In April and May the Spithead and Nore mutinies broke out. Grampus was one of the vessels caught up in the disorder, and is named in the proclamation read out on 10 June. The exact date of her arrival at Sheerness, the date of her joining the mutiny, and the date of her crew returning to duty are not known. Still, she was at Sheerness by 16 May. At the time, Grampus was preparing for a voyage to the West Indies. After the reading of the Proclamation on 10 June, the crews of a number of vessels sought to abandon the mutiny. On Grampus a fight broke out between loyalists and mutineers, a fight that the mutineer faction won. Under some reports, the mutineers abandoned her.
