When the Netherlands recently announced that it was sending one of its Walrus class submarines for the anti-piracy patrol off Somalia, many people found this puzzling. But the Dutch subs have a well deserved reputation for their ability to secretly collect information at sea using subs. Most recently it has done this off Iraq, Bosnia and in the Caribbean. The special ingredient here is stealth. Entering service in the early 1990s, the Walrus subs can spend 46 days, moving at 16 kilometers an hour, at periscope depth (with only a small air intake/exhaust snorkel above the surface). A submarine can watch portions of the Somali coast, without the pirates knowing they are being observed. This makes it easier to detect new tactics by the pirates, and counter these moves more quickly.
The Walrus class are large (2,400 ton), diesel electric boats, with a crew of 52. Submerged, they can move at up to 37 kilometers an hour (compared to 24 kilometers on the surface.) They are armed with four 533mm (21 inch) torpedo tubes and twenty torpedoes. On the surface, the crew can deploy several machine-guns. But the Walrus boats are mainly equipped to find stuff. They have a surface search radar (range of nine kilometers), and a powerful sonar (both internal, and towed.) The passive sonar can detect surface ship engines farther than nine kilometers away.
Holland is not the only nation to use their subs like this. Other nations have used their subs to catch fish poaching ships, which tend to sneak in and illegally deploy their nets, while avoiding surface patrol ships. Subs, with their highly sensitive sonar, can detect the engines of poacher ships, and even the characteristic noise of commercial fishing gear being deployed (especially trawlers, that scrape the bottom of the seas they are fishing).