Nothing Lachie Henderson faces in Friday night's preliminary final against Sydney – including Lance Franklin – will be quite as scary as a moment he had while surfing last week.

Henderson was surfing with his former Carlton teammate Jarrad Waite at Bells Beach last Friday when a surfer close to them was knocked off his board by a shark.

A two-metre shark became entangled in the surfer's leg rope, knocking him off his board and into the water.

Witnesses say the man was screaming and splashing for several seconds before the shark swam away.