Naval Assessment -1940

The first line of defence against the German invasion flotillas was the East Coast Mine Barrage. On the outbreak of the war it was laid from off Scotland to the Humber to guard against invasion from German ports only. It was planned to extend it south in December 1939 to cover the Thames estuary but work only really began on this extension in May 1940. The final barrier was 30-70 miles offshore and about 60 miles deep numbering 35,000 mines. Heavy ships would not be able to cross it without minesweeper support but shallow draft craft such as barges and E boats could. Another anti-invasion minefield was laid between Cornwall and Ireland in August 1940.

The East Coast Mine Barrage and other
defensive minefields


An Admiralty assessment of the invasion risk in May 1940 considered that Germany would select landing places which would give the greatest chance to the landing of the first wave. This would be the area in which her bombers and fighters could support the invasion – as at May this was between The Wash and Newhaven. A seaborne invasion could be delivered by:

• Ordinary transports which would require port facilities to unload
• Transports of a special type that were self sufficient in unloading
• Special landing crafts
• Surface warships
• Submarines

Support to the invasion force could be:

• Strategical – an attempt to draw off Royal Naval vessels by attacks on commercial shipping with submarines or the use of surface ships away from the selected invasion area. Mining would also be likely either defensively to protect the area of operations or offensively by blocking Royal Navy fleet bases. Submarines could also be used to intercept Royal Navy vessels sailing to engage the invasion fleet.

• Tactical – close escort was expected by destroyers and motor torpedo boats although it was considered unlikely that Germany would use cruisers for close support. Cruisers may however be used to give wider coverage along with submarines.

The Royal Navy could attack the invasion fleet either (a) before departure, (b) during passage or (c) at its point of arrival. To attack before departure would require good intelligence of the likely embarkation ports (which could then be easily bombed from the air or the approaches mined) while if the invasion fleet set sail it would require accurate reconnaissance to detect it or keeping at sea continually a large number of flotillas which the Navy simply did not have. It was therefore considered that the main dispositions of the fleet should be to deal with the invasion fleet at its point of arrival and secondly to intercept it during passage.

The most suitable vessels to strike at the landing force at its point of arrival were destroyers and motor-torpedo boats supported by smaller inshore vessels. Ports which would be suitable to base a striking force to attack any landings between The Wash and Newhaven were Dover, Sheerness, Harwich, Humber and Plymouth. The striking force could be supported by heavy ships such as ARETHUSA-class cruisers and A/A cruisers and sloops. Should the enemy attempt to force the Straights of Dover, the most suitable ports to base forces would be Portsmouth and Dover. These forces would be unsupported by heavy ships so it was considered necessary to base as many “R” class battleships as available at Plymouth. Smaller vessels should be employed to keep watch in inshore waters.

To meet an (i) invasion threat on the East Coast, (ii) attack on Ireland or (iii) break out of German ships northwards as a diversion the Home Fleet made the following dispositions in June 1940: 

  • Two cruisers at Sheerness (GALATEA and ARETHUSA)
  • Four destroyer flotillas under C.IC. Nore (consisting of 36 vessels), one flotilla each at The Humber, Harwich, Sheerness and Dover.
  • Southampton-class Cruiser at the Humber
  • At Rosyth – NELSON and RODNEY and two flotillas. These could sail south to deal with any invasion as they were the best two protected ships of the Royal Navy.
  • At Scapa – HOOD, RENOWN and GLORIOUS and 1 ½ flotillas.
  • Clyde – all available 8” cruisers. These would not be employed in the southern North Sea due to their vulnerability to air attack.
  • ARK ROYAL and one 8” cruiser at sea working westward between Ireland and Iceland
  • At Plymouth – VALIANT and REPULSE
    •   Above: Heavy ships of the Home Fleet. Left - HMS Nelson. Right - three Cruisers

      If the enemy attempted to invade Ireland the ships at Plymouth and the 8” cruisers on the Clyde would be ideally placed to deal with the situation. The deployment of heavy ships changed frequently – for example both ARK ROYAL and VALIANT were dispatched to the Mediterranean to immobilize the French Fleet at Oran on Jun 27th.

      The invasion flotillas were expected to be preceded by “barrage breakers” and minesweepers and escorted by dive bombers, E boats and submarines. It was expected the enemy’s heavy forces - modern cruisers and destroyers - would be deployed further north but their appearance in the southern North Sea should still be anticipated. Smoke and gas clouds may be used to help the invasion.

      The enemy was expected to send out a large force regardless of losses – some would get through. An attempt would be made to seize a port, most likely using airborne troops but a direct attack may be carried out by vessels specially adapted for rapid disembarkation. Numerous conveys were to be expected all with large air escorts; some would be decoys. It was also thought that Germany would not commit such a large force without the full support of her navy.

      The bulk of the Navy’s destroyers were at sea at night on anti-invasion patrols and at rest during the day. Hence they could encounter and engage any invasion fleet sailing during the night and be at most three hours sailing form invasion beaches during the day. As well as engaging enemy vessels, destroyers could also shell the enemy forces which had succeeded in landing on the beaches. Close fighter support would be required and arrangements were put in place to call for immediate fighter support.

      A fleet of 700 armed patrolling vessels was available for reconnaissance and 200 to 300 were always at sea between the most threatened area between The Wash and Newhaven. In addition an Auxiliary Patrol of 400 trawlers and other smaller vessels was at patrol between Invergordon and Plymouth. A weakness of this patrol was that it would have been relatively easy for the enemy to locate and destroy these vessels (as happened off the Isle of Wight on one night when four were sunk) – any prelude to invasion would probably have involved mopping up these vessels during the dark hours.

        Above left: Minesweepers. Right: Auxiliary Patrol vessels on patrol

      A mine sweeping force of 25 fast mine sweepers and 140 mine–sweeper trawlers operated between Sunderland and Portsmouth to maintain a searched channel between these two ports. They also operated as additional look-out vessels.

      Two submarines were also on patrol between Havre and Cherbourg. Motor torpedo boats carried out sweeps of the Dutch rivers.

      In August the Admiralty issued orders that no heavy ships would proceed into the Narrow Seas unless enemy heavy ships were operating in support of the invasion fleet. Gen Brooke commented on this in his diary: “I soon discovered that the Home Fleet, in the event of an invasion, had little intention of coming further south than the Wash. As destroyers were also been drawn off to protect the Western Approaches, the naval defence in the Channel and southern waters did not appear to be … to offer the required interference with German landing operations”.

      However, if enemy heavy ships were reported the Navy’s heavy ships would move south as quickly as possible to engage them. In September when invasion seemed imminent, the heavy ships based at Scapa were ordered to Rosyth where they remained until November.


      Admiralty papers on invasion, TNA
      Cabinet papers, TNA
      The Turn of the Tide, A Bryant, Collins, 1957
      The British Navies in The Second World War, Admiral Sir W.M. James, G.C.B., Longmans, Green and Co Ltd, 1947


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