Stu, captain's clerks, stewards, etc., were ratings classed as petty officers (as opposed to commissioned officers). Often they were taken from within the ranks of the enlisted sailors, and were proficient in taking written messages and in writing skills.
At least some examples (although in the Confederate Navy) are shown in the listing of personnel who served aboard the CSS SHENANDOAH, and which list is included in the ALABAMA CLAIMS volumes. This list clearly shows them as petty officers.