Able Seaman (AB) is a deck rating that requires sea service experience, training and exams. An AB rating is required to work on most seagoing vessels greater than 100 gross tons (about 65 feet). Sea service requirements for the AB rating varies from 360-days on any waters for AB Special, to 1080-days on ocean waters for AB Unlimited. All AB ratings require the applicant to complete a “Proficiency in Survival Craft” course to satisfy the training requirements for either Lifeboat-man or Lifeboat-man Limited.
If you do not have the qualifying experience for an AB rating than you need to get an Ordinary Seaman (OS) rating to work on seagoing vessels greater than 100 tons. The OS is an entry-level rating that requires no sea service experience, training or exams.
Holding an endorsement as Master 200 tons or less, Mate 200 tons or less, or OUPV does not qualify the mariner to sail as an AB or an OS. To sail as a deckhand on seagoing merchant vessel greater than 100 ton the mariner needs to hold either an AB or OS even if he holds an MMC (Merchant Mariner Credential) officer endorsement.