Mine types

A mine. Defusing a mine.

It has been estimated that during World War II Finland had at least 46 different mine types from almost ten countries. Most of these were anchored contact mines, but there were also magnetic and acoustic types. Anchored contact mines had two methods for triggering, electromechanic and mechanic. Horned mines were electromechanic. They were triggered by the so-called Hertz device. When a lead horn was bent, a glass ampoule with acid broke creating a battery and electric current triggered the mine. Some of the electromechanic mines had also mechanical horns that closed the circuit. Mechanical mines were triggered when the mine was moved and a lead pendulum was dislodged. A spring actuated device then triggered the mine.

Horned mines could have surface antennas tied to horns. They were lengths of rope suspended by a float. When a passing ship was fouled in the rope, horn was bent and the mine exploded. For antisubmarine mines there were several types of antennas. Some of them were based on galvanic currents. These antennas were made of copper wire and copper buoys. When a steel structure touched copper antenna, a galvanic cell was formed and resulting current triggered a relay and mine detonated. Copper antennas could extend both up and down. A hose trigger was installed around mooring wire. When submarine or sweeping equipment touched the trigger, it moved upwards, closed an electric circuit and mine exploded. To counter these mines Soviet submarines moved very slowly not to move a hose trigger and they were protected with wooden covers to prevent galvanic contact in copper antennas.

Most of the mines Finland had before Winter War were old contact mines left by Russians in 1918. These mines were tested during the 1920's to study and verify their working when dropped. This work was done in the area of Lappohja anchorage.

Mine technology

The most common type for detonating a mine was the Hertz device. This device is based on electrochemical principle. When a glass ampoule filled with acid breaks, the acid contacts two different materials and electrochemical reactions create electric current. The reacting materials were for example aluminium or zinc and carbon. The resulting voltage was about 1.5-2 volts. The device was invented by German naval engineer Hertz in 1870's. First mines using this device were constructed probably in 1876.

Hertz horn

Submarine mines needed more efficient detonator system than horns that bent on contact. A submarine can move in three dimensions and so it was necessary to increase the "dangerous area" around a single mine. This was first done by using antenna mines. They were inventend by Americans during World War I. The first large scale use was North Sea mine barrage against German submarines. The electrochemical principle of antenna mine has two weaknesses. First the system needs a metallic contact between copper antenna and steel hull. This can be prevented by thick resistive paint or wooden covers as done by Soviet Union in World War II. The second weakness is that the current developed by metallic contact decreases sharply when salt content of sea water decreases. In Baltic Sea the salinity of near coast brackish water can be less than 10% of ocean water salinity.

Antenna mine

The depth setting of anchored mines had two methods. After dropping to sea the mine remained floated and sent a sound line to bottom. The length of the sound line was set beforehand and it determined the mines depth setting. Paying out the anchor wire stopped when sound line touched bottom. The submarine mines usually had hydrostatic setting. These mines sank first to the bottom. After some time a fuse released the mine to rise. When decreasing water pressure indicated that depth setting was achieved paying out of anchor wire stopped.

Contact mines used by Finnish Navy

Mine type Origin Description Weight kg Explosive load kg
S/08 Russia Electromechanic, five horns. Spherical shape. 580 75-80 Depth setting 0.9-6.7 m, against submarines down to 18 m.
M/10 Russia Mechanic, no horns. Oval shape. 80-100 Explosive was picric acid
M/12 Russia Mechanic, no horns. Spherical shape 580 90-100 Depth setting 0.3-6.2 m
Ma-13 Russia Ground relay mine, round shape 100
M/18 Sweden Electromechanic mine 110
S/S I Germany Electromechanic submarine mine 150 From year 1915, minimum depth setting 1.5 m
S/30 I Sweden Submarine mine, four horns 860 200 Made by Motala for "Vetehinen" class, hydrostatic depth setting.
S/30 P Sweden Submarine mine, four horns 660 Made by Motala for Saukko, hydrostatic depth setting.
S/Ma-35 Finland Electromechanic, built from buoy of ground control mine Ma-13. 95 Minimum depth setting 1.2 m
S/Ma-36 Finland Electromechanic, built from conical buoy of ground control mine Ma-14, trigger mechanism as in S/08. 90-95
S/36 Germany Electromechanic submarine mine. Oval shape, large lid on top. 880 200 Purchased 1936
S/38 I and II Sweden Electromechanic, up and down horns made of lead. Spherical shape 200 Swedish type M/31, purchased 1938
S/40 Finland 100
S/41 I Germany Electromechanic, five horns up and two down. Spherical shape 300 Einheitsmine C, EMC, developed from S/S I. Minimum depth setting 1.5 m
S/41 P Germany Electromechanic, four horns up and copper antennas. Spherical shape 150
S/41 Finland Electromechanic, five lead horns up and two mechanical down. Spherical shape 300 Copy of EMC
S/42 Germany Electromechanic 30 S/SP or UMA without mechanical horns down.
S/R-42 Submarine mine 850
S/43, S/43-Pa Finland Electromechanic, five horns. Spherical shape. 100 Copy of type S/08. Pa = with surface antenna
S/SP Germany Electromechanic antisubmarine mine, lead horns up and mechanical horns down. 30 UMA, U-bootmine A. Purchased 1941.
UMB, UMB-RL Germany Electromechanic antisubmarine mine, mechanical horns down. Hose trigger in mooring wire 30 U-bootmine B
KMA, KMA-Pa Germany Mechanic beach mine. Concrete anchor with explosive load. 80-150 Küstenmine A, Pa = with surface antenna.

Magnetic mines

Mine type Origin Description Weight kg Explosive load kg
RMA Germany Magnetic bottom mine with aluminium box. 1150 800-830 Used only at water depth less than 25m.
RMH Germany Magnetic bottom mine with wooden box. 830
TMB Germany Magnetic or acoustic bottom mine, torpedo shaped. Diameter 53 cm, length 2.3 m. 740 560 Torpedomine Typ B
EMF Germany Moored magnetic mine. Aluminium, spherical shape 340 Einheitsmine F. Depth setting 15-35 m.

Anti-sweeping devices

Type Origin Description
RE/S-1 Germany Exploding type anti-sweeping device Sp.B. C, Sprengboje C 0.8 kg explosive load. Purchased 1941
RE/S-2 Germany Cutting type anti-sweeping device RB, Reissboje
RE/S-3 Germany EMC type buoy with 50 m of chain and a cutting device EMR, EMR/K

In addition at least following mine types were laid by Finnish forces:





This page is maintained by Jari Aromaa.
Last modified 13.1.2008.