 The Birkenhead was launched on 30 December 1845 by the Marchioness of Westminster . Her hull then weighed 903 tons and drew 9 @.@ 75 feet ( 2 @.@ 97 m ) , although she was at this time missing approximately 15 tons of cabin fittings . Machinery , stores , and other fittings were expected to add an additional 1 @,@ 000 or so tons , increasing her draught six more feet . She undertook her maiden voyage to Plymouth in 1846 , averaging 12 knots ( 22 km / h ) to 13 knots ( 24 km / h ) for the journey . 
