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nun-buoy

nun-buoy Naut.
  [Cf. nun n.2, quot. 1615.]
  A buoy of circular shape in the middle and tapering towards each end.

1703 W. Dampier Voy. III. i. 149 The Tide..runs very swift here, so that our Nun-buoy would not bear above the Water to be seen. 1769 Falconer Dict. Marine (1780) s.v. Buoy, Nun-Buoys, are shaped like the middle frustum of two cones, abutting upon one common base. 1816 Specif. Foulerton's Patent No. 4040, The term nun buoy is applied generally to that description of buoy which is fastened by a rope to a ship's anchor by which you may know where the anchor lies. 1875 Bedford Sailor's Pkt.-Bk. v. (ed. 2) 136 Wrecks will still continue to be marked by green nun-buoys.

Oxford English Dictionary

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