Why is the Colonel Called "Kernal"?
The Origin of the Ranks and Rank Insignia Now Used by the United States Armed Forces

Officers:

Navy Captain

Captains entered the English navy in the Eleventh Century as the commanders of soldiers serving on ships to do the fighting when needed. The ships were commanded by Masters, who were Warrant Officers. The Masters were in charge of operating the ships while the Captains just concerned themselves with combat. In the Fifteenth Century the Captains and their Lieutenants began taking over the executive functions on the ships. By 1747 the officers had full command of the ships so the British made Captain an official naval title and thereafter called the commander of any ship a Captain. In 1748 the British navy established three grades of Captain, depending on the size of ship commanded. The top grade of Post-Captain was equal in rank to an Army Colonel. The two lower grades eventually became the ranks of Commander and Lieutenant Commander in the British navy.

Captain was the highest rank in our Navy from its beginning in 1775 until 1857 when Congress created the temporary rank of Flag Officer, which gave way to Commodore and Rear Admiral in 1862. The commander of any warship was a Captain. This situation lead to three grades of Captain ranking, according to the officer's duties, with an Army Brigadier General, Colonel or Lieutenant Colonel. The top grade of Captain became Commodore or Rear Admiral in 1862 while the lowest grade became Master Commandant in 1806 and Commander in 1837. The Navy Captain thus remained equal in rank to an Army Colonel.

The eagle as a rank insignia for Captain first appeared in 1852 when he wore an eagle perched on an anchor on his epaulettes and shoulder straps. On the epaulettes he also wore a silver star, which he lost to the Commodore in 1862. The four sleeve stripes appeared in 1869. The four stripes also showed up on the Captain's shoulder marks in 1899. In 1941 he began wearing metal pin-on rank insignia on his khaki shirts. For that insignia he exchanged his eagle perched on an anchor for the spread-eagle worn by Army and Marine Colonels.