[johnno74]

This page has some info: http://www.uboatwar.net/equipment.htm

The jumper wires that are such a distinctive part of most Second World War U-boats were in fact short-wave radio antennae (3-30MHz). Stretching both fore and aft of the conning tower, the forward portion was used to transmit while the stern twin wires were for reception. The antennae were connected to the radio within the U-boat by a thin insulated wire entering the conning tower facade. Coupled with their primary use was also the value of using the jumper wires as a means of attaching life lines for crew working on deck in bad weather.