Marine radars can only detect objects above the horizon. The curvature of the Earth limits their detection range.
Some marine radars are marketed using their maximum ranges, which far exceed the typical over-the-horizon distances to another vessel. Flat-Earthers noticed the fact and used it as “evidence” against Earth’s curvature. In reality, the specified ranges are radar signal ranges. Earth’s curvature still limits the actual detection range.
While a marine radar with an advertised range of 50 nm will not detect vessels at that range, it is still useful and can identify higher-ground inland areas.