The incident had to have happened between 1769 and 1776 as the same story is recounted another book that he wrote/edited Correspondence of "First Citizen"--Charles Carroll of Carrolton, Ang "Antilon"--Daniel Dulany, Jr., 1773: With a History of Governor Eden's Administration in Maryland. 1769-1776 (1902).
Riley wrote in The Ancient City:
Tradition tells us, that they built the "Brig, Lovely Nancy"—at the launch of which the following incident occurred: "She was on the stocks, and the day appointed to place her on her destined element, a large concourse of persons assembled to witness the launch, among whom was an old white woman named Sarah McDaniel, who professed fortune-telling, and was called 'a witch.' She was heard to remark— 'The Lovely Nancy will not see water today.' The brig moved finely at first, and when expectation was at its height to see her glide into the water, she suddenly stopped, and could not be again moved on that day. This occurrence created much excitement amongst the spectators ; and Captain Slade and the sailors were so fully persuaded that she had been 'bewitched,' that they resolved to duck the old woman. In the meantime she had disappeared from the crowd; they kept up the search for two or three days, during which time she lay concealed in a house."
Another source, A NOTICE OF SOME OF THE FIRST BUILDINGS WITH NOTES OF SOME OF THE EARLY RESIDENTS, by Rebecca Key, published in the Maryland Historical Magazine, vol. 14 gives us what reports to be a first-hand account of the incident. Key was born in 1754 so the dates are consistent."The 'Lovely Nancy,' did afterwards leave the stocks, and is said to have made several prosperous voyages.
The only vessel whose name I recollect was called "The Lovely Nancy" after Mrs. Roberts, an intimate acquaintance with whom I used to play in childhood. I remember the name from an incident connected with the launching. She was on the stocks and a large concourse of people assembled to see the launching. An old woman named Sarah McDaniel (white), a fortune-teller and witch, who was standing by said: "The 'Lovely Nancy ' will not see water to-day." She moved finely for a while but stuck at last and Captain Slade with his sailors, fully under the impression that the vessel had been bewitched, determined to duck the old woman. They searched for her busily two or three days during which time she lay secreted in my father's kitchen, which stood adjacent to his dwelling on the lot opposite to Mrs. Walshe's residence.It is interesting to note that the woman's statement had an effect on the minds of the crew, so much so that the Captain sought to get revenge on her. And while, if he had caught her, he would have probably gotten away with dunking her, killing her would have been out of the question. Maryland at the time was under the laws of England and the The Witchcraft Act of 1735 removed the death penalty for witchcraft and instead punished anyone who "pretended" to use witchcraft with imprisonment for up to a year.The ruling classes, under the influence of the spirit of the age, ceased to believe that old women casting spells could have any effect on the health of a person or the safety of ships at sea, but the law still recognized that the common people could be tricked and thus victimized by those who pretended to practice the black arts.