 The Birkenhead was never commissioned as a frigate , as two factors came into play while she was still under construction , that resulted in her being converted into a troopship . Firstly , the Royal Navy 's warships were switched from paddle wheels to more efficient propeller propulsion , following an experiment organised by the Admiralty in 1845 in which the benefits of the propeller over the paddle wheel were dramatically demonstrated . Secondly , the Admiralty had doubts about the effects of cannon shot against iron hulls — in a number of trials carried out at Royal Arsenal in 1845 , at lower velocities shot made a jagged hole that was hard to plug . 
