AIS MOB devices are not meant to replace but rather augment PLBs. That said, since they alert nearby AIS-equipped vessels, such as the boat from which the person fell overboard, to assist in the search-and-rescue effort, these devices can reduce rescue times substantially. A PLB, on the other hand, works worldwide, well beyond VHF range, to summon rescuers. It’s important to keep a PLB around if venturing far out to sea and away from other boaters. The downside is that it might take hours for first responders to arrive on scene.