The Main Induction system is a vital component to modern submarine operations necessary to manage the introduction of external air into the vessel. Its origins are rooted in the submarine snorkel mast that is one of the most innovative developments and important advances in underseas warfare. Because prior to its advent submarines were designed to routinely operate on the surface and they submerged primarily to evade enemy ships and aircraft, or to launch daylight attacks. Thus, they were essentially surface craft capable of submerging for relatively brief time durations generally limited to several days. Consequently, prior to the extensive use of radar, submarines were safer while operating on the sea surface because although sonar could detect them underwater it was ineffective in finding them when surfaced.
Consequently, the sustained development in radar technology during WWII compelled the German U-boats that were engaged in the Battle of the Atlantic to operate submerged more frequently while running on their battery powered electric drives. However, when Germany defeated the Netherlands in 1940 their capture of two submarines revealed that they were equipped with a device dubbed a snuiver (sniffer) that the Royal Netherlands Navy had been experimenting with and it was comprised of a simple pipe system that enabled diesel propulsion at periscope depth while exhausting gases and also recharging the submarines’ batteries. However, the Nazi Kriegsmarine initially regarded the snorkel only as a means to replenish the atmosphere inside their boats and they rejected the necessity to run the diesel engines while submerged. Nevertheless, by 1943 more U-boats were being sunk owing to their operational limitations hence the snorkel was retrofitted to the VIIC and IXC classes and designed into the new XXI and XXIII types with operational use begun in early 1944 then by June 1944 nearly half of the German boats stationed in their French bases had snorkels retrofitted. On Type VII U-boats the snorkel folded forward and was stored in a recess on the port side of the hull, while on the IX Types the recess was on the starboard side. However, the XXI and XXIII types both had telescopic masts that rose vertically through the conning tower close to the periscope.
Afterwards, the snorkel mast was widely adopted and improved by other navies as well as the USN because their submarines too had to surface in order to start-up their air reliant diesel engines. However, snorkel-equipped submarines could remain submerged at periscope depth with only the top of the mast exposed above the water-line while running their diesels. Several rudimentary snorkel-like devices had been installed on board USN submarines in times past including on the Alligator in 1862 that had air supplied from the surface via two tubes with floats connected to an air pump inside the submarine and was the first operational U.S. submarine to have any kind of an air replenishment capability while it was submerged; then in 1863 the CSS Hunley had a similar system too. The original John Holland designed steam powered Plunger in 1898 was to have a coiled hose system with a float attached to permit air to be drawn in from a deeper depth than either the Alligator or Hunley. However, the contract to build the submarine was cancelled by the U.S. Navy in favor of the gasoline powered USS Holland SS-1.
Ultimately, the USS Irex SS-482 received the first ‘full up’ snorkel system installed on a U.S submarine at Portsmouth Naval Shipyard in December 1946. Next the system was evaluated in extensive testing during the period July 1947 to February 1948 making her the first modern USN submarine to have an operational snorkel mast.
Although early snorkels where a significant innovation their use posed some limitations including that U-boats running submerged with their snorkel masts raised were limited to six knots to avoid damaging or breaking the tube. And, the Gruppenhorchgerät, the boat's sonar hydrophone array, was rendered useless by the thunderous roar of diesels. And, although somewhat faster there are still speed limitations imposed today on current-day submarines running at scope depth with any of their array of masts or antennae raised.
A crucial feature of the snorkel was the head-valve located atop the mast that minimized the entry of sea water down the tube. However, early designs utilized automatic ballcock valves that slammed shut in rough seas forcing the engines to suddenly draw a high volume of combustion of air from within the boat resulting in an abrupt reduction in atmospheric pressure inside the submarine and many crewmen suffered extreme ear pain and occasionally ruptured eardrums. The problem still exists on today’s submarines however the effect is mitigated through the use of high-vacuum cut-off sensors that shut down the diesels when any sudden pressure drop is detected and snorkels that have a fail-safe design utilizing an electrical circuit that controls a compressed air system that holds a "head valve" open against the pull of a strong spring. So, when waves wash over the exposed contacts, the control circuit breaks, venting the compressed air and causing the head valve to slam shut. The valve is immediately reopened by compressed air when the contacts are again clear of the water thereby reducing the time the head valve remains closed.
Another downside was that a submarine's diesel exhaust plume could be seen on the surface up to a distance of about three miles. As well its “feather" the wave created by either the snorkel or periscope moving through the water could be easily spotted in calm seas. And, although the “feather” problem still exists for modern nuclear-powered submarines they tend to stay deep only coming to periscope depth for short periods in order to receive radio traffic from their squadrons. And, the exhaust problem is allayed because nuclear submarines run their backup diesel generators at sea only in the advent of a reactor scram or other emergency situations.
It’s essential that contemporary nuclear-powered submarines be capable of operating quietly, swiftly, independently and for extended time periods while deep beneath the surface of the world's oceans in order to successfully carry out their highly sensitive and covert deterrent and intelligence patrols. And, a primary impetus to the development of nuclear power for submarines was for them to remain hidden by eliminating their risk of detection due to the necessity for combustion air to run conventional diesel engines. As such the development of the snorkel mast has provided a vital interim contribution towards that goal and it has now become the primary component of an all-inclusive external air management system designated as the Main Induction System.
The Main Induction System comprises the snorkel mast as well as a low-pressure blower (LPB) that are piped and valved together in order to facilitate the operation of the backup diesel-electric generator for emergency propulsion and to recharge batteries while submerged. Additionally, the LPB is employed to ingest clean external air into and exhaust internal air out of the boat in order to efficiently emergency evacuate contaminated compartments from smoke and poisonous gasses from fires or other emergency situations all while remaining submerged at “scope” depth. In addition, the LPB is contributory to “driving” the submarine to the surface rather than blowing the Main Ballast Tanks (MBTs) by slowly pressurizing the tanks to expel water and/or to blow the tanks dry once surfaced even after a conventional high-pressure blow.
So, from a modest concept of a rudimentary air tube the snorkel mast has morphed into one of the most important engineering achievements to affect submarine operations and to allow America’s submarine force to stay submerged undetected and on station almost indefinitely with the only limiting factors being systems failure, provisions supplies, and crew sustainability.