Fisher's belief that battlecruisers would take the place of light cruisers to protect commercial shipping soon proved impractical, as their high construction cost precluded their availability in sufficient numbers to do so, and destroyers were too small for scouting duties. The group of 21 Town-class cruisers begun in 1910 proved excellent in scouting in all types of weather and could carry enough fuel and ammunition to guard the shipping lanes. The Arethusa class, launched three years later, was also successful.