My Dad lent him his sextant ...
We have a family link to this story... my father Richard Hiseman - a former merchant seaman and navigator - knew Donald Crowhurst, and lent him his sextant which Crowhurst took on his voyage.
The sextant, like Crowhurst, was never seen again.
Referenced elsewhere on these pages, Nicholas Tomalin and Ron Hall's classic book The Strange Last Voyage Of Donald Crowhurst (https://www.amazon.co.uk/Strange-Voyage-Crowhurst-Sailors-Classics/dp/0071414290) provides incredible and poignant detail, as does the brilliant DEEP WATER drama documentary from Osmond/Rothwell (I think the Colin Firth movie is based on this doc).
Looking forward to seeing this in cinemas, although it'll be a grim couple of hours.