A term referring to a method of propulsion for submarines. Boats of this type typically have two motor sets onboard, one set of Diesels and one of electric motors. The Diesels are used on the surface (or when
schnorkeling) for
propulsion and to charge large banks of
batteries aboard. When submerged, the Diesels are shut down (as they would suffocate the crew and run out of air in perhaps twenty seconds) and the electric motors are engaged to propel the boat using the batteries.
Diesel-electric submarines are not nearly as 'old-fashioned' as their name suggests. Modern diesel-electrics are typically quieter than nuclear submarines when on electric power, and noise-deadening technologies (such as isolation rafts and anechoic coatings) invented for use on the larger nuclear submarines work on their smaller cousins as well. The prime disadvantage to these submarines is that the amount of charge that can be contained in their batteries is quite limited; a diesel-electric boat must break off a sustained engagement to surface and recharge batteries as well as refresh its air supply. A nuclear submarine makes its own air, and can remain underwater and at full speed practically indefinitely.
On the other hand, diesel-electrics are much, much cheaper than nuclear submarines, and the technology is quite easily obtained. They are ideal defensive assets, and are still better than anything else save a nuclear attack sub for sinking shipping, especially in chokepoints where the shipping lanes are restricted (such as the Straits of Malacca or the Persian Gulf). Russia makes the Kilo-class (Project 636) diesel-electric for sale abroad. There are quite capable German and Dutch makes (Kockums Type-209, etc.) as well as British, Scandinavian, Spanish, French, Chinese, Japanese etc. available for sale around the world, if you have a plutonium American Express card and, in some cases, an end-user certificate.