An able seaman (AB) is a seaman and member of the deck department of a merchant ship with more than two years' experience at sea and considered "well acquainted with his duty".[1] An AB may work as a watchstander, a day worker, or a combination of these roles. Once a sufficient amount of sea time is acquired, then the AB can apply to take a series of courses/examinations to become certified as an officer.[2]
At sea an AB watchstander's duties include standing watch as helmsman and lookout. A helmsman is required to maintain a steady course, properly execute all rudder orders and communicate using navigational terms relating to heading and steering. A watchstander may be called upon to stand security-related watches, such as a gangway watch or anchor watch while the ship is not underway.

An AB dayworker performs general maintenance, repair, sanitation and upkeep of material, equipment, and areas in the deck department. This can include maintenance of the ship's metal structures such as chipping, scraping, cleaning, priming, and painting. Areas frequently in need of such maintenance include the hull, decks, superstructure, cargo gear, and smoke stack. Dayworkers also frequently perform maintenance on lifeboats, rescue boats and life rafts, and emergency and damage control gear. For many vessels, being a dayworker is a position granted to senior ABs, since it generally allows more time for rest and relaxation.
An AB may be called on to use emergency, lifesaving, damage control, and safety equipment. Able seamen perform all operations connected with the launching of lifesaving equipment. An AB is expected to be able to operate deck machinery, such as the windlass or winches while mooring or unmooring, and to operate cargo gear.
Able seamen require advanced training, including lifeboatman certification.
The ship's boatswain, if carried, is typically a senior AB. The boatswain is in charge of the able seamen and ordinary seaman that comprise the unlicensed deck crew, and reports directly to the chief mate.

連邦規則集46 CFR 12.05 [ 3 ]では、米国商船隊の熟練船員を5つのカテゴリーに分類している。
特定の訓練プログラムや練習船での勤務期間は、上記の勤務期間に代えることができます。五大湖の熟練船員(18ヶ月勤務)、あらゆる水域の熟練船員(12ヶ月勤務)、あらゆる水域のタグボートおよび曳航船の熟練船員、総トン数500トン以下の非旅客船の湾および海峡の熟練船員( 12ヶ月勤務)、および外洋バージの熟練船員(12ヶ月勤務)には、特別な勤務証明書が発行されます。
米国商船隊については、連邦規則集46 CFR 12.05 [ 3 ]において、熟練船員の資格認定のための試験要件が定められており、これには以下が含まれる。
2004年の調査によると、ITF契約なしで航海する典型的な資格のある熟練船員(AB)の総報酬は約800ドルですが、ITF契約があれば総報酬は月額1,300ドルになります。[ 4 ]
商船隊で活躍した著名な船員には以下のような人物がいる。
現代の文献の中には、ABは「 able seaman」だけでなく「able-bodied seaman」の略語でもある、あるいは「able seaman」の 代わりに使われると主張するものもある。当初、船員名簿や記事では、 「able seaman」はより分かりやすい「AS」ではなく略語「AB」で表記されていた。これは、「ordinary seaman」(OS)との混同を避けるためであったと考えられる。その後、略語は「AB」と表記されるようになり、民間語源的には「able-bodied seaman」という表現が生まれた。「 able seaman」という「正しい」用語は、法律文書、船員の書類、船上では今も使われている。