Souburg Air Base near Flushing (hereafter referred to as AFB Flushing), together with Flushing harbour, its naval facilities, and the vessels anchored there, constituted prime strategic targets. Since the Germans expected French and British forces to land in Zeeland, the Luftwaffe allocated considerable resources to conducting frequent air raids against the area.
The port of Flushing was defended by two anti-aircraft platoons, each equipped with four heavy machine guns, in addition to the coastal artillery batteries. These AA machine-gun platoons were positioned within the harbour area itself. The harbour was further protected by a coastal battery armed with three 7.5 cm guns and two naval 3.7 cm guns.
During the evening of 10 May, the first French units to arrive reinforced the anti-aircraft defences of Flushing with six Hotchkiss 2.5 cm AA guns. Air defence in the area was further strengthened by the Dutch naval vessels present in the harbour, several of which possessed medium anti-aircraft batteries. Consequently, the greater part of the anti-aircraft capability around Flushing came from naval rather than land-based forces.
The harbour and roadstead of Flushing were occupied by a small number of larger Dutch naval vessels, including the light cruiser Hr.Ms. Sumatra and two modern gunboats, as well as two small and quite obsolete brown water gunboats, Hr.Ms. Bulgia and Hr.Ms. Vidar, both armed with 3.7 cm guns. Also present were the modern large minelayers Hr.Ms. Hydra and Hr.Ms. Van Meerlant, each equipped with three 7.5 cm guns, together with two auxiliary minelayers armed only with 12.5 mm heavy machine guns.
A number of smaller auxiliary and support vessels were also stationed in the harbour, most fitted with either 3.7 cm or 5 cm deck guns. In addition to the naval and militarised civilian vessels, the harbour and anchorage at Flushing contained many civilian ships and boats of various sizes, including several large ferries and mailboats.
The dedicated anti-aircraft armament of the Dutch naval vessels present in Zeeland was generally rather limited, with the exception of the cruiser and both modern gun boats. Only these three larger surface units possessed dedicated anti-aircraft batteries.
The light cruiser Hr.Ms. Sumatra was equipped with six 4 cm Bofors guns and eight 12.7 mm Vickers machine guns. Hr.Ms. Flores carried a single dual-purpose 7.5 cm gun together with four 12.7 mm machine guns, while Hr.Ms. Johan Maurits van Nassau mounted two 4 cm Bofors guns, two twin-mounted 12.7 mm Vickers machine guns, and four 7.9 mm Vickers machine guns.
Taken together, this did not constitute particularly impressive anti-aircraft firepower. It was supplemented only by several additional heavy machine guns aboard the smaller vessels and by the two land-based anti-aircraft platoons stationed in the harbour area. Some of the small army detachments guarding the harbour installations could, if necessary, also employ their heavy machine guns in an anti-aircraft role. But heavy machine guns typically had little effect beyond 1,5 km height and in fact were only effective against very low flying or diving aircraft.
Heavy anti-aircraft artillery of 7.5 cm calibre was poorly represented in this sector and so were the medium weapons. Apart from the battery near Air Base Flushing, all anti-aircraft weaponry - whether naval or land-based - consisted solely of light or medium-calibre guns. Enemy aircraft operating above approximately 4,500 metres therefore enjoyed a considerable degree of freedom of action.
During the early morning, the harbour of Flushing came under attack from German Junkers Ju-88 medium bombers. At the same time, Heinkel He-115 seaplanes laid magnetic mines in the Scheldt estuary, while Messerschmitt Bf- 110 fighters repeatedly carried out low-level strafing attacks against targets of opportunity. Two German aircraft were shot down during these initial operations. One He-115 minelaying aircraft crashed into the Western Scheldt after being hit by gunfire from Hr.Ms. Johan Maurits van Nassau, while a Bf-110 fighter, heavily damaged over Flushing Air Base, was forced to make an emergency landing near Goes.
The German magnetic mines presented the Royal Netherlands Navy with an entirely new challenge. Although the more modern Dutch warships were equipped with degaussing systems (1) - or were regularly degaussed as a passive protective measure against magnetic mines - the Dutch minesweeping fleet lacked vessels capable of effectively clearing such weapons from the waterways. This represented a serious and arguably unforgivable shortcoming in Dutch naval procurement and planning during the interwar years.
Fortunately for the defenders, the mines had all been dropped without proper knowledge and thus inaccurately; most landed outside the deeper navigational channels. Later, following the arrival of French and British minesweepers specially equipped to deal with magnetic mines, several of the devices were successfully destroyed. Nevertheless, these insidious weapons would still exact a toll on shipping in and around Flushing and the Western Scheldt.
(1) German magnetic mines were equipped with sensors capable of detecting disturbances in the Earth’s magnetic field caused by passing ships. When a sufficient distortion of the magnetic field was registered, the mine detonated. In shallow waters, the resulting explosion could be devastating, often enough to destroy or seriously damage a vessel. Magnetic mines were therefore particularly effective in coastal and shallow-water environments. Because these mines relied on the Earth’s magnetic field for activation, different calibrations were required for operations in the Northern and Southern Hemispheres, where the polarity of the Earth’s magnetic field is reversed.
The Germans measured magnetic field strength in gauss, from which the term degaussing was derived. The earliest degaussing methods involved dragging an electrical cable carrying a strong current along the hull of a ship. This process altered and neutralised the vessel’s magnetic signature and became known as “wiping the ship.” However, ships gradually acquired new magnetic charges simply by operating at sea within the Earth’s magnetic field. As a result, they had to be “wiped” regularly. Later, larger warships were fitted with permanent degaussing systems using electromagnets. These systems maintained a stable and controlled magnetic profile, thereby minimising the distortion detected by the mines’ magnetic sensors and reducing the likelihood of detonation.
Minesweepers capable of clearing magnetic mines first had to possess a minimal magnetic signature themselves. Ideally, such vessels were constructed with wooden or otherwise non-steel hulls in order to reduce the risk of triggering the mines. The principal sweeping method used during the early stages of the war involved towing a heavy electrical cable carrying pulsed currents behind a minesweeper or, preferably, suspended between two sweepers in what became known as the “Double L Sweep.” The electromagnetic field generated by the cable simulated the magnetic signature of a passing ship and detonated the mines at a safe distance.
The Dutch naval minesweepers in 1940 were not only built of steel, but also lacked the specialised equipment required to sweep magnetic mines effectively. By contrast, the British and French navies already possessed a number of vessels equipped for this task. The Allies had begun developing countermeasures because they had first encountered the latest German magnetic sea mines as early as September 1939.
In addition to naval sweepers, the Allies also employed aircraft fitted with degaussing coils. These aircraft could fly at low altitude over shallow coastal waters where magnetic mines were suspected, using their electromagnetic equipment to trigger and neutralise the mines from the air.
The larger Dutch naval vessels were ordered to leave the port of Flushing on 10 May. The light cruiser Hr.Ms. Sumatra was immediately dispatched to Yarmouth in England, while both Hr.Ms. Flores and Hr.Ms. Johan Maurits van Nassau were ordered to sail for Rotterdam, initially to provide artillery support against Waalhaven Air Base that had been seized by German airbornes. Hr.Ms. Flores would later return to Flushing, becoming the only major Dutch naval vessel remaining in this theatre.
Two brand-new North Sea ferries were also quickly commissioned, brought to operational readiness, and sent to England. These were Hr.Ms. Koningin Emma and Hr.Ms. Prinses Beatrix. During the war, both vessels were converted for amphibious operations and taken over by the Royal Navy. Renamed HMS Queen Emma and HMS Princess Beatrix, they participated in numerous commando raids, including the infamous Dieppe Raid of August 1942. Both ships served the Allied cause throughout the war and survived. In 1946, they were returned to their Dutch owners.