46 CFR §11.452

1. (a)The minimum service required to qualify an applicant for an endorsement as Master of Great Lakes and inland self-propelled vessels of less than 200 GRT is 1 year of service on vessels. Six months of the required service must have been as Master, Mate, or equivalent position while holding an endorsement as Master or Mate of self-propelled vessels, or Master or Mate (Pilot) of Towing Vessels, or OUPV. To obtain authority to serve on the Great Lakes, 3 months of the required service must have been on Great Lakes waters; otherwise the endorsement will be limited to the inland waters of the United States (excluding the Great Lakes).
2. (b)To obtain an endorsement for sail or auxiliary sail vessels, the applicant must have 6 months of service on sail or auxiliary sail vessels. This required service may have been obtained prior to issuance of the MMC endorsement as Master.