With a few exceptions federal regulations require sailors working on deck of seagoing vessels greater than 100 gross registered tons (GRT) to be credentialed. meaning they must hold a Merchant Mariner Credential (MMC) endorsement as either Able Seaman (AB) or Ordinary Seaman (OS). These are called National Rating Endorsements; and holding an MMC Officer endorsement does not qualify the mariner to serve in any position requiring a Rating Endorsement.
All ABs must also hold the MMC rating endorsement of Lifeboat-man or Lifeboat-man Limited. The Lifeboat-man rating requires 108 days of qualifying experience on vessels operating on ocean or navigable waters.
There are several different AB ratings, all require documenting sea service experience. Any mariner holding an officer endorsement will have enough sea service documented to qualify for one of these ratings.
- AB Unlimited may sail on any size of a seagoing vessel. This AB rating requires 1080 day (3-years) sea service on ocean waters.
- AB Limited may sail on seagoing vessels of less than 1600 GRT, and makeup 50% of the required ABs on vessels greater than 1600 GRT. This AB rating requires 540 days on vessels greater than 100 GRT operating on ocean waters.
- AB Special may sail on seagoing vessels less than 500 GRT, and makeup 50% of the required ABs on vessels greater than 500 GRT. This rating requires 360 days on working on deck on ocean or navigable waters.
- AB Offshore Supply Vessel (OSV) may sail on any oil industry vessel, and requires only 180 days of experience.
- AB Fishing Vessel (FV) may sail on any fishing industry vessel, and requires only 180 days of experience.
- AB Sail may sail on any sailing vessel, and requires 180 days experience on sailing vessels.
If you do not have the qualifying sea service experience for the AB endorsement and you want to sail on seagoing vessels greater than 100 GRT than you will need the OS rating. This entry level rating does not require any training courses, exams, or qualifying experience.