Crab Island is an island in the Caribbean Sea on which the young Peter Duck was washed-up after being shipwrecked, about 1870. While living as a castaway on the island, Peter Duck saw two piratical types bury a chest at the foot of a tree.
A footnote in Peter Duck says Crab Island is Not to be confused with the larger Crab Island east of Puerto Rico (PD3): The island is now called Vieques; see Wikipedia: Vieques, Puerto Rico
Trinidade is the largest island of the Trindade and Martim Vaz archipelago located about 1,200 kilometers (740 mi) east of Vitória in the Southern Atlantic Ocean, belonging to the State of Espírito Santo, Brazil. The archipelago consists of six islands; Trindade being the largest island, with an area of 10.1 km² and Martim Vaz the second largest, with an area of 0.3 km² (30 hectares). See Wikipedia: Trindade and Martim Vaz
Trinidade (win an E, Portugese for 'trinity') should not be mistaken for the Caribbean island of Trinidad (without an E, Spanish for 'trinity').
The Cruise of the Alerte
In 1889 E F Knight sailed to Trindade in a 64 foot yawl named the Alerte, in search of a buried treasure reported to have been left there by pirates in 1821. He wrote the book The Cruise of the Alerte about his voyage, with detailed descriptions of Trindade.He had previously visited the island in December 1880. There is a graphic description of the island and his adventures while exploring it in his book The Cruise Of The Falcon
Arthur Ransome, a long-time fan of Knight's sailing books, used the descriptions from Knight's books as a basis for Crab Island, except that he set the island further north in the Caribbean Sea; in a sort of in-joke, Ransome placed the island near the location of the without-an-E Trinidad. (See Wikipedia: Trinidad and Tobago)