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.