page_title: Lightvessel
section_title: Mooring
hierarchical_section_title: Lightvessel / Construction / Mooring
caption_attribution_description: Photo by William J. Grimes
context_page_description: A lightvessel, or lightship, is a ship that acts as a lighthouse. They are used in waters that are too deep or otherwise unsuitable for lighthouse construction. Although some records exist of fire beacons being placed on ships in Roman times, the first modern lightvessel was off the Nore sandbank at the mouth of the River Thames in England, placed there by its inventor Robert Hamblin in 1734. The type has become largely obsolete; lighthouses replaced some stations as the construction techniques for lighthouses advanced, while large, automated buoys replaced others.
context_section_description: Holding the vessel in position was an important aspect of lightvessel engineering. Early lightships used fluke anchors, which are still in use on many contemporary vessels. These were not very satisfactory, since a lightship has to remain stationary in very rough seas which other vessels can avoid, and these anchors are prone to dragging.
  Since the early 19th century, lightships have used mushroom anchors, named for their shape, which typically weigh 3-4 tons. They were invented by Robert Stevenson. The first lightvessel equipped with one was an 82-ton converted fishing boat, renamed Pharos, which entered service on 15 September 1807 near to Bell Rock, and had a 1.5 ton anchor.  The effectiveness of these anchors improved dramatically in the 1820s, when cast iron anchor chains were introduced (the rule of thumb being 6 feet of chain for every foot depth of water).
caption: Lightship Portsmouth (LV-101) shows its mushroom anchor. It can be seen at downtown Portsmouth, Virginia, and is a part of the Naval Shipyard Museum.
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