The English navy/airforce was way more powerful and plentiful, yet Germany had a few powerful ships but they where hidden away in Norwegian fjords under heavy AAA protection.

Germany always had the supply problem and was planning to get it from the east (Russia, Poland, etc). Germany broke the truce with Russia for that reason. Everything went well until winter set in.

Germany had to use submarines to even make a dent, surface ships simply would have been blown out the water.

The German campaign was about isolating and subduing the English, basically to starve them into submission.

You should read the story about the Bismark (one of the two most powerfull battleships ever made) and how it came to its end. Even by today's standard the Bismarck was a monster.

This was because of Germany's reliance on submarines in World War II. It had perhaps 10-20 surface vessels capable of capturing enemy ships (and most of these were soon sunk), but fielded some 1200 submarines.

Submarines were "assassin" vessels. They were typically half the size of the merchantmen that they sank, and they carried crews half the size of merchant crews.

A surfaced submarine was no match for any surface warship, or even a reasonably well-armed merchantman. Submarines were basically too small and "weak" to fight a conventional battle and capture ships.

What gave submarines their power was their ability to "hit and run" while underwater. This, together with an "assassin" weapon such as torpedoes (as opposed to gunfire) made it possible for submarines to sink, but not capture enemy ships.

How do you imagine that's gonna to happen?

You are talking about convoys, not single and unarmed merchant ships etc. which have been in fact captured by German raiders like Schwern answered in detail.

A British convoy is surrounded by destroyers/frigates and several columns of ships transporting goods, the most valuable (most volume and expensive cargo) in the middle. Germany had no power to challenge Britain's warship superiority on the sea. Worse, many convoys were armed to the teeth and had planes for reconnaissance/hunting.

So the only way to attack convoys were German submarines. Those were extremely weak on the surface, even one well-armed merchant was capable to destroy a submarine by weaponry or ramming. Trying to capture a ship from a convoy was suicide. So the only way was attacking and sinking ships with torpedoes. Even if there were only unarmed freighters, you can only capture one of the targets because they will disperse immediately and alarm every warship in the vicinity.

All in all, you could not capture enemy surface ships with a submarine without extreme risk.

But even in the unlikely case that the submarine could surprise a freighter and force them to give up, what now?

• Sending a force to capture and imprison the crew means that once a British warship appears on the horizon (and you must expect it because the radio operator send a distress call), your force must return to the submarine as fast as possible, endangering the submarine.

• Disabling the freighter's machines (so it could not ram the submarine) and tethering it to the submarine is also not recommendable. The most dangerous enemy of the German submarine was the plane, the only means of escape is diving as fast as possible (They tried later to fight planes on the surface with flaks, with disastrous results). Tethering means that the pilot has a perfect target.

• The submarine has no room. There were only beds for a part of the crew because there were continous shift changes. It was strictly forbidden to allow survivors on boards, the only exception were allied pilots because their knowledge was extremely valuable for the German command. So you cannot move the freighter crew to the submarine.

When German Raiders Did Use Captured Ships

In addition to the other answers, there were a few examples of German raiders sending captured ships back early in the war. These were auxiliary cruisers, fast, long ranged merchantmen fitted with enough hidden weapons to overpower lone merchant ships. If they encountered a warship they'd disguise themselves as a neutral merchant. Many began the war outside of Germany and raided until destroyed. Germany had ten at the start of the war and they sunk hundreds of thousands of tons of shipping in the early war. As the war progressed, there were fewer German-held or friendly ports they could provision at, the tightening blockade made breaking out more and more difficult for a slow merchant ship, and increased Allied patrols made it more difficult to operate at sea.

Captured ships were rarely returned just to supply Germany, but rather to offload POWs and send captured documents home. Each came at a cost: a prize crew. A raider only had so many crewmen it could send to guard and man the captured ship. Each prize they sent home meant less crew for raiding. Eventually they'd run out of crew and have to return, likely never to raid again. A raider was more valuable staying at sea, tying up the enemy's resources looking for them, and sinking the enemy's supplies.

In addition, capturing a vessel took time; time when the raider would be vulnerable. The crew would have to be subdued. The ship surveyed. The machinery worked out and possibly repaired. All this time the raider is sitting stopped next to the merchant that possibly got a distress call off. A auxiliary cruiser's greatest strength is looking like an innocent merchant ship. If a warship or aircraft comes along and sees two ships stopped in the ocean where raider activity has been reported there is no question what is happening. It's safer to smash and grab: disable the ship, grab what you can, sink it, and exit the scene.

Instead, the supplies were used by the raider themselves, fuel and food being the most precious. Sometimes a raider would capture a merchant ship and use it as a floating supply dump until it was empty, then they'd scuttle it. Sometimes they'd supply other raiders and submarines as in the case of the tanker Ketty Brøvig which supplied not only Atlantis who captured her, but also the heavy cruiser Admiral Scheer, and the Italian submarine Perla.

Pinguin and Storstad

One of the most creative uses of a captured vessel was the Norwegian tanker Storstad by Pinguin, the most successful German raider of WWII. Pinguin captured the Storstad and found a nice remote spot to convert her to a mine layer (one of the astonishing things you learn in these histories is just how much work you can do on a ship while at sea) and commission her into the German Navy as the Passat. At the point in the war the Germans were still following the rules of warfare at sea and to use Storstad as a mine layer it had to be commissioned as a warship lest they be considered pirates. Together they quite effectively laid mines off South Australia, the two ships being able to cover more ocean.

With that job done, they returned her to her original merchant ship form and decommissioned her. She was crewed by German prize crew and Norwegian volunteers. Now she was a nice, innocent "Norwegian" merchant to scout for Pinguin, which she did effectively. A merchant ship that happened to be radioing position reports of nearby vessels that a warship might pick up was fine.

After sinking 11 ships Pinguin found herself with 405 POWs aboard. They were loaded onto Storstad and after using her to refuel other German raiders they were sent on their way back to France.

Pinguin and the Norwegian Whaling Fleet

Pinguin had a grand time with the Norwegians, capturing a whaling fleet intact was her greatest coup. This whaling fleet was operating for the British and Pinguin picked up their radio chatter. She waited until the factory ship Ole Wegger was transferring oil to the tanker Solglimt and then slipped alongside and quietly captured both and most of her whalers. Pinguin then quietly captured another factory ship, Pelagos and her whalers.

Rather than sink them the Norwegians were told to continue their work. They were working for Germany now and would be paid. At this point in the war Norway was occupied by Germany, and whalers like to be paid, so they did.

Pinguin then ran north-west at high speed for five days brazenly broadcasting signals home. The British, as expected, picked this up and, as expected, began searching along this new course. Her ruse complete, Pinguin returned to the whaling fleet who had been diligently working meantime, rounded them up, and headed off in the opposite direction.

Lacking prize crews for all 15 ships, but whale oil being extremely valuable, the oil was transferred to two ships and they were sent to France. The rest rendezvoused with the tanker Nordmark, supply ship Alstertor, and the captured refrigerated ship Herzogin. One whaler was converted to a minelayer, and the rest were sent to France with prize crews from Nordmark.

Pinguin was resupplied with ammunition from Alstertor and food from Herzogin. The captured Herzogin had been supplying a good portion of the German navy with fresh meat and eggs, but was running out of things to burn to keep her refrigeration plant running. Her bridge, lifeboats, masts and decks had all been torn up and burnt. After supplying Pinguin she was scuttled.

Atlantis

Examining the cases where Atlantis, the second most successful German raider, sent captured vessels back or used them for extended periods, gives an idea of the reasons they would do so.

• Tirranna - sent to France with supplies
• Durmitor - sent to Italian Mogadishu with POWs and captured documents
• Ole Jacob - sent to Japan with SS Automedon's secret cargo
• Speybank - sent to France, converted to a minelayer
• Ketty Brøvig - used to supply other German and Italian ships

The allies blockaded Germany in WW2. Even if the Kriegsmarine could capture ships (and Orangesandlemons's answer correctly explains that this was unlikely) - there was no way to actually bring them to German ports.

The British navy would just re-capture/sink them on the way.

The general answer, "It's easier to sink than capture," has already been given. Now consider the specifics.

In the early stages of the war:

In two words - "Prize crew".

Having captured a ship, it is necessary to plant on it a sufficiently large crew to either operate it by themselves or to oversee the original crew and keep them in line. This means the prize crew must be vigilant 24 hours a day to avoid an insurrection by the original crew. At a minimum, let's say 6 men, two shifts of 3 to man the bridge, hoping that there are no weapons hidden on the ship. And note that this is actually an impractically small number, but it will serve as a starting point.

The most common U-boat type in the North Atlantic was the Type VII, with a normal crew of 42 to 52, depending on model. If the sub were set up to handle 20 captures in a cruise, this would require an additional 120 men be carried, along with provisions, and the subs simply didn't have the space. Even a projection of 10 captures would more than double the departure loading, and was completely impractical.

In the later stages of the war:

"Convoy system"

With virtually all of the shipping organized into convoys with armed escorts, capture was effectively impossible. U-boats were extremely effective against unarmed merchantmen, but a pack of destroyers was just not in the cards.

It is far easier to sink than capture, especially when your main tool is the submarine. If you go further back in history, capture was indeed often the name of the game. But in a WW2 context Germany could not go head-to-head with the Royal Navy, hence submarines being the primary tool. Beyond this, ships were far more traceable and could be avoided; once more making submarines the better choice.