[SabreHawk]

Well I didn't find much but I did find this. :

Originating from the Vetehinen design of 1931, the Type VII comprised ten medium sized submarines (U-27 to U-36, all commissioned from August to December 1936) that were both single-hulled and single-ruddered.
The type was equipped with four bow torpedo tubes and a distinctive externally mounted stern tube. Additionally, large external ballast tanks (to lift the heavy hull for surfacing) gave the Type (and its future variants) a characteristic shape. Able to dive fast and deep with a theoretical crush depth of over 700 feet, the Type was a potent threat to Allied shipping. However, operational combat soon showed up four major weaknesses:
1) engine mountings fractured, forcing boats back to base;
2) the single rudder gave poor underwater maneuverability; 3) the external stern torpedo tube could not be reloaded;
4) and a short range of only 6,200 miles.
Still, when war began in September 1939, the ten Type VIIa U-boats - grouped into the Saltzwedel Flotilla - were at the forefront of combat. The first sinking by a submarine was of the "Athenia" (made by U-30) and days later the aircraft carrier "HMS Courageous" was torpedoed by U-29. Also, the largest ship sunk by any U-boat during WWII was by Hans Jenisch in U-32 when he sank the already damaged "Empress of Britain" in October 1940.
But as the battle extended into the open Atlantic after the capture of French seashore bases, the design's shortcomings became more and more obvious and - by the end of 1940 - the four Type VIIs U-boats that had not been lost in action so far were relegated to training.
Eventually all Type VIIa U-boats but two (U-29 and U-30 which were both scuttled in Kupfermühlen Bay in 1945) were sunk during WWII.