Great white sharks infrequently attack and sometimes even sink boats. Only five of the 108 authenticated unprovoked shark attacks reported from the Pacific Coast during the 20th century involved kayakers.[89] In a few cases they have attacked boats up to 10 metres (33 ft) in length. They have bumped or knocked people overboard, usually attacking the boat from the stern. In one case in 1936, a large shark leapt completely into the South African fishing boat Lucky Jim, knocking a crewman into the sea. Tricas and McCosker's underwater observations suggest that sharks are attracted to boats by the electrical fields they generate.