Please note the snokel was detectable by radar. Wake or not the Allies could pick it up.
http://inventors.about.com/library/i...submarine9.htm
Please note the part of snokels and radar near the bottom of article.
From article "Using a snorkel a submarine could run its diesel engines and recharge its batteries while operating just below the surface. Air for the diesel engines was drawn into the submarine through the snorkel that was extended to the surface. To some extent the snorkel reduced vulnerability to detection and attack, but it protruded above the surface and could be detected by radar. The Germans introduced the snorkel too late in the war to make a difference."