Source: Historical Section, COMNAVEU. "Administrative History of U.S. Naval Forces in Europe, 1940-1946." vol. 5. (London, 1946): 301-337 [This manuscript, identified as United States Naval Administrative History of World War II #147-E, is located in the Navy Department Library's Rare Book Room.]

CHAPTER VII
DEFENSIVE MEASURES -- NEPTUNE OPERATION

PART I
ENEMY NAVAL DISPOSITIONS, 1944

1. Before pursuing the adventures of the assault forces, the comprehensive measures executed for their protection and support will be reviewed. The protection of NEPTUNE from enemy counter action was essential to the success of the operation. Allied forces were most vulnerable to enemy counter action when they were embarked and at sea. Some 5,000 allied vessels, carrying approximately nine army divisions with full combat equipment, were at sea at one time.¹ These vessels were formed into some 75 convoys and groups, passing along narrow coastal lanes, moving across the channel through the narrow mineswept channels of the SPOUT² or crowded into the congested confines of the assault area. Had the enemy not been deterred by a comprehensive program of defense, this enormous armada would have presented to enemy air and naval forces a very profitable target.

2. Enemy naval forces available to attack NEPTUNE consisted of two battleships (by D-day, both were seriously damaged), two pocket battleships, one unfinished aircraft carrier, two heavy cruisers, four light cruisers, thirty-seven seaworthy destroyers, eighty-three torpedo boats, and some 200 U-Boats.³ In addition, the enemy had some 215 miscellaneous small war vessels stationed in or near the channel.⁴

3. It was assumed that once the enemy was aware that the Allies had committed themselves to the Normandy invasion, he would expend his forces ruthlessly in an attempt to defeat it. His heavy units in the north were not expected to be used directly in the Channel area, except, possibly, as a last desperate measure. They were likely, however, to make diversionary sorties into the Atlantic. But his light surface forces might be concentrated in the Channel against NEPTUNE. These forces were expected to restrict their attacks to darkness or periods of low visibility, and to operate primarily, on the assault and convoy flanks. Enemy U-Boats were expected to concentrate rapidly in the Channel and its western approaches and to operate without regard to loss. The enemy also maintained a mine barrier along the continental coast line, and was expected to undertake further offensive and defensive minelaying especially in sea areas around the U.K. assembly areas and the assault areas, both by aircraft and naval minelayers.

4. The allied naval plan to defend NEPTUNE against the enemy naval threat was:

   1. To prevent distant enemy forces from moving toward the approaches to the Channel,

   2. To seal off both ends of the, Channel, so that enemy forces in its environs could not penetrate,

   3. Within the Channel, to patrol the flanks of the Convoy route, and to screen the assault area against enemy channel based forces,

   4. To escort each and every convoy,

   5. To sweep see mines out of all waters required for NEPTUNE forces, and

   6. To defeat enemy air attacks by an integrated program of air cover and AA defense.

PART II
MINELAYING (OPERATION MAPLE)¹

A. Plan for Minelaying

5. In order to assist in the protection of allied vessels engaged in the assault, naval and air forces executed an extensive minelaying operation (known as Operation MAPLE), with the general purpose of impeding the movement of hostile vessels against the invading forces. The objects of Operation MAPLE were:

   1. To impede the movement of light enemy vessels stationed inside the Channel,

   2. To impede ingress into the Channel by enemy naval forces situated in the Atlantic and North Sea,

   3. To compel enemy naval forces moving toward the NEPTUNE area to follow a course to seaward of the extreme range of enemy shore batteries and shore based fighter cover, in order that allied surfaces forces would be free to intercept their progress,

   4. To disrupt enemy shipping generally during the critical. period, in order to reduce seaborne movement of enemy reinforcements and supplies toward the battle area,

6. Minelaying was carried out by the following naval forces:

   (a) HMS Apollo, (b) HMS Plover, (a) 10th, 50th, 51st and 52nd M.L. Flotillas, (d) 9th, 13th, 14th, 21st, 22nd and 64th M.T.B. Flotillas. In addition mines were laid from the air by Halifax, Sterling and Lancaster bombers of numbers 1, 3, 4, 5 and 6 Groups, Bomber Command, R.A.F. Command of the Minelaying operation was exercised by the normal authorities responsible for minelaying in British home waters. The Naval forces were accordingly under the direction of the Commanders-in-Chief, The Nore, Portsmouth and Plymouth, and the Admiral Commanding, Dover. Minelaying aircraft were under the direction

of the Air Officer Commanding-in-Chief, Bomber Command. General control of all minelaying authorities was exercised through the usual Admiralty Channels. Coordination between the minelaying operation and the general NEPTUNE naval plan was arranged by ANCXF, who stated his requirements and coordinated the general plan. The MAPLE minelaying plan was developed by ANCXF and is set out in his naval plan Appendix XVII. Mines were laid with the object of:

1. Creating normal hazards to enemy shipping,

2. Endangering enemy light craft, and particularly E and R-boats,

3. Presenting the enemy with a difficult minesweeping problem.

The supply of mines was arranged by the Admiralty.

7. With the exception of the field laid in the Straits of Dover, and an area mined by aircraft to the north of the Frisian Islands, all the minefields were offensive in character. The two semi-defensive minefields, referred to above, were intended primarily to counter any movement of enemy heavier forces from the east. Dormant preparations were also made to reinforce these fields, and to lay mines in the Kiel Canal or its approaches (Operation BRAVADO).

B. Minelaying Operations.

8. The Operation was divided into six phases:

   Phase (1)

   (up to 17 April) Normal minelaying operations were executed by naval minelayers and aircraft, using standard mines;

   Phase (2)

   (17 April - 9 May) Routine offensive laying standard mines was continued, and, in addition, special types of mines were introduced into fields laid off Ijmuiden, the Hook, the Scheldt, Boulogne, Fecamp, Le Havre, the Brittany coast and the Frisian Islands. A proportion of the mines laid were timed to become effective at various dates so as to escape being swept before the operation was due to begin.

Phase (3)	(9 May - 28 May). Operations continued as in phase (2), with additional fields being laid in the vicinity of Ushant, and on the general line between the Gasquets and Ushant. Minelaying with special mines, timed to become effective at varying dates, was augmented in the fields to the north of Le Havre and to the north and northwest of Cherbourg. During this phase, aircraft executed an extensive series of minelaying operations in the Kattegat, Baltic Heligoland Bight, Frisian Islands, and in the Bay of Biscay, with the object of retarding the movement of enemy vessels from these more distant waters toward the Channel and assault area. On the night of 12/13 May, a minelaying operation was executed by Mosquito aircraft in the Kiel Canal. Similar operations were executed on the nights of 15/16 and 26/27 May, when mines were laid by aircraft in the approaches to Aarhus, Aalborg and The Sound, as a counter to possible enemy movements from Norway. As these operations involved some risk of compromise to the new types of mines, it was not originally intended that they should be laid from aircraft before 20 May. But to enable full advantage to be taken of especially suitable minelaying conditions, this date was anticipated by five days in the minelaying in the Baltic.
Phase (4)	(29 May - 4 June). During this phase, operations were primarily directed to the laying of special mines off Ijmuiden, the Hook, the Scheldt, Calais, Le Havre, Cherbourg, St. Malo, Morlaix, Brest, and the Biscay ports. Aircraft also executed further lays off Aalborg and in the approaches to Aarhus.
Phase (5)	(Nights of 5/6 June). Operations in Phases 1-4, were, of necessity, related to a fixed date. Those in Phase 5, on the other hand, were planned to be carried out concurrently with the assault. The original plan provided for the laying of mines: (a) off Point de Barfleur, (b) southwest of Le Havre, (c) off Etretat (in conjunction with Operation TAXABLE), and (d) off St. Malo. In the event, only the lay off Etretat was executed. In view of the shipping congestion in the approaches to the assault

	area and the resulting complication of the projected lays, the other D-day minelaying operations were abandoned.
Phase (6)	(6 June onwards). During this phase, the majority of the Coastal Force minelayers were diverted to escort and patrol duties. Further limited operations off Le Havre were executed, however, under the direction of NCETF. Aircraft continued to lay mines off the Channel, Biscay, the Channel Island ports, as a deterrent to the use of these harbors by U-Boats.

9. During the course of Operation MAPLE, a total of 6,850 mines were laid. Of these, 42% were laid by naval forces in 66 operations and 58% were laid by aircraft in 1800 sorties.¹ These operations made an effective contribution to the general immunity from surface and U-Boat attack enjoyed by the assault forces. A considerable number of casualties were inflicted on the enemy, and his minesweeping organization was stretched to the limit. Minefields in the vicinity of Ushant and off the Brittany coast had the desired effect of driving U-Boats into open water, where they could be dealt with by allied anti-submarine forces. The special operation in the Kiel Canal resulted in a complete dislocation of the enemy organization at an important moment. The entire operation cost the allied cause, in casualties, only one Motor Torpedo Boat ² and 19 minelaying aircraft.

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Footnotes:

p.410 #1 For numbers, formation and organization of these vessels, see ON 1, Appendix II, III, IV and ANCXF Report, Para. 2. For details of numbers of convoys, see ON 7, Appendix 1, ON 16, Section B, and ON 9. This great number of convoys lasted for only a few days just before and just after the assaults. During the Build-Up, however, the extraordinary average of 32 Convoys and smaller groups was maintained. See ANCXF Report, Para. 64.

p.410 #2 The routing and movement of the assault forces is discussed in detail in Chapter VI, Section 3. The SPOUT was the name given to the system of cross channel convoy lanes. Its northern end, which was about 10 miles wide, was located about 15 miles south of NAB Tower (near Portsmouth) and marked by the "Z" buoy. Its southern end, which was about 30 miles wide, debauched into the assault area at 49°40' north latitude. Convoys hugged the coast line until they reached the northern end of the SPOUT; they then turned south and crossed the channel in various lanes within the SPOUT.

p.410 #3 This was the estimate of enemy naval strength as of 1 Nov 1943, made by the U.S. Chiefs of Staff in CCS 300/3 of 18 Nov.43. The estimates for destroyers and below were only approximations.

p.410 #4 This was the estimate for February 1944, given in XFNP Page 3.

p.412 #1 For details of NEPTUNE minelaying, see XFNP, Appendix-XVII and ANCXF Report, Appendix 15.

p. 415 #1

Mines Laid	Areas
1,133 799 4,184 734	Baltic and Kattegat Heligoland Bight and Frisian Islands Ijmuiden to Brest Biscay Ports
	Minelayers
1,248 1,369 250 3,983	Apollo Coastal Forces Plover Aircraft

p. 415 #2 On the night of 18/19 May, MTB 203 struck an enemy mine off Etaples and was sunk. Casualties were one missing and five injured.