In the event of an invasion of the European continent, the Germans will make a determined effort to prevent the Allies from occupying the important ports. Among the defenses of these ports are fields of marine mines (both free and controlled), booms, and blockships. The latter would be sunk to obstruct channels when a major landing operation is started. This section will give available details on those obstacles which are essentially the concern of engineer and other ground troops.
b. Booms and Nets
The Germans have been increasing recently the already large number of booms installed across harbor entrances and approach channels. Such booms have also been placed across the entrances of underground concrete shelters for submarines and motor torpedo boats. They are usually laid in one to four lines.
The antiboat booms consist of linked timber rafts of various sizes and
shapes, studded, in some instances, with long, sharp spikes. Other
types of booms are made from a series of plain timbers linked
together, a double line of timbers separated by floats, or a series
Antisubmarine nets often consist of wire cable nets suspended from steel barrel-floats and mooring buoys. The meshes of the nets are usually about 1 yard square. These nets are about 240 feet in length and from 60 to 120 feet in vertical depth.
The Germans are known to have a considerable number of concrete barges, sometimes called "Bruges barges" after the city where many of them were built. In designing these the Germans have taken into account the ultimate purpose of submerging them to block channel and harbor entrances. Ordinary merchant ships will also be used for this purpose. In approximate dimensions the barges, which are of cellular construction, are 27 feet long, 16 feet wide, and 12 feet high. As an aid in identification, it may be noted that two types of roofs have been observed on the superstructure—flat and gabled.
d. Mined Installations
In and around harbors and canal openings the Germans will probably
have mined the quays, seawalls, breakwaters, pillboxes, shelters, river
and harbor bridges, and other typical installations that might be useful
if seized by an invading force. The Germans may deliberately leave
facilities of this kind intact in order to blow them up after they have
been occupied by landing forces.