As a ship's boat the pinnace is a light boat, propelled by oars or sails, carried aboard merchant and war vessels in the Age of Sail to serve as a tender. The pinnace was usually rowed, and could be rigged with a sail for use in favorable winds. A pinnace would ferry passengers and mail, communicate between vessels, scout to sound anchorages, convey water and provisions, or carry armed sailors for cutting-out expeditions. The Spanish favored them as lightweight smuggling vessels while the Dutch used them as raiders. In modern parlance, pinnace has come to mean a boat associated with some kind of larger vessel, that doesn't fit under the launch or lifeboat definitions.
Rendering of a pinnace under a small square sail, Seth Eastman c. 1850
With the introduction of steam propulsion came the steam pinnace. Coal burning warships were particularly vulnerable when at anchor, immobile until they could get a head of steam. Steam pinnaces were designed to be small enough to be carried by the capital ships they were allocated to and in addition to other duties were armed to act as picket boats.
^cf Knox, Dudley, ed. (1940). Naval Documents Related to the Wars With Barbary Powers, Naval Operations from 1802 to 1803II. U.S. Gov't Printing Office. pp. 267; 270. (examples: "[a]t 5 sent our pinnace along side of a French Man of War (lying at Tunis) with a letter to Consul Eaton. . ."; "[a]t 8 the pinnace returned from the island, she found no bottom within 20 or 30 yards of the shore."; "[a]t 2 lower'd down our pinnace alongside of an American vessel lying in the bay. When the pinnace returned Lieu't Stewart gave us the following interesting news . . .")(extracts from journal of U.S. Frigate Constellation, Captain Alexander Murray, U S Navy, 6 Sept. 1802).