In early 1939, Germany had only fifty-seven operational U-boats, but by the end of the war they had built well over 1,000 more. U-boats sunk over 14,554,000 tons of Allied shipping with the loss of over 50,000 Allied seamen. But by the end of the war, the allies had sunk over 630 U-boats and captured five thousand U-boat crew.

Of the 40,000 Germans who served in U-boats, 28,000 were killed. No other service in any previous war had suffered such a high casualty rate. The Battle of the Atlantic lasted for the full five and a half years of the war, and as in the First World War, the U-boats came close to defeating the allies.

In 1904 the Krupp Shipbuilding Works in Germany built the first of a series of submarines. It was called the U-1, with the "U" standing for Untersee Bote (Undersea Boat). The first U-type submarines were powered by gas engines and electric batteries which limited their range of travel. In 1906, the first diesel engines were fitted which increased the travel range.

Even as Germany was perfecting this new form of war weapon so were Great Britain, the United States and France. However, Germany expanded the technology using principles of hydro dynamics so that the vessel was shaped like a tube but tapered at the ends. It had an inner hull which provided stability and was different from the designs of other countries. The German U-boat had developed into a weapon of war which could travel underwater and remain relatively undetected.

At the beginning of the First World War in August 1914, Germany had about 20 operational U-boats in its High Seas Fleet. When hostilities with Russia began, the U-boats were deployed in a defensive screen in the North Sea.

German engineers added the periscope for the use of the commander to sight targets from below the water surface. Then torpedo tubes were fitted and guns were mounted on deck. By 1914 submarines had a cruising range of 2,500 miles at speeds of 15 knots on the surface and 10 knots submerged.

The primary targets of the U-boat campaigns in both world wars were the merchant convoys bringing supplies from Canada and the United States to Europe.

On May 7, 1915, German U-boat, U-20, sank the liner RMS Lusitania. Though there was a great deal of outrage at the sinking of an "innocent" merchant ship at the time, there is evidence that the Lusitania had munitions aboard, making it a valid target under international law. But, this attack turned American public opinion against Germany and was a significant factor in getting the United States involved in the war on the Allied side nearly two years later.

After the First World War, the Treaty of Versailles restricted Germany’s independent tonnage of ships and forbade the construction of submarines. The Krupp family covertly continued the designing of submarines in other parts of the world while secretly supplying parts to Germany.

Before the start of the Second World War, Germany had started rebuilding U-boats and training crews, hiding these activities as "research" or other covers, so that when the Second World War started, Germany already had several U-boats ready for warfare.

Aboard a submarine, a German sailor could stand watch on a bridge with the Atlantic waves crashing over him for only a few hours at a time because the conditions were very dangerous and many were washed overboard. The only peace and quiet on a U-boat was when it was submerged but then it was smelly with stagnant air, oily equipment and diesel fumes. The interior was like a long cramped hallway where mess tables were used as desks and then converted to beds. There was one toilet for fifty men.

Wolfpacks, were large groups of U-boats with anywhere from 3 to 20 submarines. The idea was to gather U-boats in patrol lines to scout for convoys. When a convoy was spotted the first boat was designated "shadower" and would chase the convoy and report its heading and speed. The other boats could position around the target vessel and attack.

About 135 Wolfpacks were formed during 1940-1943, each one lasting up to two weeks. During both world wars, U-boats sunk nearly 8,000 merchant ships and warships with a loss of hundreds of thousands of lives.

At the beginning of the Second World War, U-boats couldn't carry enough fuel on their own to cross the Atlantic and remain to attack Allied ships. A critical development made it possible for U-boats to haunt the shipping lanes of the Atlantic for longer periods of time. It was the submarine tanker which could re-supply U-boats at sea. These tankers could carry 700 tons of diesel fuel and 45 tons of other commodities which could supply active U-boats for months. Germany had ten of these tankers and it became a high priority for the Allies to destroy this supply system.

When the Germans overran Holland they discovered the snorkel used by the Dutch Navy which they adapted for the U-boats. At night, the U-boat could rise from the depths but not fully to the surface and raise the snorkel to admit fresh air to the sub and start the diesel motors to recharge its batteries. The snorkel-equipped boat was able to more effectively hide from the Allied sub hunters.

U-boats also patrolled the Atlantic coast of Canada and the United States. The German submarine, U-1228, was sent into the Gulf of St. Lawrence where its crew attempted to repair a faulty snorkel. Without the tactical advantage that the device provided, its commander, feared his chances should he pass through the Cabot Strait.

On the moonlit night of November 24, 1944 he tested his repairs, found them ineffective and decided to return to Germany. As he issued orders that would send the U-boat back into the Atlantic, the HMCS Shawinigan was sighted. U-1228, which had not yet recorded an attack on enemy shipping, let loose a torpedo. The HMCS Shawinigan disappeared in a plume of water and a shower of sparks. All 91 members of her crew were killed.

The HMCS Shawinigan was one of three Canadian warships that were lost with all hands during the war. Shawinigan's final resting place is in the Cabot Strait, between Newfoundland and Cape Breton Island.

The Royal Canadian Navy was vital to helping the Allies to victory during the Battle of the Atlantic. More than 25,000 merchant ships made it safely to their destination under Canadian escort, delivering more than 165 million tons of supplies to Europe. The RCN sank or helped sink more than 30 U-boats during the war, but paid a heavy price.

The RCN lost 14 warships to enemy attack and another eight ships to accidents at sea. About 2,000 Royal Canadian Naval seaman lost their lives escorting merchant ships across the Atlantic during the war.