I know I really should get out more, but I have to point out that no prisoners from the hulks were hanged at Wapping. Until 1783, public hangings in London were done at Tyburn (now Marble Arch) and then up to about 1868 outside Newgate Prison. CJB probably has in mind hangings at Execution Dock at Wapping, which were done under the jurisdiction of Admiralty courts to punish pirates and mutineers.

Not much fun being a pedant - so much is intensely irritating.