Pinnaces are a unique kind of ship. I don't have a ton of data for this article so I apol0gize but i have a few scraps. They were a light fast ship that had oars and sails. They were commonly used as spies or to take mail back and forth. They also escorted fleets.
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.
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.