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Insidious Weapons

by Cristiano D'Adamo

The SLC (Slow Moving Torpedo) was the best known assault weapon of the Regia Marina during World War II. This new torpedo, nicknamed "maiale" (pig or hog, depending on the author), was equipped, amongst other things, with diving planes, ballast tanks, and compressed air for the release of ballast, thus allowing for full underwater navigation, just like a submarine.

The models built after the original 1936 prototype employed an electric motor with power increased from 1 to 1.6 HP. Batteries were increased to a total of 150 Amp. Specific efforts were made in bettering equipment; a magnetic compass, for instance, was preferred to a gyroscopic one, due to the difficulties encountered in miniaturizing such a complex device. The magnetic compass was improved, rendering it less susceptible to magnetic interference.

The breathing apparatus used by the operators was also notably improved. After having noted the unreliability of the Davis hoods used on submarines, studies were began on specific apparatuses capable of providing longer autonomy and complete safety.

To avoid bubbles rising to the surface, which would have revealed the presence of a diver, the Regia Marina employed special devices, specifically built by Pirelli, which utilized a sealed system. These breathing apparatuses were not fueled by compressed air, but instead used pure oxygen. The use of oxygen eliminated the presence of bubbles since it did not produce exhaust gases. During breathing, the expended oxygen is turned into carbon dioxide (CO2) which is then purified by a lime-based filter (sodium carbonate and lime). A closed system allows for the apparatus to function until the lime is saturated and cannot any longer absorb the carbon dioxide (CO2). The ARO, as it was designated by the Navy, eliminated the risk of the bends typical of the compressed air apparatus (designated as ARA), and the fastidious decompression stages required during ascent.

Nowadays we know that breathing pure oxygen stresses the central nervous system and can cause epileptic episodes, but no one recollects such occurrences during the training conducted by the Italian divers. Therefore, it is believed that the oxygen in use was not 100% pure. As a matter of fact, it is recorded that divers experienced "breathlessness" typical of carbon dioxide (CO2) intoxication. Regardless, divers experienced some maladies caused by oxygen breathing at certain depths. The maximum submerged depth of an ARO apparatus should have not exceeded -15 meters (- 45 feet), but it was known that depths of up to -30 meters (-90feet) were often reached.

Adapted from: 'Imagini di Storia - Decima MAS: I mezzi d'assalto della Marina Italiana - N7 gennaio 1995

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