depth charges hydrophones sonar blimps hunter-killer submarines HMS R-1 anti-submarine weapons submarines hydrophone U-boats
The deaths of British merchant sailors and the seeming invulnerability of U-boats led to the development of depth charges (1916), hydrophones (sonar, 1917), blimps, hunter-killer submarines (HMS R-1, 1917), forward-throwing anti-submarine weapons, and dipping hydrophones (the latter two both abandoned in 1918).