After we had rowed, or rather driven about a league and a half, as we reckoned it, a raging wave, mountain-like, came rolling astern of us, and plainly bade us expect the coup de grâce.
Then all in one welded commotion came an invisible push from astern, while forward the boat seemed striking on a ledge; the sail collapsed and exploded; a gush of scalding vapour shot up near by; something rolled and tumbled like an earthquake beneath us.
The dinghy was trailing astern at the end of its painter, and Merrion looked at it as he passed. He saw that it was a battered-looking affair of the prahm type, with a blunt snout, and like the parent ship, had recently been painted a vivid green.
Adverb: "In the direction of the stern; backward (motion)"
1751, Thomas Birch, “A Discourse of the Invention of Ships, Anchors, Compass, &c.”, in The Works of Sir Walter Ralegh, Kt., volume 2, London: Dodsley, page 79:
We have given longer Floors to our Ships than in elder Times, and better bearing under Water, whereby they never fall into the Sea, alter the Head and shake the whole Body, nor sink astern, nor stoop upon a Wind, by which the breaking loose of our Ordnance, or the not Use of them, with many other Discommodities, are avoided.
1996, Graham Danton, The Theory and Practice of Seamanship, 11th edition, New York: Routledge, page 152:
Immediate action upon stranding: The engine(s) should be stopped and put astern if the tide is falling. Some students worry that this action could tear the bottom out of an already damaged ship and she will sink as she moves astern. It is most unlikely that going astern will move a ship off rock or coral but in the improbable event of it occurring, and foundering seeming imminent, the vessel can be re-grounded, or beached.