Submarine detection device used by surface ships in World War 2?

Answer 1

Anti-Submarine Warfare (ASW) in WWII used many of the same tools for detection that are still used today - active and passive sonar, aerial reconnaissance, surface radar for detecting periscopes, and plain old-fashioned eyeballing from lookouts with binoculars and incentive.

Surface search radar was used against German U-boats pretty effectively until they mounted detection devices to alert them to radar signals long before they reached their effective range. This was the forerunner of today's modern Electronic Warfare systems. Though active sonar can help detect a submarine, it wasn't really used unless the Destroyer running down a submarine had already attacked, as active sonar is also great at broadcasting your location to any submarine within a very wide radius. Sound travels faster in water than in air, and further as well. While passive sonar is okay, even on today's modern surface ships finding a submarine while traveling at any speed over 10 knots makes too much noise to be very effective, and it was much worse in those days.

Aerial recon is what really changed the tide against the U-boats in the Atlantic. Long range bombers from England and convoy ships outfitted with up to 4 launchable planes for recon could spot a submarine at 200 feet or more depending on water conditions, and either attack with bombs or direct surface warships to the target. This hasn't changed, only improved. Today's ASW aircraft comprise both Helo's and long range fixed wing aircraft (e.g., P-3 Orion) that can drop sonobuoys that have both active and passive sonar to find a submerged target. They are also outfitted with, and can launch torpedoes when they find one.

However, much has changed in submarine technology today since WWII. Back then, submarines were extremely limited in their ability to remain submerged for very long. With the advent of Nuclear Power and improved battery technology for Diesel Boats today, the ASW advantage is in the submarine's ballpark. The inherent quietness of modern boats, combined with the ability to use the ocean's many environmental hiding places, makes it difficult at best for ships, planes and helo's to find a submarine. In fact, the best detection device for finding a submarine is another submarine.