The Oliver Cromwell was built at Saybrook, now Essex, during the first year of American independence. Connecticut had authorized a small state navy in 1775 to defend its coast, protect trade, and prey on British shipping. The colony's long maritime tradition supplied shipwrights, sailors, timber, rope, ironwork, and riverfront yards. In January 1776 the government approved a large warship, and builder Uriah Hayden began construction near the present Connecticut River Museum site.

Work proceeded quickly. The keel was laid in April, and the vessel was launched on June 13, 1776, before the Declaration of Independence had been adopted. Roughly eighty feet along the keel, with a beam of about twenty-seven feet and a burden near three hundred tons, Oliver Cromwell carried twenty guns and a large crew. Building and fitting such a ship required extensive skilled labor and supply coordination, making it both a military project and a major industrial undertaking for the river town.

Under captains including William Coit and Timothy Parker, the ship cruised against British commerce. State naval vessels operated alongside the Continental Navy and privateers, seeking prizes whose cargoes and sale proceeds could sustain the war effort. Cruising was dangerous and uncertain: crews faced storms, disease, scarce supplies, powerful Royal Navy patrols, and disputes over prize law. Oliver Cromwell captured multiple vessels and helped demonstrate Connecticut's determination to contest British control at sea.

A hurricane damaged the ship during a southern cruise, stripping away masts and showing that weather could be as dangerous as enemy fire. On June 6, 1779, British vessels engaged Oliver Cromwell near Sandy Hook. After a hard fight, the Connecticut warship struck its colors and was captured. The British renamed her Restoration and later HMS Loyalist, turning an American-built ship against the side that had commissioned it.

The vessel's career continued to cross national lines. French forces captured Loyalist in 1781 and transferred her back to American hands under the name Loyaliste. Those changes of flag illustrate the Atlantic scale of the Revolution, where ships, crews, and prizes moved among state, national, British, and French services. The war at sea extended far beyond famous fleet actions and depended heavily on commerce raiding and the capture, refitting, and reuse of vessels.

The Essex marker identifies the construction site rather than a surviving ship. No original Oliver Cromwell remains, but models, documents, and the waterfront setting help interpret her story. The marker ties a local shipyard to global conflict and recognizes the labor behind naval warfare. Here a riverside community transformed raw materials and maritime skill into one of Connecticut's largest Revolutionary weapons, then watched that vessel enter a complicated service life under several names and nations.