September 14, 2007: The recent success of a newly commissioned South African Type 209 sub against NATO ships was yet another example of how deadly diesel electric subs are, even against modern warships with trained crews and good ASW (anti-submarine warfare) equipment. However, one element that is often left out of these exercises, is the search for the subs before the surface ships are located by the subs. Diesel electric subs spend most of their time on the surface, or near it (with just an exhaust device above the water, to let air in and diesel exhausts out). When on or near the surface, these boats can be easily detected from the air. Diesel Electric boats are also quite vulnerable when still in port, and any foe would strive to use bombs or missiles to sink these subs before they can put to sea. This is when the subs are most vulnerable, and training exercises demonstrate this so frequently that ASW training instead concentrates on the hard part, finding a submerged sub that is stalking surface ships.

The really dangerous submarines are those using a non-nuclear AIP (air independent propulsion) system. This enables the sub to stay under longer, thus making the sub harder to find. AIP allows the sub to travel under water for at least 4-5 days at low speed (5-10 kilometers an hour). While nuclear subs also have AIP, their nuclear power plant is noisier, and the subs are larger. Both of these elements make nuclear boats easier (but not very easy) to detect. Some of the newer AIP designs allow boats to stay under for several weeks. If such boats are equipped with good (and very expensive) passive (silent) sensors, they can be the most difficult subs to detect. This has caused all the major naval powers to increase research in ASW, and increase ASW training.