Written by
James Davies

The Dunkerque

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Overview

With the end of the First World War the French navy was in a much weakened state.  40% of its ships had been lost during the war, and not replaced due to priority being given to the army.  In the immediate post-war years financial constraints prevented any large ships from being built, and construction focused on destroyers and cruisers.  Tentative approval to the construction of new battleships as part of an overall naval programme was given in 1924, and design studies began in 1926.  Germany’s announcement of the construction of pocket battleships in 1928 provided a focus for designers, and the new French battleships were intended to counter this threat.  Approval for construction was finally given in late 1931.

The main guns were chosen to be 330 mm (13 in), and it was decided to mount them in two quadruple turrets forward of the superstructure.  This gave considerable weight saving over a quadruple turret design by reducing the length that needed armour protection and reducing the total number of turrets.  It also had the advantage of allowing the full weight of fire to be used in a chase situation, as it was expected that the pocket battleships would attempt to flee when confronted with such an obviously superior vessel. The potential vulnerability of this arrangement was reduced by having an armoured bulkhead between the two turret halves, and by having separate magazines for each turret half.  To address the potential weakness to attacks from astern the superstructure was designed to maximise the arc of fire available to the guns.

The selection of secondary armament was somewhat revolutionary.  They were the first capital ships to choose HA/LA (high-angle / low angle) dual-purpose secondary armament, for use both as anti-aircraft and anti-surface weapons in stead of the traditional separate secondary guns for surface targets and smaller tertiary guns for air targets.  This had the advantage of giving more guns for either air defence or surface defence, provided that both were not required at the same time.  As with the main guns, these were placed in quadruple mounts and separated in to half-turrets by an armoured bulkhead.  The guns were 130 mm (5.1 in) calibre and were placed at the stern of the ship to provide some protection in that area from surface attack.  Close range protection against aircraft was provided by ten 37 mm (1.5 in) cannon in five double mounts, as well as machineguns.

The armour gave the ship a theoretical immunity zone of 16,600 to 28,400 m (18,160 to 31,060 yds) against a 280 mm (11 in) gun, meaning that at ranges below 16,600 m a 280 mm shell could be expected to penetrate the side armour, and above 28,400 m a 280 mm shell could be expected to penetrate the deck armour.  It must be stressed that this is a theoretical calculation, as in practice several factors (particularly the sea state) affect the actual resistance to shells at the instant they hit, and the immunity zone is constantly changing as the ship heaves, rolls and pitches.  It’s also worth noting that the armour for the Dunkerque was designed to resist the relatively small calibre guns of the pocket battleships, rather than the much larger guns of true battleships.

The ship was designed for 29.5 kts at ‘normal’ (design) displacement, following the normal design principle that the ship should be able to out-run anything that it could not out-fight.  This gave a 2 kt advantage over the German pocket battleships, and matched the speed of contemporary cruisers.  The design was very fuel efficient, and sufficient fuel was provided to allow over 16,000 nautical miles at 17 knots, enabling the navy to meet its worldwide colonial commitments.

Hangar space for two Loire 130 seaplanes was provided, along with a single catapult and a single crane, on the stern of the vessel.  A third aircraft could be carried on the catapult, however this is not believed to have occurred in practice.  The Dunkerque was the first capital ship to have aircraft included as part of their initial design, and they were intended for reconnaissance and to spot the fall of shot.

These ships were well suited to their chosen role of protecting French commerce worldwide from the German pocket battleships.  Faster, better armed and better protected than the pocket battleships there is little doubt that they would have overwhelmed them in an engagement.  There were some minor shortcomings, however: the action at Mers el Kébir confirmed their vulnerability to battleship-calibre guns (although it confirmed the wisdom of the armoured bulkhead dividing the turret halves), and the loading arrangement of the secondary armament was not a success.  Nevertheless, many of the design concepts were brought forward in to later classes of French battleships, and they influenced the design of both the British and the Italian ships of the time.

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