U-161 what is your definition of shallow water?To avoid cavitating your speed should be no more than the square root of your depth.
If you are being quiet=stealthy then torpedoes shouldn't be chasing you!
Compare the size of a torpedo and the flow of water around it to that of a submarine.An adcap is 21 inches wide-a submarine is 34 feet.In the submarine you are disturbing a much greater body of water and this is obviously interacting with the seabed causing a disturbed flow.This in turn produces drag around the form of the submarine.
http://www.dsto.defence.gov.au/publi...TO-TR-1622.pdf