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Longest patrol ever was made by Eitel-Friedrich Kentrat in U-196. On his first patrol in that boat, he was at sea for 225 days. His second patrol in that boat was also longer then 5 months.
Another long patrol (Left Kiel in May 1944) in a IX U boat was U 862 Commander Heinrich Timm. (over the course of two operation patrols and twelve months) U862 travel further from home than any other German U-boat of the Second World War. She sailed to Australian & later New Zealand waters . Along the way she sunk 7 merchant ships, shot down 2 aircaft and was indirectly responsible for the loss of two others when elsewhere the average exchange rate was two U-boats lost for each Aliied ship sunk.