Naval factors and harbour obstacles

Taken from the Military intelligence service war department Washington,
U.S Army Military History Institute 15th June 1943.

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NAVAL RESPONSIBILITY

The water approaches to beaches, harbours, and other possible landing points constitute the forward area of German coastal defences, and responsibility for this area rests in the German Navy. To engage enemy forces before they reach land, the Navy not only employs surface units of all types, but also controls a certain number of land installations, including long-range
batteries, some antiaircraft batteries, and a line of lookout stations. The responsibility of the German Army for land defence begins on the beach, the fortifications and other positions of which are manned by defensive coastal commands as well as by offensive troops.

NAVAL PATROLS

Fast vedette and patrol boats are used in all the coastal waters of Western Europe. In cooperation with the lookout stations on shore, they guard against seaborne as well as air attacks (The naval meaning of vedette is similar to the military meaning. A vedette boat is a small vessel used to watch an enemy, and give notice of danger.) One of their chief functions at present is to flash warnings of the approach of Allied air formations that sweep across the continent. The patrol boats go out in great numbers, particularly at night. As sentinels on a watery outpost line, they may prove highly effective in nullifying the vital element of surprise on which any seaborne task force would rely heavily in approaching a well defended beach. In addition to the larger units of the fleet, the Germans have also a considerable number of smaller craft available for the dual purpose of antiaircraft and coast defence. Standard types of these smaller vessels are submarine chasers, S- and R-boats (motor torpedo boats and motor minesweepers), Sperrbrechers, and Siebel ferries. The Sperrbrechers and Siebel ferries will be discussed here because they are potentially effective weapons against landing craft.

SPERRBRECHERS

a. General
The Sperrbrecher ("obstacle breaker"), of which Germany has a fairly large number along the western coast of Europe, is a converted, medium-sized merchant ship that bristles with guns. The number of its weapons, of various types, ranges from 8 to 13, the average number being 9. A typical example of the armament of one of these ships is as follows: two 4.1-inch guns, on platforms erected on the poop deck and the forecastle; two 37-mm guns on a twin mount, on a platform aft; 20-mm antiaircraft guns, on four high platforms; and machine guns, on the stump mainmast and on the bridge.
Sperrbrechers are especially useful as antiaircraft ships, but they are often seen acting as escorts for convoys of U-boats and of merchant vessels. As such, they provide protection against mines and aircraft. They also serve as auxiliary antiaircraft defences in harbours.

b. Means of Recognition

Some characteristics which usually distinguish the Sperrbrecher from the normal merchant ship are as follows:

(1) One of the masts has been removed.

(2) One or more of the masts have been cut down one-half to one-third the usual height, in order to carry a platform for
a light antiaircraft gun or searchlight.

(3) The forward well deck is partly covered by a light deck. Which is about the height of the bulwarks; this deck is believed to cover the electrical mine-sweeping gear. The German swastika is nearly always painted in a prominent position on the deck mentioned above, on a hatch, or on the roof of the deckhouse, so that it can be clearly seen from the air. It is between 10 and 15 feet in diameter.

(4) Motor launches, additional boats, and life-saving floats are carried on deck.

(5) A light mast is usually placed amidships, attached to the navigating bridge or funnel, to carry the wireless aerial and the signal halyards.

(6) A small number of derricks are carried; sometimes only one is visible, in a position to handle motor launches.

(7) The large number of guns is always a conspicuous feature.

(8) Gun platforms on these vessels are usually circular; only a few square ones have been noticed.

SIEBEL FERRIES

a. General

The Siebel ferry is actually an Army landing craft, or "invasion barge," of the type that was mentioned frequently at the time when a German seaborne attack upon Great Britain appeared probable. It is discussed here because a large number of them, heavily armed and apparently under control of the German Navy, are operating in western European coastal waters to supplement port and coastal batteries. (See below)

Sketch of Siebel ferry (based on a photograph).

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The Siebel ferry is conspicuous because of its hull, which consists of a pair of steel sectional pontoons, each of which is made up of nine lightly constructed sections and one larger stern section. There is a 20-foot space between the two pontoons, which are equipped with wooden runners to facilitate landings and to protect the craft against damage. The craft is powered by two engines-perhaps airplane engines, installed in the stern section of each pontoon, which drive underwater propellers. In some cases these engines may be supplemented by air propellers so that the craft can operate in shallow waters where underwater propellers would foul. The Siebel ferry has a speed of 8 to 9 knots, and it is believed to be highly manoeuvrable and fairly seaworthy. Its shallow draft reduces its vulnerability in minefields. The heavy type described in the next paragraph is 75 feet long and 56 feet wide, but there is no information available to show how the three known types differ as to size and construction features. (See fig 2 below)

b. Types

The three known types of this vessel are as follows: The heavy (Kampffdhre), the light, and the transport (Trossfiihre) type. The heavy and light types are heavily armed and are notable, from the point of view of landing troops, for three features:

(1) They are fully mobile offshore and therefore are not good counter battery targets.

(2) They cannot be plotted like normal coast defences.

(3) They might be very effective against landing craft and assault landing troops.

fig 2: Plan of Siebel ferry.

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The heavy type usually carries three 88-mm dual-purpose guns, and two 37-mm or two 20-mm antiaircraft machine guns, and one 4-meter-base stereoscopic range-finder with predictor. (Fig. 2.2 shows the layout of guns.)
The armament of the light type consists of four 37-mm or four 20-mm guns, one in each of the four corners of the deck, and possibly a fifth gun in the center. The 20-mm guns may be of the four-barrelled type. The transport type has only open deck space and is designed for transportation of personnel, supplies, and vehicles, and for ship-to-shore lighter age. Its armament usually consists only of one antiaircraft machine gun.

Plan of Siebel ferry Fig 2.2

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HARBOR OBSTACLES

a. General

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 defences 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 harbour 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 anti-boat 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 of T-shaped rafts. Nets are suspended from some of the booms. 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.

c. Blockships

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 harbour 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 harbours and canal openings the Germans will probably have mined the quays, seawalls, breakwaters, pillboxes, shelters, river and harbour 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.

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