Meaning of "Grummett."

A Constant Reader is desirous of addressing such of your correspondents as are well versed in maritime history,—Mr. Bolton Corney to wit,—on the following subject. In the early ages of our Navy there was a distinct rating, called "Grummett," on board each man-of-war, and he was generally, as may be seen in the Cottonian MSS., placed after the "maryners and gonners." Now, the reader will be highly obliged to any one who will trace the designation to its source, and give information as to what were the special duties of the Grummett, or Gromet.