Flushing harbour and the navy
The AFB Souburg at Flushing [hereinafter called AFB Flushing] and the harbour with its maritime facilities and vessels were prime strategic targets - especially since the Germans expected French and British forces to land in Zeeland - and as such the Luftwaffe had reserved quite considerable forces for frequent raids in this area.
Flushing port was defended by two AA platoons with four heavy machineguns each and coastal guns. The two AA machinegun platoons were stationed in the harbour. One coastal artillery battery with three 7,5 cm guns and two navy 3,7 cm guns had been situated in the harbour. In the evening of 10 May the first arriving French units reinforced the Flushing AA land defences with six guns of 2,5 cm. The air-defences were supported by navy units, of which some had medium AA batteries. As such the better part of the AAA measures came from sea, not from land.
The harbour and road-stead of Flushing were occupied by a small number of larger Dutch navy units [one older light cruiser and two modern gunboats] and two small obsolete river gun-boats [HrMs Bulgia and HrMs Vidar, both with 3,7 cm guns]. Two mine-laying vessels [HrMs Hydra and HrMs Van Meerlant], quite well armed with three 7,5 cm guns each and two auxiliary mine-laying vessels, obviously lightly armed with only 12,5 mm MG's. A number of even more insignificant facility and auxiliary ships were present too, all fitted with either a 3,7 cm or 5 cm deck gun. Obviously - aside from navy and militarized civil vessels - plenty of bigger and smaller civil ships and boats were moored in and outside Vlissingen harbour, amongst which several bigger ferries and mail-boats.
The AA armament of the few Dutch navy ships was quite poor, with exception of the cruiser. Only the three bigger surface units had some dedicated AA batteries on board. The HrMs Sumatra light cruiser had six 4 cm Bofors and eight 12,7 mm Vickers machineguns. The HrMs Flores had one multi-role 7,5 cm gun and four 12,7 mm MG's and the HrMs Johan Maurits van Nassau had two 4 cm Bofors guns, two twin mounted 12,7 mm Vickers and four 7,9 mm Vickers machineguns. All in all not a very impressive fire-power that was only assisted by a few more heavy machineguns on board other ships and the two land based AA platoons. Some of the small army units stationed along the harbour as guarding troops could - if necessary - also apply their heavy machineguns for AA duties. The heavy [7,5 cm] AAA was poorly represented in this sector, since next to the battery near the AFB Flushing all AAA - sea- or land-based - was of light and medium calibre. Enemy above and beyond 3,500 meter range was therefore quite free to operate.
War breaks out
In the morning Flushing harbour was attacked by German Ju-88's medium bombers. Also magnetic mines were dropped in the Scheld by He-115's and Bf-110 fighters kept on coming down strafing spotted targets. Two planes were shot down. One German He-115 mine laying plane crashed into the Westerscheld after being hit by a volley of the HrMs Johan Maurits van Nassau and a Bf-110 fighter had to make an emergency landing near Goes after sustaining heavy damage over Flushing AFB.
The German magnetic mines posed a new challenge to the Dutch navy. Although the modern ships were fitted with degaussing devices (1) or frequently wiped (degaussed) in order to (passively) protect them from triggering these devices, the sweeping fleet of the Royal Dutch Navy was not equipped with vessels that were able to effectively clear these magnetic mines from the waters. An unforgivable strategic flaw of navy material management during the interbellum. Fortunately enough the mines were all dropped off target and fell outside the deeper waterways. Later - upon the arrival of French and British mine sweepers that were equipped for sweeping magnetic mines - some dropped mines could be destroyed. Still, a price of the shipping around Vlissingen would be demanded by these devious devices.
(1) Degaussing device: German magnetic mines were fitted with a sensor that detected changes in the earth magnetic field around the device. Should enough difference be detected [distortion of the earth magnetic field] the mine set off. The blast would kill a ship, especially in shallow waters. As such the mines were most effective in shallow waters. Obviously - since the earth Magnetic field was mandatory to trigger the device - different devices were required for the southern and northern hemisphere due to the reversed earth magnetic field.
The Germans used "Gauss" as a measurement of strength of the magnetic field and as such "degaussing" was born. First appliances of degaussing were done by dragging an electrical cable along the ship's hull with large quantity of amps flowing through it. It induced the magnetic field of the ship. This method was called "wiping the ship". Yet ships tend to pick up magnetic charge from the earth magnetic field by simply sailing the seas. As such ships had to be wiped on a regular basis. Later bigger units were fitted with electromagnets to guarantee a continuous and stable (non)magnetic profile that saw to minimizing the distortion of the "North Pole down" indicator of the mines...
Mine sweepers that were able to sweep this kind of mines first needed to have a bias magnetic profile themselves. Preferably they were constructed out of wood or any non-steel hull. The sweeping device used in the early days of the war was the towing of a heavy pulsed electrical cable behind the sweeper or - preferably - between two sweepers [called the "Double L Sweep"]. The Dutch navy sweepers in 1940 were not only made of steel but also they lacked the devices to sweep magnetic mines. The British and French navies were equipped with some of these devices. The fact that the Allies had already taken active counter-measures was caused by the fact that they already became acquainted with the latest German magnetic sea-mines in September 1939. The Allies also used airplanes fitted with degaussing coils, which could fly rather low over the shallow waters in which mines were expected.
Evacuation of maritime units
The bigger Dutch navy units were ordered to leave Flushing port at 10 May. The light cruiser HrMs Sumatra was sent to Yarmouth [England], whereas both the HrMs Flores and HrMs Johan Maurits van Nassau were ordered to set sail for Rotterdam [at first instance to shell Waalhaven AFB]. HrMs Flores would later return and as such become the only remaining larger Dutch navy unit present at Flushing.
Two brand new Northsea ferries were also manned, steamed up and sent to England. It were the HrMs Koningin Emma and the HrMs Prinses Beatrix. These ships were later adapted for amphibious operations and commandeered by the Royal Navy. Re-baptised as the HMS Queen Emma and HMS Princess Beatrix they would serve during commando raids and during the notoriuous operation at Dieppe [August 1942]. Both ships would serve the Allied cause throughout the entire war and survive. In 1946 they were returned to their Dutch owners.