In August, United States Navy escorts began escort fast convoys (ON and HX designations) between North America and Iceland under the Argentia Agreement. The NEF was restricted to escorting slow convoys (SC and ONS designations). Following the introduction of the US destroyers, German tactics changed, moving concentrations of U-boats around the Atlantic in response to battle and circumstance. In September the U-boats shifted north and encountered convoy SC 42. Escorted by one of NEF's groups and heavily augmented during the ensuing battle, it became the worst loss of the period by Allied escorts with 15 merchant vessels sunk. During the battle, the Canadian escorts were able to sink one U-boat .