I knot measures how fast a ship moves. Twelve knots is equal to traveling 12 nautical miles per hour. That might not seem like much but its pretty fast when your on the water.
http://answers.ask.com/Reference/Other/how_fast_is...

A knot is 6080 feet therefore 1.1515 miles multiply by 40 = 46.0606 MPH.
http://www.chacha.com/question/howfastis40knot...

40 knots is equal to 46.0311779 mph
http://www.kgbanswers.co.uk/howfastinmphis40...
