Only one completed the journey both ways.[1][2] Karl Dönitz, the Commander-in-Chief, U-boats, Befehlshaber der Unterseeboote (BdU) was always reluctant to send his boats into the Mittelmeer but he recognised that natural bottlenecks such as the Straits of Gibraltar were more likely to result in shipping being found and attacked than relying on finding it in the vast Atlantic Ocean.