In the early war my be. Terence Robertson in his book "La herradura dorada" may be in english "The golden horseshoe" (due to the emblem of Kpt Krestchmer and his sub) wrote, Kpt Krestchmer tells he was able to pass at night between two merchants separated by 1 km without be spoted.
Remember Kpt. krestchmer perform many sucessful night attack on surface at early war times.