EDIT: (Correction): The structure of your question indicates that you're referring to WW1 (which led me in that direction with my previous answer, now edited out). In WW2 there was never a problem with Britain's Navy against Germany - it was a moot point, anyway, since Churchill and Roosevelt had conspired to drag Germany into a war. The United States was assisting Britain in violation of America's Neutrality Laws. They would escort passenger ships through Britain's treacherous waters, report the whereabouts of German ships and U-boats, and in early 1941, was given permission to fire against the Germans, thus involving them directly before war had been officially declared between the United States and Germany.
In an interesting FYI, Hitler offered to lend Britain some of Germany's troops to help them against the Japanese, as impossible as it may sound. I don't have a reference to it (I thought I did), but he did it in yet another attempt to make peace with Britain.