Depends what time of the war we are talking here. Hitler had a strict DON'T INVOLVE THE AMERICANS mindset, he knew if the Americans got involved, the war would be over. Nonetheless, they had U-boats patrol the Atlantic to cause trouble to allied shipping lanes, which included ships that the US were using to give supplies and other war material to Britain and France. But near 1941, they increased due to the fact Japan declared war on the US, thus invoking the Triple Entente pact they had, consisting of Japan, Germany, and Italy.
