U-511, Type IXC
During the summer of 1942 the boat under the command of Kptlt. Steinhoff took part in some of the most interesting experiments of the entire world war. Steinhoff's brother, Dr. Erich Steinhoff was working at Peenemünde on the rockets program and they worked it out so that U-511 was used for rocket tests.
A rack for 6 30 cm rockets was installed and somewhat extensive tests were carried out. These were concluded with a successful launch of rockets from a depth of 12 meters. These amazing tests did not convince Dönitz' staff of their merit and they were not carried out. The rockets in question, the 30cm Wurfkörper 42 Spreng, was by no means advanced enough to target ships but it might have been used to bombard shore installations such as oil refineries in the Caribbean. That idea was born in late 1944 when type XXI boats were to tow V-2 launchers which would attack shore bases. The launchers, nor the XXI boats, were not available in time though.
from
http://uboat.net/boats/u511.htm
see also
http://www.prinzeugen.com/V2.htm for more info on the The U-Boat Rocket Program