You may find some information with your great-grandfather's service record at The National Archives. Go to the website and search the catalogue at
https://discovery.nationalarchives.gov.uk/advanced-search . Put 'John Parnell' in the first line and 'ADM' into one of the References boxes (this will restrict the search to naval records). I tried also including a ship name but 'Curcoa' didn't work and neither did 'Curacoa' (which I think it ought to be).