Small warships - destroyers, destroyer escorts, corvettes, patrol boats, coast guard cutters - all were generally called "sub chasers". They had Sonar (the British called it Asdic) which stands for "Sound Navigation and Ranging". An operator listens through headphones while the Sonar apparatus sends out "pings" (sounds) underwater. When these sound waves encounter the solid steel hull of a sub, they echo back, and a skilled operator can tell if it IS a sub (not a whale, or submerged wreckage) and where it is in relation to the ship, maybe about how deep. Sub crews knew about this detection equipment, and could hear the pinging inside their boat. Very nerve-wracking. Some clever sub captains would try to hide by going far enough down to get under a different thermal layer in the water, which might conceal them. Some sub chasers also had hydrophones, where the operator again listened through headphones, but there was no pinging, just listening. Sub crews were also aware of this and were extremely, excruciatingly careful to make no noise inside their boat when they knew they were being hunted, because a dropped tool, for instance, might be heard by the hydrophone operator. The sub crew would speak only when absolutely necessary, and then in whispers. This could go on for hours, or even a day or two.

If a sub chaser thought he had a sub located he would make a "depth charge run". There were usually two racks on the stern (back end) of the ship, situated long ways, front to back, each holding maybe a dozen depth charges. A depth charge looked like a 55 gallon oil drum, laid on its side. They held about 400 pounds of ammitol - a high explosive, and had hydrostatic fuses, which would detonate the explosives at a preset depth - where it was believed the sub was lurking. If these depth charges were coming close to the sub, the men inside could hear the fuse click, followed instantly by the explosion. If one of these went off close enough to the sub it would crush the pressure hull and, at the great pressure at those depths the water would instantly basically explode into the boat so fast that the flesh of the crewmen would be atomized. So the agony was not prolonged, anyway. Submarines usually had their own hydrophones and a good operator might be able to tell when the subchaser had located them and was starting a depth charge run. The captain might order a right angle, 90 degree course change, and possibly a change of depth. The splashes as the depth charges rolled out of their racks into the sea might be heard in the sub. The explosion of the depth charges rendered hydrophones useless for several minutes and so roiled the water that Sonar gave back confused echoes, so, if not destroyed, this was a chance for the sub to try to sneak away.

Some subchasers had "depth charge projectors" along the sides, which could fling depth charges out some yards to the side.

Late in the war there was a new, improved weapon, called hedgehog. It fired 24 individual bombs simultaneously, which fell into the sea in a pattern a hundred yards or so from the subchaser which fired them, then sank. These only went off if they hit something solid, like the hull of a sub.

WWII subs ran on diesel engines when on the surface, and on huge arrays of batteries when submerged. Generally a sub needed to surface every night to recharge its batteries. A sub could go much faster under diesel power on the surface than it could on batteries under water. The British and Americans were much more advanced than the Germans and Japanese in radar. Huge flying boats, and heavy bombers of the four-engine type used to bomb Europe were outfitted with radar and carried bombs and/or depth charges. They had huge searchlights mounted under the wings. They would try to catch a surfaced sub charging its batteries on their radar, then cut the engines and glide silently to attack position, to prevent the sub from becoming alarmed and crash-diving, then turn on the lights, and bomb the sub. The Germans figured out they were being caught by radar, and invented a radar detector, called Metox. For a time they dodged the radar equipped planes. Then the allies figured out that the Germans had a radar detector, and developed a detector of the radar detector, so the Germans' Metox was betraying them to the planes.

Near the coast of the US blimps (gas bag, non-rigid airships) were also used to scout for subs and to try to bomb them. In the middle of the Atlantic there was an area where even long-range bombers operating from American and the British Isles could not cover. To close this gap "baby flat tops" were made, by taking some of the hundreds of Liberty ships the US ship yards were cranking out and turning them into little aircraft carriers, which could carry about twenty planes. These CVEs (Escort Carriers) sailed with the convoys as part of the group of escorts protecting each convoy. Convoying itself was a basic protection measure - there were many more supply ships needing to cross the ocean than there were subchasers to protect them, so the merchant ships would be collected until there was a large group, then six or eight subchasers would shepherd the entire flock across. Sometimes American subchasers went half way then British units met them in the middle of the ocean and took them the rest of the way. Fast ships, like 30 knot per hour passenger liners converted to troopships often sailed alone, as they were so much faster than subs, which could only do about17-18 knots on the surface, it would take really bad luck and sailing right in front of a sub to get sunk, and none were. Convoys could only go as fast as the slowest ship could manage - usually about 8 knots.

By late in the war the Germans had developed snorkel boats, which could run along below the surface on their diesel engines with the snorkel pipe sticking up out of the water to suck in air. (When a wave rolled over the pipe it left the crew gasping for breath and in agony for their near-ruptured eardrums as the diesel instantly sucked all the air out of the boat). But by then Allied radar was so refined even the snorkel pipe could be detected. This led the Germans to experiment with rubberized, radar-wave absorbing paint for the snorkel pipes, but not too successfully.

From start to finish it was a technological struggle, in the end won decisively by the Allies. About 90% of German U-boat sailors died with their boats by the end.

In the Pacific, US submarine sailors made up 1% of the US Navy, and sank 55% of all Japanese ships which were sunk. But they too paid a high price.

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