411 Dive Log Australasia April 2025

d’Assalto (10th Assault Vehicle Flotilla) was searching for a fixed base as close to Gibraltar as possible, for their clandestine diving operations. A young Maiale operator, Licio Visintini, hit upon the idea of using the OLTERRA for this purpose. So, in 1942 Italian Naval personnel arrived secretly in Algeciras to begin converting the OLTERRA into a human-torpedo base. Their cover story was that they were repairing the tanker to be ready for use again at the end of the war, whilst in fact, engineers created a compartment inside the hull of the OLTERRA to house the Maiale torpedoes and other equipment. A hinged-door was cut through the side of the ship to allow the Maiale divers to exit and re-enter the submerged compartment in secret, at night. The OLTERRA was thus Italy’s farthest out-post and base for their frogmen. Shipments of “repair parts” coming in for the OLTERRA “refurbishment” thus became a secret supply source of combat equipment for diving operations. Nightly, divers straddled their low-speed Maiale torpedoes and drove them out to the allies’ ships off Gibraltar, disconnected their warheads under the target ships and returned safely to the OLTERRA. The Maiale, were 6.7m x .533m and carried 225 kg of explosives. They had a range of 32 kms and could dive to a depth of 30m. In all, they sank 14 ships at Gibraltar. Free swimming Italian frogmen also maintained harassment, attaching smaller mines to transport ships, which did not have the thick armor of battleships. Some mines were magnetic and others were floated against the bottom of the hull. A more ingenious model was timed to explode only when the ship got underway. To counter these saboteur attacks, a British Royal Navy Volunteer mine-disposal officer, Commander Lionel Crabb OBE GM (1909-1956) was appointed to search for and remove the Italian explosives at Gibraltar. As fast as the Italians put mines on the ships arriving at ‘Gib’, Crabb’s underwater working “P-Parties” searched for and removed them. Crabb’s divers used British Davis Submarine Escape Breathing Apparatus (DSEA), which was a short duration rebreather, suitable only for diving under ships for inspection, as it required frequent scrubber chemical replenishment.

Oxygen poisoning was known in those days but training was often poor and divers disappeared in combat, leaving no clues to their loss. Oxygen toxicity occurs not only with depth, but with long exposure, which was always a problem with military diving operations. Both the Pirelli Lung and the DSEA were pendulum-type rebreathers and produced tendency for carbon dioxide to build up in the unit’s single breathing hose. A combat saboteur diver was very busy indeed, he had to both control his exertion and remember, periodically, to purge the system’s CO2 build-up. Many busy, stressed and distracted military divers died from CO2 build-up or lost their buoyancy and descended swiftly to their death. When the Italians capitulated, Cmdr. Crabb supervised the captured Italian frogmen to risk their lives clearing German mines in Italian waters. After the war, Cmdr. Crabb continued clearance operations in the Middle East and mysteriously disappeared on a special mission under a visiting Russian warship at Portsmouth Harbor in 1956. If you would like more information about diving pioneers, contact the Historical Diving Society Australia-Pacific by email at: info@historicaldivingsociety.com.au or visit our website www.historicaldivingsociety.com.au or Facebook https://m.facebook.com/groups/120950924589540/

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DIVE LOG Australasia #411 - April25