turrets (one triple and one quadruple forward, and one triple aft), four twin 138 mm mountings (the caliber used on the most recent destroyers), and eight twin 100 mm turrets for antiaircraft use (as on FRENCH CRUISER Algérie, the latest heavy cruiser). Top speed would have been 29 knots, with the armor only able to withstand 203 mm shells. The final design resembled Algérie, with a distinctive forward tower with a single funnel abaft. This profile would carry over into the final Dunkerque design.

[[FRENCH BATTLESHIP Dunkerque]] emerges
French plans were upset when the Germans laid down the first in February 1929. Although the cruisers secretly displaced far more than allowed by treaty, they were nonetheless well-suited for commerce raiding, and could outrun all major capital ships (except for HMS Hood (51) and the two s of the British Royal Navy). The French immediately prepared draft designs for a counter to the Deutschlands. Protection against German 280 mm shells was the paramount requirement, although it seemed that the armament from previous projects and a speed of 30 knots might still be maintained. The end displacement was between 23,000 and 25,000 tons, which fit within the limitations being proposed by Britain.

The 1930 London Naval Treaty extended the "battleship holiday" from the Washington Naval Treaty until the end of 1936, although France and Italy were allowed to lay down new battleships in accordance with the older treaty. France, however, refused to adhere to the new restrictions – mainly related to cruisers – due to new German construction. In response, Italy also refused to accept these new restrictions. Bilateral negotiations between France and Italy took place in early 1931 to resolve the issue, and resulted in an agreement on 1 March that allowed each country to build only two 23,333 ton battleships until 1936. It was not possible to go any further, with the Italians being dissatisfied with their 23,333 ton design, and also studying a 35,000 ton design.

The French did decide to go forward with 23,333 tons as a compromise between an inadequately armored 17,000 ton ship and a prohibitively expensive 35,000 ton ship. The design was 213 m long with a 27.5 m beam, and was armed with two quadruple 305 mm/55 caliber turrets forward and three quad 130 mm Dual Purpose turrets aft. It had a 230 mm armored belt with 150 mm horizontal protection, and was designed for a speed of 30 knots. This was submitted to Parliament in May, but was rejected after two months of debate – it seemed excessive to counter the (official) 10,000 ton Deutschland with a ship of twice that size. In July, the navy was allowed to carry out revisions for resubmission.

Vice Admiral Durand-Viel became Chief of Staff in January 1931. He ordered a study replacing the 305 mm/55 guns with 330 mm/50 guns so as to outmatch the old Italian battleships. This increased displacement (by 3,000 tons) to 26,500 tons, length by 2 meters, beam by 2.5 meters, and reduced speed to 29.5 knots. Two more double 130 mm DP turrets were added, and the thickness of the armored belt and decks were also increased slightly. This design was approved by Parliamentary committees in early 1932. The FRENCH BATTLESHIP Dunkerque was finally ordered on 26 October, and laid down on 24 December.

Responses to Dunkerque
The Germans response to Dunkerque were the, both laid down in 1935. They were larger and better armored, but had only nine 280 mm guns. These guns were improved versions of the type used by the Deutschland class, with longer barrels and higher muzzle velocity. A heavier armament had been pre-empted by negotiations for the 1935 Anglo-German Naval Agreement. As the Dunkerque-class ships were designed to withstand 280 mm shells, the French saw no need to reply with a new class.

Rather, it was the Italians who provoked the next class of French capital ship. On 26 May 1934, Duce Benito Mussolini announced that Italy would exercise its treaty right to build new battleships – this was confirmed a few days later when Agenzia Stefani reported the laying down of two 35,000 ton s. The French almost immediately (on July 16) ordered a second Dunkerque, the FRENCH BATTLESHIP Strasbourg, with increased vertical armor. She was laid down in November. Also in the meantime, the French began work on their own 35,000-ton design, which would emerge as the.

Design
Dunkerque was an innovative design, notable for concentrating the main armament of eight 330mm/50 Modèle 1931 guns forward. As built, FRENCH BATTLESHIP Dunkerque had a standard displacement of 26,500 t, a 215 m length, a 31 m beam, and a 8.5 m draught. Her propulsion machinery developed 107,000 shp, allowing a speed of 29.5 knots, with the armor belt at a maximum thickness of 241 mm (9¾-inches). FRENCH BATTLESHIP Strasbourg was the same except for heavier vertical armor, with a displacement of 27,320 t, and 0.15 m more draught.

Dunkerque bore distinct similarities to the battlecruiser HMS Tiger (1913), which was discarded in February 1932 due to the London Naval Treaty. This explains why Henri Le Masson considered the Dunkerque ships as being closer to battlecruisers than to battleships. The 28500 LT British battlecruiser had eight 34.3 cm Mk V guns, a 660 ft length, a 90 ft beam, a 32 ft draught, and 108,000 shp for 29 knots. Dunkerque differed mainly by having an all-forward main artillery arrangement (which saved 27% on turret armor), and a more modern propulsion plant. This allowed the French ship to have 10 cm more armor (330 mm versus 229 mm) on its turret faces, and to sport horizontal armored decks.

Similarities could also be seen when compared to the British s. Both classes saved weight by locating the machinery further aft to cut shaft length, and used inclined armor belts to increase their effective thickness. Both classes concentrated the secondary battery astern for superior firing arcs, and placed the main fire control atop the bridge structure. However, the two designs reflected different objectives. The Nelsons prioritized firepower and protection over speed, with the Dunkerques being the opposite. Dunkerque reflected the evolution of French design, demonstrated elsewhere by the changes from the s to the cruiser FRENCH CRUISER Algérie.

Main artillery
The quadruple arrangement had been foreshadowed in the French and  projects, just before World War I. The all-forward main artillery arrangement had been first introduced by the Royal Navy on the s, but these warships had only three turrets carrying nine guns, and the angles of fire for the rearmost guns were limited by the turrets to their front, as it had not seemed possible to accept the supplementary weight of armor above the main armored deck for the barbette of a third turret in superposition of a second turret yet raised.

Retained on Dunkerque, the two quadruple 330 mm foreturret arrangement gave unrestricted forward fire. Therefore, the entirety of the main artillery was able to fire forward, as the ship closed on her enemy, in an angle where she made the smallest possible target. When Dunkerque  was laid down, it was more powerful than any existing German or Italian warship, and was intended to be engaged as part of a scout wing of the slower and heavier British battlefleet, so the concept of the main artillery concentrated in the bow seemed justified. Six years later opinions had changed, and the French Admiralty came back to a main artillery arrangement on the bow and the stern (on the Gascogne battleship project), and in 1940 June, in the tactical situation of the battle of Mers-el-Kebir, this main artillery disposition was actually a severe handicap.

The biggest drawback of the quadruple turret was that a single unlucky shot that immobilized one of the turrets would effectively put half the main artillery out of action. So the French quadruple turrets of both the Dunkerque and s were divided internally to localize damage, and this was proved effective when Dunkerque, at Mers-el-Kebir, was stricken by a 381 mm shell on the upper 330 mm turret, which put out of action only the right-half turret.

In order to avoid the possibility of a hit which simultaneously damaged both turrets, they were positioned 27 m apart from one another - there were 19 m between the A and B turrets, and 23 m between B and C turrets, of the s. The diameter of a barbette is all the more large because of the number of turret guns and their caliber. With a 32 m beam, the s supported barbettes for three-406 mm gun turrets. The 1911 French designers of the s had thought that it was possible to install quad 340 mm turrets on ships with a beam of 27 meters. The Dunkerque designers resolved that quad 330 mm turrets were the maximum possible with 31 m beam. Nevertheless, the four barrels were not mounted independently in individual mounts because this would have meant an unduly large barbette diameter. For that reason, the right- and left-hand pair of barrels were each placed in a common mount. This was not the case on the fore and aft 14-inch quadruple turrets of the British King George V-class battleships, which had a 34 m beam. On Dunkerque and Strasbourg, as later on in the FRENCH BATTLESHIP Richelieu and Jean Bart, the guns of the half turrets were so close (1.69 m) that a "wake effect" between shells fired simultaneously by a half-turret was leading to excessive dispersal. This was not corrected before 1948 on the FRENCH BATTLESHIP Richelieu.

The weight of one quadruple 330 mm turret built by Saint-Chamond was 1,497 tons, nearly the same weight as one triple 381 mm turret of the. The maximum gun elevation was of 35°, the muzzle velocity was 870 m/s, and the range at maximum elevation was 41,500 m, giving a relatively flat trajectory. The nominal rate of fire was one round every 40 seconds (1.5 rounds per minute), which could be increased to one every 30 seconds (or 2 rounds per minute) under ideal conditions. The turrets were intended for loading at any elevation angle, although as it occurred, shells jammed in the breech at higher elevations when the other guns were firing, and thus as a practical matter, loading took place at a 15° elevation. The rate of elevation was 6°/s, and the rate of train was 5°/s.

The 330 mm shell was 1.65 m long and weighed 570 kg, which was nearly twice the weight of the 280 mm shell (300 kg), or of the  class AP shell (336 kg). The weight of Italian battleship shells was 452 kg for the 305 mm guns, and 525 kg for the 320 mm guns, after reconstruction. The British battleships which bombed the French battleships at Mers-el Kebir fired 875 kg shells. The 330 mm shell was Armor Piercing Capped (APC), registered in the French Navy as Obus de Perforation Modèle 1935 (OPf Mle 1935) existing in two variants, OPf and OPfK, the later OPfK variant incorporating a dye bag and fuze (dispositif K) to color (red in Dunkerque, green for Strasbourg) not only their splashes but their hits, and thereby facilitating spotting for ships operating in formation while in combat. A High Explosive (HE) variant of shell, referenced OEA Mle 1935 (Obus Explosif en Acier), could have been designed and tested, but would have not been ordered, as no reference of this shell has been found in the war ammunition inventories of the ships.

The heavy guns caused a great deal of difficulty – when firing slightly aft of beam, the ship's command position was troubled by the noise of the explosion, the fire, and the smoke to such an extent that the full arc of fire could not practically be utilized. The absorption of the great recoil forces also presented difficulties, and these ships were very lightly constructed, and they suffered repeatedly from damage caused by the firing of their own guns.

Secondary artillery
The secondary artillery was dual purpose, anti-ship and long-range anti-aircraft, for the first time on capital ships, as was followed by the Royal Navy on the King George V-class battleships. Three quadruple armored turrets (turrets V, VI, and VII), weighing 200 tons, were aft, one axial (turret VII) on the aircraft hangar, and the others (turrets V and VI) laterally disposed, abeam the after superstructure. Like the 330 mm turrets, the quad 130 mm turrets had a 20 mm steel bulkhead dividing the turret in two independent half-turrets, in which the two guns were placed on the same mount with distance-apart gun axes of 0.55 m. The two double turrets (turrets III and IV) amidships had only 20 mm anti-splinter plating.

In the anti-surface mode, the 130 mm guns were firing 33.4 kg Semi Armored Piercing (SAP) shells (referenced in the French Navy as OPfK 130 mm Mle 1933), with a muzzle velocity of 800 m/s, out to a maximum range of 20,800 m at an elevation of 45°, against aircraft, and 29.5 kg time-fused HE shells (OEA Mle 1934) with a muzzle velocity of 840 m/s, with a ceiling of about 12,000 m at the maximum elevation of 75°. The rate of fire was 10–12 rounds per minute. The maximum training speed was 12°/s, and the maximum elevating speed was 8°/s.

However, these guns didn't have sufficient power for their anti-ship mission. The 130 mm (5.1 inches) guns had been used in single mountings (Mle 1919 and 1924), as an anti-ship battery on the destroyers of the and  classes (commissioned 1926 to 1931). The later and s (commissioned after 1929), however, were fitted with stronger 136.8 mm (5.5 inch) guns, also in single mountings (Mle 1923 and Mle 1927), and the 1929 «protected cruiser» project was designed with anti-ship 138.6 mm LA guns, in dual mountings, which were actually installed (Mle 1934) for the first time on the s in the late 1930s.

The Axis navies (Kriegsmarine and Regia Marina) fitted their battleships with distinct batteries for anti-ship and anti-aircraft purposes, preferring the 150 mm caliber for the anti-ship artillery on the and the s, and the 152 mm/55 Model 1934 or 1936 on the.

In the anti-aircraft mode, the 130 mm Mle1932 guns were considered to have poor efficiency against close-rapid aircraft (i.e. dive bombers) due to its too-slow rate of fire of 10 rounds per minute. The US Navy and the Royal Navy opted for dual-purpose batteries, and chose calibers equivalent to the French 130 mm caliber – slightly less, 127 mm (5 inch) for the US Navy, or slightly more, 133.3 mm (5¼ inch) for the Royal Navy. The US Mark 12 127 mm/38 caliber gun – used in dual mountings on the and  battleships, s, and many cruisers, had a higher rate of fire of 15 rounds per minute (and even 22 rounds per minute during short periods), but it was usually under the control of the advanced Mark 37 Gun Fire Control System, which provided accurate and timely firing solutions against both surface and air targets.

The Royal Navy 133.3 mm Dual Purpose guns, first used on the King George V-class battleships and later on in the s, originally had characteristics equivalent to the French 130 mm guns, but they were found reliable only on the last unit of the King George V battleship class, the HMS Anson (79), and on the Bellona sub-class cruisers, after they were linked to the improved RP10 and RP10Mk2 fire control, which increased training and elevating speeds to 20°/s, and with the High Angle Control System in its best performing versions.

Dunkerque-class battleships were also the first French capital ships to have Remote Power Control (RPC) for training and elevating on the main and secondary turrets, but the Sauter-Harlé-Blondel RPC training gear proved unreliable, and the system never worked properly. A so-called "«electromagnetic detection device»", the French ancestor of radar, were to have been fitted on the Strasbourg in 1942, as on the Richelieu as early as 1941, and also on the Jean Bart, but only for sea and air warning, and not as a gunnery fire-control radar.

Anti-aircraft artillery
With five 37mm CAD Mle 1933 semi-automatic mountings and eight 13.7 mm machine guns in quadruple mountings, these ships lacked the numerous quick-firing light artillery that was mounted on the Richelieu during her refit.

The 37 mm/50 guns were frequently used as light anti-aircraft batteries on the French Navy warships in the late 1920s. In the single mount CAS Model 1925, they were fitted on the 10,000 ton heavy cruisers and destroyers. Hand-loaded, using 6-round box magazines, and even in the twin semi-automatic CAD Model 1933, the rate of fire – which was intended to be 20 to 30 rounds per minute – was practically only of 10 to 15 rpm. This appeared to be too slow as compared to the rate of fire of guns of equivalent caliber, such as the British Pom Pom gun or the Swedish designed Bofors 40 mm/L60 gun, the rate of fire of these being between 120 and 200 rpm. To address such issues, it was decided to develop a new gun 37 mm/70 Model 1935, with an automatic AA twin mount (ACAD Model 1936), linked with one fire control director (or two mountings for one fire control director). The intended rate of fire was 165 to 172 rpm. The development of this gun and its twin automatic mounting proved to be more difficult than was thought, and thus it appeared that it would not be possible for the Dunkerque to be fitted with these before her completion.

During her trials, Dunkerque received only six 13.2 mm/76 quadritube Model 1929 Hotchkiss mountings (amidships and abeam tower). At the time of completion, she was fitted with six single 37 mm CAS Model 1925 mountings, which were replaced in early 1939 by four twin semi-automatic 37 mm CAD Model 1933s (two abeam turret II and two between the funnel and the aft tower, with four directors, two above the fore pair of 37 mm mountings, and two just before the second pair). A fifth 37 mm CAD Model 1933 was added in August 1939 on the Dunkerque, located abaft the 130 mm quad turret VII (centerline aft). Strasbourg received only four twin 37 mm semi-automatic CAD Model 1933 mountings, and instead of the fifth twin 37 mm mounting, received a quadruple 13.2 mm MG mounting.

Fire control
A massive fore control tower was, for the first time, fitted with an internal lift and topped by three conductors (numbered n°1, 2, and A from top down), which were mounted on the same axis. The accumulation of heavy weight (85 tons) so high up in the top was felt to be noteworthy. When Dunkerque  was torpedoed at Mers-el-Kébir on 6 July 6, 1940 this proved to be a shortcoming, as the main fire control directors were unseated from their ball races because of the "whiplash" effect on the mast around which they were mounted.

A secondary control tower, topped by two conductors (numbered n° 3 and B), was also positioned abaft the funnel.

Range finding
For the 330 mm battery, there was a stereoscopic triplex 12 m OPL (Optique de Precision de Levallois-Perret) rangefinder, which was replaced in 1940 by a 14-meter model, in director A. This system weighed 40 tons, and was placed in a lower position atop the fore tower. A stereoscopic duplex 8 m OPL range finder in director B was located in a lower position atop the aft tower, while two stereoscopic duplex 12 m OPL range finders were located one in each main artillery turret.

For the 130 mm battery, there was a stereoscopic duplex 6 m OPL range finder for the anti-ship gunnery in director 2 which weighed 25 tons in a central position, and a stereoscopic duplex 5 m OPL range finder for the anti-aircraft gunnery in director 1 which weighed 20 tons in an upper position, both atop the fore tower, and a stereoscopic duplex 6 m OPL range finder for director 3 was located in an upper position atop the aft tower, with three stereoscopic duplex 6 m OPL range finders, one for each quad 130 mm turret.

In addition to the main gunnery directors, a stereoscopic OPL 5-meter tactical rangefinder (to be used by the flag staff) was located atop the conning tower of Dunkerque. On Strasbourg, it was located atop the fore tower, and the silhouette slightly different, as it integrated the conning tower and the bridge installations.

Watch
For the optical watch, at the lower level (veille basse), for close sea targets, there were three lookout positions port and starboard, on the first deck abeam the bridge. For aerial targets, the middle lookout station (veille moyenne) was on platform 3 of the forward tower, having five lookouts on both sides of the ship. Against mines and torpedoes, the upper lookout station (veille haute) was on platform 8 of the forward tower, with five lookouts on both sides of the ship.

For night firing, there were seven searchlight projectors on Dunkerque, four on a raised platform around the after base of the funnel, two on platform 6 on the rear of the fore tower, and one in a forward position on the same level of the tower. Strasbourg had six searchlight projectors, four in the same positions as on the Dunkerque, abaft the funnel, and two in forward positions on the fore tower.

Aviation facilities
The aircraft installations (hangar, catapult, and crane) on Dunkerque were particularly complete and well-designed, and were major advances over the facilities retrofitted to older battleships in the 1920s. For example, on the fast battlecruiser HMS Hood, a stern catapult was fitted in 1929, but which had to be removed in 1932 as it was frequently awash in the North Atlantic heavy seas (due to the lack of freeboard aft). A single 22 m long trainable catapult was located along the center line on the quarterdeck. It was operated with compressed air, and could launch a 3.5 ton aircraft at 103 km/h. Adjacent to the hangar, there were workshops for repair and maintenance.

The aircraft were Loire 130 single-engined flying boats. Two planes were to be stowed, wings folded, in a two-tier hangar on the two platforms of a lift, and a third on the catapult, with wings deployed, and the possibility to stow a fourth plane atop the aviation hangar. The planes were moved on rails from the hangar to an elevator, then placed on the catapult. At the end of its mission, the Loire 130 landed in the sea and taxied alongside the ship. It was then lifted aboard by a recovery crane with a capacity of 4.5 tons.

The aviation fuel was stowed in tanks located in the upper part of the stern. These incorporated certain safety features, including refrigeration and sprinkler systems and the replacement of used fuel by an inert gas.

Armor protection
The proportion of the weight of the protection, relative to the design displacement, reached 35.9% in the ship, which was the highest value recorded until then (the armor weight was only 34% of design displacement on the s).

The protection design in the class was also very modern, as it used the "all or nothing" armor scheme, unlike contemporary German warships. The citadel, being about 126 m long, corresponded to about 60% of the ship's length, which while very similar to the 57% figure of the Nelson battleship class, left unprotected a long forward part of the ship.


 * The belt armor was designed to withstand shell hits from the 280 mm German naval guns. Inclined 11°30', it was 225 mm thick, with the fore bulkhead 210 mm, and the aft bulkhead 180 mm
 * the main deck was 115/125 mm, and the lower deck 40 mm
 * the conning tower was 270 mm on its face and sides, 220 mm on its back, and 150/160 mm on its hood
 * the main turret barbette was 310 mm, with face inclined to 30°, with 330 mm on its rear, 345 mm on turret 1, 335 mm on turret 2, and 150 mm on its roof
 * the quad 130 mm turrets had 120 mm on the barbette, 135 mm on the face, 80 mm on the roof, and 20 mm on the double 130 mm turrets

The French Navy Board had recommended that the second unit of the Dunkerque class had an improved vertical armor, and this was implemented.


 * On Strasbourg, the armored belt had a 283 mm thickness and was inclined 11°50', the forward bulkhead was increased to 260 mm, and the after bulkhead to 210 mm
 * For the 330 mm gun turrets, the barbette armor was increased to 340 mm, on the forward faces to 360 mm, on the turret I rearwall to 352 mm, on the turret II rearwall to 342 mm, and on the roof to 160 mm

Thus, the protection weight on Strasbourg was increased by 749 tons, and reached 37.2% of the normal displacement.

Underwater protection
The underwater protection consisted of a "sandwich" of void spaces, light bulkheads, liquid loading compartments, compartments filled with rubber-based, water-excluding compound called ébonite-mousse, and a heavy internal holding bulkhead to absorb the explosion of a torpedo warhead.

The compartment outboard of the inclined armor belt had a maximum depth of 1.5 m and was filled with ébonite mousse. Inboard of this compartment, there was a 16 mm bulkhead, enclosing a void compartment 0.9 m deep, then an oil fuel bunker 3.9 m deep, then a 10 mm bulkhead containing a void compartment 0.70 m deep, backed by a 30 mm torpedo bulkhead of special steel. Abeam the magazines fore and aft, the torpedo bulkhead thickness was increased to 40–50 mm, and the fuel bunkers were replaced by a same-depth compartment filled with ébonite-mousse. The maximum depth was around 7.5 meters. This figure exceeded the depth of underwater protection on existing battleships, where it was no more than 5 meters.

This underwater protection proved to be effective enough when, at Mers-el-Kébir on 6 July 1940, fourteen of the forty-four depth charges carried on the patrol ship Terre-Neuve, which were equivalent to 1,400 kg of TNT or eight air-launched torpedo warheads, exploded within a few meters of the Dunkerque hull.

Propulsion
The speed of a warship depends not only of the power of her propulsion plant, but also her hydrodynamic characteristics, which vary with the ratio of length to beam. But the length of the ship also has consequences on the displacement, because of the weight of the hull itself, but also because of the weight of the hull armor.

With a main armament of eight twin 15-inch (381 mm) gun turrets, HMS Hood (51) had a 42,000 ton displacement, a 262 m hull, and a propulsion machinery developing 144,000 shp, yielding a 31 knot maximum speed. The s had a more powerful armament, but with a weight-saving arrangement of triple turrets on a shorter citadel, and stronger protection. But as her displacement was limited to 35,000 tons, the hull was only 216 m long, and with machinery space reduced. The propulsion plant power of 42,000 shp allowed only a 23 knot speed.

The requirements imposed on the French designers of Dunkerque were different – a 26,500 tons displacement, allowing accommodation of an armament and a protection able to counter the fire of pocket battleships. However, a 215 m hull could be built, with its longest part (200 m), in the building dock of Brest Navy Yards and, imperatively, with a maximum speed of 29.5 knots. The propulsion was assured by six Indret boilers and four Parsons turbines on Dunkerque. On Strasbourg, the Indret boilers were built under license by the Penhoët Shipbuilding Yards. These small-tube boilers were operated at a pressure (temperature) of 27 kg/cm² (350°C). They were 5.33 m long, 5.34 m high, and 6.50 m wide, installed two boilers side-by-side in three boiler rooms.

Boiler Room 1 was underneath the fore tower, with (from starboard to port) boilers n°10 and n°11, followed by the forward Engine Room housing the geared turbines for the wing shafts. Boiler Rooms 2 and 3 were adjacent beneath the funnel, with boilers n°20 and n°21 in Boiler Room 2, and boilers n°31 and n°32 in Boiler Room 3. The after Engine Room housing the turbines for the center shafts was abaft Boiler Rooms 2 and 3. A 18 mm bulkhead separated the forward Engine Room from Boiler Room 2, dividing the machinery plant in two independent units. This "unit machinery" layout was considered essential for relatively lightly-armored ships to enable them to continue operating following action damage, and was first introduced by the French Navy in the first Treaty cruiser, the FRENCH CRUISER Duquesne.

In each Engine Room there were two sets of turbines, each driving a three-bladed propeller with a diameter of 4,20 m, on Dunkerque, and a four-bladed propeller with a diameter of 4.045 m on Strasbourg. Each set comprised a two High Pressure (27 kg/cm²) turbines (HP1 and HP2), a Medium Pressure (8.5 kg/cm²) turbine, and Low Pressure forward and reverse turbines, which were linked in series with HP 1 as a cruise turbine. The ship could steam at 15.5 knots on two shafts, and 20 knots on four shafts, at one-quarter power, and with HP 2 being engaged at between 34% and 50% of maximum power. Four turbo generators of 900 kW each were distributed between the engine rooms.

Designed horsepower was 107,000 shp, for maximum speed of 29.5 knots. During Dunkerque's speed trials, in May 1936, 30 knots were sustained over eight hours, with 114,000 shp and 30,000 tons displacement, and 31.6 knots were reached with 135,585 shp forcing. In July 1938, during the trials of Strasbourg, the results were similar at 30 knots while developing 115,000 shp, and 30.90 knots with 131,960 shp forcing.

The maximum fuel load for peacetime cruising was 4,500 to 5,000 tons, but in wartime, this figure was reduced to 3,700 tons to maximise the effectiveness of the underwater protection system, as filling the liquid-loading compartments to the brim creates additional pressure on bulkheads, instead of absorbing the pressure of an explosion. The radius of the class was 7,850 nmi at 15 knots, 6,300 nmi at 20 knots, and 2,450 nmi at 28.5 knots.

During the Dunkerque sea trials, it appeared that funnel smoke interfered with the use of the aft control tower rangefinders. Thereafter she was fitted (between March and May of 1938) with a more important funnel cap, nicknamed as a "bowler hat". Strasborg received the same funnel cowling in September and December 1938. War service records showed that the ship's bow suffered damage in the rough seas of the North Atlantic winter, according to one of her Flag Officers. The German s had the same problem, particularly during the winter of 1939 to 1940, even after they had been fitted with a reinforced "Atlantic bow" in the period 1938 to 1939.

Prewar service
Dunkerque was laid down in the Brest Navy Yard on 24 December 1932, in the Salou graving dock n°4. She was floated out on 2 October 1935, and the ship with 17 meters missing (as the graving dock was only 200 m long) was towed to Laninon graving dock n°8, where her bow was fitted. Sea trials were carried out from mid-April 1936 to late April 1937. In mid-May 1937, she represented France in the Naval Review of 1937, at Spithead, Portsmouth, to mark the coronation of King George VI and Queen Elizabeth. On May 27, a further review took place off the Isle of Sein, where the French Mediterranean and Atlantic squadrons were assembled following combined exercises. Dunkerque hosted there the Navy Minister and the new Chief of Staff of the French Navy, Vice Admiral Darlan. On September 1, 1937, Dunkerque joined the Atlantic Squadron as the flagship of Commander-in-Chief Vice Admiral Gensoul, and to take part in various exercises with the Atlantic Squadron in 1938.

Strasbourg was laid down on the n°1 slipway of the civilian Penhoët Shipbuilding Yards at Saint-Nazaire, which had been built to accommodate the 313-meter long keel of the liner Normandie. She was launched in December 1936, to be fitted out, and left Saint-Nazaire for Brest on 15 March 15, 1938 for acceptance trials, which were carried out. Strasbourg entered service in April 1939, joining the Atlantic Fleet, and formed with Dunkerque the 1ère Division de Ligne ("the 1st Division of the Line"). White bands were painted on the funnel in March 1939 – a single one on Dunkerque (as Division flagship), and with two on Strasbourg.

After an official visit on 3 and 4 May to Lisbon (Portugal) for the commemoration of the discovery of Brazil by Alvares Cabral, both battleships – accompanied by three modern light cruisers of the 7,600-ton type and forming the 4th Cruiser division, visited in late May and early June, Scottish harbors and Royal Navy bases, Liverpool, Oban, Staffa, Loch Ewe, Scapa Flow, and Rosyth, and returning to Brest after a four-day call at Le Havre.

During the "Phony War"
In the first days of September 1939, the Force de Raid – under Vice Admiral d'Escadre (Squadron Vice Admiral) Gensoul on Dunkerque, and including the 1 st Line Division, the 4th Cruiser division, and eight large destroyers – was based in Brest, and sortied immediately as German «pocket battleships» were reported (wrongly), to be trying to pass in the Atlantic. However, it was soon decided to split the Force de Raid into hunting groups against the German surface raiders, and incorporating some Royal Navy warships.

In October–November 1939 Force X, under Vice Admiral Duplat, on the French heavy cruiser FRENCH CRUISER Algérie, with FRENCH BATTLESHIP Strasbourg, French heavy cruiser FRENCH CRUISER Dupleix, and the HMS Hermes (95) (aircraft carrier) was based at Dakar, and vainly undertook sweeps against the GERMAN CRUISER Admiral Graf Spee. When Strasbourg returned to Brest, 800 of her powder charges remained stored in Dakar. This powder was, during the battle of Dakar, used for the FRENCH BATTLESHIP Richelieu, and was subsequently wrongly incriminated in the explosion of the Richelieu's upper turret 380 mm barrels.

In November and December, the FRENCH BATTLESHIP Dunkerque and the French 4th Cruiser Division, joined HMS Hood (51) to intercept, under orders of Vice Admiral Gensoul, the GERMAN BATTLESHIP Scharnhorst and the GERMAN BATTLESHIP Gneisenau, which had sunk the Armed Merchant Cruiser HMS Rawalpindi north of the Faroe Islands on 23 November 1939. As the German battleships renounced to break into the Atlantic, Dunkerque suffered bow damage in a huge North Atlantic storm and had to be docked for repairs. The Dunkerque, in December 1939, took part in the shipping to Canada of a part of the Banque de France's gold reserve.

Faced with the dubious Italian attitude during the spring of 1940, the Force de Raid was dispatched, on 2 April 1940, to the Mediterranean, but was ordered to return to Brest some days later to cover the Allied reaction to the German landings in Norway on 9 April 9, 1940. Finally, the Force de Raid was definitively transferred to Mers-el Kebir (24 April 1940).

Mers-el-Kébir
The only test in battle for the FRENCH BATTLESHIP Dunkerque and the FRENCH BATTLESHIP Strasbourg came in the attack on Mers-el-Kébir, after the fall of France, from the Force H battleships, HMS Hood (51), HMS Revenge (06), and HMS Valiant (1914), though the French battleships had not been designed to confront these heavily armed battleships.

In the late afternoon on 3 July 1940, the surprise was terrific – strategically, the French crews did not believe, up to the last moment, that the British warships were actually going to fire on them, and tactically, because their ships were tightly moored, their guns trained towards land, and as the British warships were free of their movements, ranged at 16,000 m, in a high sea.

As the old super-dreadnought FRENCH BATTLESHIP Bretagne, struck in her magazines, capsized and sank, killing nearly 1,000 seamen, Dunkerque, after painfully breaking her mooring ropes, quickly suffered four-15 inch shell hits. The first shell rebounded off the upper 330 mm turret roof, killing all the men in the right half-turret (the left half-turret remained operational). The second shell damaged the aircraft installations, and the last ones – piercing the armored belt – damaged boilers and destroyed the electric power plant. Therefore, the ship had to be moored on the other side of Mers-el-Kébir roadstead.

FRENCH BATTLESHIP Strasbourg, escorted by five destroyers, headed to the pass, only narrowly missed by the 15 in shells of the British guns. Steering east-north-east at 28 knots, she escaped the pursuit by HMS Hood (51) and the Fairey Swordfish torpedo bombers of HMS Ark Royal (91), reaching Toulon in the evening of the following day.

The armored belt of the Dunkerque was proved unable to resist the 381 mm shells, but the damage was not as deadly as might have been feared, because the British fire ceased after less than twenty minutes, the French admiral having signaled that he had ordered his ships to cease firing. Admiral Esteva, Amiral Sud, Commander in Chief of the French Navy in North Africa, in a later radio message to the French Admiralty, told of «moderate» damage, and boasted that Dunkerque would soon be able to return to Toulon on her own steam. The British Admiralty, knowing this, ordered Admiral Somerville, the Force H Flag Officer, to attack again, to put Dunkerque permanently out of action. With Dunkerque being beached just in front of a village, and Admiral Somerville fearing that gunfire might cause serious collateral damage to civilians, an attack was launched with torpedo bombers on 6 July. Unluckily again, one of the torpedoes hit a small patrol ship moored alongside Dunkerque that was carrying forty-four depth charges. The explosion of fourteen depth charges ripped an enormous hole in the battleship's hull, and Dunkerque sank in shallow water. As the 330 mm magazines had been ordered to be flooded at the beginning of the air attack, Dunkerque escaped a total loss. The total number killed on Dunkerque during the two attacks of 3 and 6 July 1940 reached 210.

Scuttling at Toulon
With FRENCH BATTLESHIP Bretagne lost, and the FRENCH BATTLESHIP Dunkerque and FRENCH BATTLESHIP Provence severely damaged, the FRENCH BATTLESHIP Lorraine interned at Alexandria, the FRENCH BATTLESHIP Courbet and the FRENCH BATTLESHIP Paris seized in Great Britain, and with only four heavy cruisers (of seven), and three light cruisers (of twelve) under Vichy control in Mediterranean waters, and with the Atlantic harbors under German occupation, a reorganisation of the Vichy French naval forces had to be carried out. The 1st (fast battleships) and 2nd (slow battleships) Divisions de Ligne and the Atlantic Fleet were dissolved in August 1940, and the new Forces de Haute Mer (High Sea Forces) were created, with Admiral de Laborde appointed as Commander in Chief on 25 September 1940. He raised his flag on FRENCH BATTLESHIP Strasbourg after the upper part of the fore tower had to be fitted out to better accommodate an admiral's staff. Vice Admiral Gensoul – though he received the fifth star of a full Admiral – never got a further sea command.

The Strasbourg, flagship of the "High Seas Forces", nearly never even went to sea, due to the stringent restriction of fuel, but she was eventually sent to cover, in November 1940, with four cruisers and some destroyers, the return of the Provence to Toulon. IN 1941, she received three more single 13.2 mm Browning CAS MG, and was fitted in 1942 with a détecteur électro-magnétique, the French predecessor of a sea and air warning radar.

In February 1942, having been summarily repaired, the Dunkerque returned under her own steam to Toulon, where she was dry-docked.

After the Wehrmacht had occupied the Zone libre in retaliation for the successful Allied landings in North Africa, the Dunkerque and the Strasbourg were scuttled at Toulon, on 27 November 1942, when the Germans attempted to seize the French warships that remained under Vichy control.

The Strasbourg, having been salvaged by the Italians and partly dismantled, was returned by the Germans to the Vichy authorities after the Armistice between the Allies and Italy. On 18 August 1944, she was bombed and sunk by US Air Force aircraft, as the Allied forces, having landed three days previously, advanced to liberate Toulon. The Dunkerque was left in a very decrepit condition, and she and the wrecked Strasbourg were scrapped after the war.