1944\01\14. After suffering serious damage via aircraft on it's maiden (new career to replace the previously mentioned sinking) voyage near grid AL85, U-81 was able to limp back to it's home port of St. Nazaire, via the Piening route.
Had to run a gauntlet of Allied aircraft patrols off Cape Finisterre, making it difficult to recharge it's batteries after daytime submerged travel.
But once sufficiently east of the coverage, had smooth sailing hugging the coast of Spain the rest of the way back to port, much to the chagrin of the Spanish authorties.
No sinkings, no torpedoes fired. But made it back. In 1944, that's a plus.