Knapp was an industrial designer. He studied in Leerdam, Holland, immigrated to Australia and in 1952, won the Beetle Elliott Prize for a moulded radiogram. He was a foundation member of Society of Designers for Industry, President SDI 1965-1966 He was a designer for Ferris Industries Australia, Crown Crystal Glass, HMV. Later he was a principal of Knaap-Theon Associates and Displaycraft, Sydney.