text:  During the 1860s , steam engines improved with the adoption of double @-@ expansion steam engines , which used 30 – 40 % less coal than earlier models . The Royal Navy decided to switch to the double @-@ expansion engine in 1871 , and by 1875 they were widespread . However , this development alone was not enough to herald the end of the mast . Whether this was due to a conservative desire to retain sails , or was a rational response to the operational and strategic situation , is a matter of debate . A steam @-@ only fleet would require a network of coaling stations worldwide , which would need to be fortified at great expense to stop them falling into enemy hands . Just as significantly , because of unsolved problems with the technology of the boilers which provided steam for the engines , the performance of double @-@ expansion engines was rarely as good in practice as it was in theory . 
  