Born to a family of rich merchants, Forfait studied at a Jesuit college in Rouen, where he was awarded prizes in Mathematics and Hydrography upon graduation.

In 1783, he embarked on the 110-gun Terrible, part of a Franco-Spanish fleet assembled before Cádiz under Admiral d'Estaing, but the end of the American War of Independence occurred before it saw action.

[4] After the Treaty of Paris, he returned to work at the Naval Academy and notably authored a Traité de la mâture[note 1][5] upon request by Castries, then Secretary of State of the Navy.

He was then sent to England to study the British shipbuilding techniques, and authored the Observations sur la marine de d'Angleterre[note 2] upon his return.

He then returned to Le Havre, where he designed 3 heavily armed gunboats to repel British bomb vessels.

At the collapse of the Peace of Amiens and the outbreak of the War of the Third Coalition in May 1803, Forfait was tasked with improving the Flottille de Boulogne.