Complex sentence: The hydrophones are placed at equal distance on the cable, on the first few hundred meters that are near the ship's propeller there are usually none since their effectivity would have been reduced by noise and turbulences generated by the propulsion.
Split into simpler sentences: The hydrophones are placed at equal distance on the cable. On the first few hundred meters near the ship's propeller there are usually none since their effectivity would have been reduced by noise and turbulences generated by the propulsion.