A packet is a boat that plies a regular route carrying passengers, freight, and mail. In nineteenth century sailing terminology, "ship" refers strictly to those large vessels carrying three masts and fitted with a full complement of square sails. Packet ships were the workhorses of the open seas during the sailing packet era from 1818 to 1858. The classification of William Frothingham as a packet ship therefore conveys both her form and her function.
William Frothingham was built in 1851 at Belfast, Maine, by White & Conner. She was built for John W White et al., Belfast. She was a medium model ship, 163 feet long. She measured 33.5 feet across her beam and had a depth of 21.5 feet. She displaced 830 tons of water with a draft of 18 feet fully loaded. Her hull was fashioned from white oak with iron and copper fastenings. Her two decks were capable of housing at least 269 passengers. Above her main deck she sported a half poop. William Frothingham was typical of the Maine-built type of vessel that became known as the "Down Easter". These three-masted, full-rigged ships gained the reputation as being fast, able, and reliable ocean carriers. The vast majority of Down Easters found service as transients or general traders on the Seven Seas.