Mr Duck

Peter Duck is an elderly seaman who sailed on many clipper ships before retiring to own and captain the wherry the Arrow of Norwich. He joins Captain Flint and the Swallows and Amazons on their schooner Wild Cat as an able seaman.

Peter Duck's tale of being shipwrecked when and watching two men bury a mysterious box on the deserted Crab Island has led to much speculation about treasure. This inspired the notorious Black Jake to search unsuccessfully for the treasure and keep pestering Peter for more information. When Black Jake sees the Wild Cat setting sail with Peter aboard, he is convinced that Captain Flint has persuaded him to to lead them to the treasure on Crab Island.

Peter Duck is a calm individual who is not really interested in treasure-hunting. He acts as a curb to Captain Flint's enthusiasm and provides essential adult supervision of the children.

Does Peter Duck exist?

Peter Duck is described as Titty's imaginary friend in Swallowdale (SD4).

In 'Their Own Story' Peter Duck makes a sudden entrance (TOS2) then engages in the ongoing conversation about telling a story, contributing the name Wild Cat for the putative schooner. He got this name from an island he had seen on a chart in "...that book you was talking about" (he meant SA). Mr Duck continues to serve as able seaman and occasional milk supplier to Polly Ann on her Norfolk cruise.

Source

Peter Duck's appearance and early sailing history is based on Captain Sehmel who sailed with Arthur Ransome and Evgenia Shelepina (the Cook) in Racundra's First Cruise.