Captain Carl Sehmel, an old sailor from Riga, Latvia was the Ancient Mariner on board Racundra as described in Racundra's First Cruise. He also looked after Racundra when the Ransomes were away from Riga. He had sailed with Lady Brassey in the Sunbeam in the 1880s (Life page 257-8,285).
He was the model for Mr Duck in Peter Duck, and also for Simon Fastgate in Coot Club, with the same views on bacon which should be thick an' soft an' jewsy (CC19) (Life page 257-8,336)