In February 1943 the strength of Allied defences were ominous for Dönitz. The battle of convoy HX 224 was ended upon the intervention of air power from Iceland. Dönitz sent 20 boats to attack SC 118 and both sides suffered heavy losses—11 merchant ships for three U-boats plus four damaged. It was "what both sides considered one of the hardest fought battles of the Atlantic war". Despite sending 20 crews into action, Dönitz was concerned that most captains did not press home attacks. The majority of the ships sunk were by one crew, commanded by Siegfried von Forstner—he sank seven.
