46 CFR § 11.502 - General requirements for national engineer endorsements.
(a) For all original and raise of grade of engineer endorsements, at least one-third of the minimum service requirements must have been obtained on the particular mode of propulsion for which the applicant seeks endorsement.
(b) If an applicant desires to add a propulsion mode (steam, motor, or gas turbine) to his or her endorsement while holding a license or MMC officer endorsement in that grade, the following alternatives are acceptable:
(1) Four months of service as an observer on vessels of the new propulsion mode.
(3) Six months of service as oiler, fireman/watertender, or junior engineer on vessels of the new propulsion mode.
(4) Completion of a Coast Guard-approved training course for this endorsement.