Today we consider such concepts as “captain”, “boatswain”, which are synonyms.
Etymology and Meanings
The word "boatswain" comes from English, in some sources from the Dutch (Dutch) word boatsman, which means "boatman". This value was used before the revolution, and after it was excluded from the language.
So, the boatswain is a military non-commissioned officer rank that existed in the Russian fleet, as well as in the fleet of some other countries of the world. This title is applied in the military units of modern states. Simply put, the boatswain service is a non-commissioned officer position.

In the explanatory dictionary of S. Ozhegov and N. Shvedov, the following definition is given: "The boatswain is the youngest person to whom the deck team obeys. He needs to know the names of all the sailors who are subordinate to him, also non-commissioned officers, to monitor their behavior, to know about their skills and the skills of the maritime service. In addition, this person must know the equipment of the ship, be able to work with a compass, control the steering wheel and sails. If the ship has a boatswain team, then among its employees is appointed senior or chief - boatswain. On a merchant ship, the boatswain dit for rescue equipment, boats, anchors, their capacity for work. He directs the loading and securing of cargo, distributes work among the sailors. "
The full dictionary of foreign words by M. Popov says that the boatswain is the senior rank between the sailors, their closest boss. The same dictionary gives a more concise definition. The boatswain is a sergeant in the navy.
Meaning of the word "sergeant major"
This title came from the German feldwebel and was used in the tsarist army. It means the rank of non-commissioned officer, who is the assistant commander for economic affairs and internal routines. It corresponds approximately to the rank of senior sergeant. In Russian, the word has been used since the 17th century. This rank was introduced by Peter I and was used not only in the Navy, but also in the cavalry, where it was called a little differently - the Wahmister.
Later, the title of sergeant major was replaced by “second lieutenant” and “ensign”.
The word "boatswain" refers to such concepts as "captain", "sergeant major", "non-commissioned officer", "wahmister", "sergeant".