When submarines first started firing torpedoes, ships attempted to use a high speed zigzagging strategy to avoid being hit. As the speed of submarines improved, ships resorted to other methods. Light steel nets were hung around warships beneath the water line to deflect incoming torpedoes. These nets were ineffective and were soon removed from warships.
When a submarine surfaced it was possible for ships to ram them. During the First World War nineteen U-boats were sunk in this way. Attempts were also made to shell submarines but it was difficult to hit such a small target before it dived. The Royal Navy also used submarines to hunt U-boats. Eighteen were torpedoed and destroyed in this way.
In 1915 Allied ships began using depth charges. These waterproof bombs exploded at a chosen depth. At first these were not very effective and between 1915 and the end of 1917, depth charges accounted for only nine U-boats. However, they were improved in 1918 and that year were responsible for destroying twenty-two U-boats.
The most successful method of dealing with submarines was placing mines at various depths along busy sea-routes. An estimated 75 U-Boats were destroyed in this way. Minefields were also used to blockade hostile submarine bases.