The OMA (Oberflächen Mine A) was a moored, contact, surface mine the development of which was undertaken in late 1942. It utilized a novel type of mine case and was de-signed in five models, which were designated OMA I, OMA II, OMA III, OMA IV, and OMA/ K. Of these five models only the OMA I and OMA/K were used operationally. The OMA II and OMA III were abandoned in the preliminary development stage, and the OMA IV was unperfected at the close of the war.

The first moored, contact, surface mine developed by the Germans war a jury-rig aff-air designated UMA/K; it was laid in 1942. In the fall of the same year the development of a more effective mine was undertaken, this type being designated OMA I. Original re-quirements for the OMA I called for its use in water depths up to 65 feet, but these re-quirements were subsequently increased to 325 feet. The development of this mine was completed in February 1943, and it was laid in the fall of 1943.

In 1944 the German Navy Department, alarmed by the large number of OMA I mines breaking loose, reduced the water-depth requirements to a mine suitable depth of 50 feet and requested an appropriate modification of the mine. The resulting modification was designated OMA/K.

With the development of OMA/K the problem of preventing excessive drifters was suc-cessfully met for depths up to 50 feet. (The OMA/K, when tested in the extremely rough waters of the Elbe Estuary, lasted for more than five months.) However, since the buo-yancy of the OMA case precluded the use of a double chain mooring in depths over 50 feet, the problem remained unsolved for use in greater depths.

Late in 1943 the development of a moored, contact, surface mine for depths up to 985 feet was undertaken. This mine was to consists of an EMC anchor, a large steel float with a guide arrangement, an OMA Mine case with a normal spindle-type arming and dis-arming switch, and a 175-round weight. A schematic representation of the assembly is shown in Figure 31. This assembly was designated OMA IV. Because of the relative com-plexity and low priority of this mine, its development was incomplete at the close of the war.

Neither the OMA I nor the OMA/K was fitted with the normal disarming devices found in moored mines. This was due to the fact that these mines had a slack mooring cable and could not be fitted with switches of the normal mooring-spindle or hydrostatic types. Therefore, since these mines broke adrift fairly often, their use created an unacceptable hazard to German shipping and shore installations. To remedy this situation the develop-ment of the OMA II and OMA III was undertaken. The OMA II was to incorrorate a me-chanical-type disarming device and the OMA III an electrical type. However, both types presented a great number of difficulties and were abandoned in the preliminary develop-ment stage.

Since disarming devices of the normal type could not be applied to the OMA I and OMA/K the German Navy Department ordered that they be fitted with a ZE IVa. (60-day disarming clock).

The five models of this mine utilized the same type of mine case. The characteristics of the case are as follows:

Method of firing

Total weights

Weight of charge (cast)

Positive buoyancy

Total height

Diameter less skirt

Delay in arming

20 - 35 minutes (Soluble washer)

The significant design feature of this mine case was the steel skirt fitted to the un-derside of the case. The purpose of this skirt was to eliminate dipping of the mine.

The OMA I and the OMA/K were identical except for the types of mooring and anchors used. The OMA I mooring consisted of a length of chain secured on one end to the mine case and on the other end to a length of steel cable leading from the mine anchor. The OMA/K mooring consisted of a 35-foot bight of chain secured on one end to the mine case and on the other end to a length of chain leading from the mine anchor. The an-chors were substantially the same, except that the cylindrical mine steel around which the mooring was coiled was higher on the OMA I than on the OMA/K.

Figure 30 – OMA/K Mine

Figure 31 – OMA IV Mine