The armed merchant cruisers (AMC) of the British Royal Navy were employed for convoy protection against enemy warships. They ultimately proved to have limited value and many, particularly ocean liners, were later converted into troopships, a role for which they were more suited. Documentary evidence quoted by the BBC researched from the early stages of the First World War suggests that the express liners had greater speed than most warships (few warships of the period could exceed 21 knots), which made them suitable as AMCs. The downside proved to be their high fuel consumption; using them in a purely AMC role would have burned through the Admiralty reserve supplies of steam coal in less than three months. The ships were vulnerable to enemy fire because they lacked warship armour, and they used local control of guns rather than director fire-control systems, which reduced their effective fire power.