Adam Clarke on Acts 27:29

Cast four anchors out of the stern - By this time the storm must have been considerably abated; though the agitation of the sea could not have subsided much. The anchors were cast out of the stern to prevent the vessel from drifting ashore, as they found that, the farther they stood in, the shallower the water grew; therefore they dropped the anchor astern, as even one ship's length might be of much consequence.