Lead Line: A weighted rope with a lead weight at one end was lowered into the water to measure the depth. If the weight made contact with an underwater object, it could indicate the presence of a submarine.
Soundings: Similar to the lead line, sailors would take regular measurements of the water depth using a sounding line. Sudden changes in depth could suggest the presence of an underwater hazard or submerged object.
Visual Observation: Sailors kept a watchful eye for any signs of enemy submarines, such as periscopes, wakes, or any other suspicious activity on the surface of the water.
Listening Devices: Before the advent of sonar, various primitive listening devices were used to detect underwater sounds, such as hydrophones and underwater microphones. These devices were often placed in the water near a coastline or harbor to help detect approaching enemy submarines.
Nets and Barrages: In some cases, navies used nets or barrages to block off potential submarine routes. These barriers were designed to prevent submarines from entering or leaving an area.
Acoustic Signal Systems: Early acoustic signaling systems, such as the Submarine Signal Company's "Fathometer," were used for echolocation and depth measurements. These systems employed sound waves to estimate the distance to underwater obstacles or the seafloor, providing rudimentary sonar-like capabilities.