The April mystery ship is the tug Knocker White, which was built by T. van Duijvendijk at Lekkerkerk, Netherlands as the Thames lighterage tug Cairnrock in 1924 for Harrisons (London) Ltd. She originally had a 400ihp steam compound engine from Crabtree & Co, Great Yarmouth. In 1960 she was sold to Alfred E. White, who fitted a pair of Petters diesel totalling 600bhp and geared to a single shaft. Renamed Knocker White, she was operated by W. E. White & Sons (Towage) Ltd of Erith until the end of the 1970s, when the company largely ceased operations. Sold for scrapping at Erith in 1983, she was saved and purchased for restoration by the Museum of London, where she remains, moored in front of the museum in West India Dock. With little background, the photo is hard to locate, but was definitely taken on the Thames or Medway.
David Asprey, Shoeburyness, Essex