Premise: True as a Turtle is a 1957 British comedy film directed by Wendy Toye and starring John Gregson, Cecil Parker, June Thorburn and Keith Michell. In the film, a young couple embark on a voyage on a ketch named "Turtle". John Coates wrote the screenplay, based on his novel of the same name.
Hypothesis: Coates was inspired to write a story about the ketch because of the sailing trips he took with his father.