Editing Screw-pile lighthouse (section)

== History ==
[[Pile lighthouse|Non-screwpile (straightpile)]] tubular skeletal tower lighthouses were built, usually of [[cast-iron]] but also of [[wrought-iron]] piles, both onshore and offshore, typically on soft bottoms such as mud, sand, and swamp. [[Alexander Mitchell (engineer)|Alexander Mitchell]] invented the screwpile, a major improvement over the standard straightpile construction type. With his son, he patented his wrought-iron screwpile design in England in 1833.<ref>{{cite book|title=The Repertory of Patent Inventions, And Other Discoveries and Improvements in Arts, Manufacturers, and Agriculture|date=July–December 1847|publisher=Repertory of Arts and Manufacturers|page=116| url=https://books.google.com/books?id=YwILAQAAIAAJ&pg=PA116}}</ref> The Walde Lighthouse in northern France (Pas-de-Calais), established in 1859, was based on Mitchell's design. Although discontinued in 1998 and shorn of its lantern, it is the only remaining screwpile lighthouse in France.

=== Screw-pile lighthouses in the United States ===
The first screwpile lighthouse type built in the United States was at [[Brandywine Shoal Light|Brandywine Shoal]], [[Delaware Bay]], an area served by a lightship since 1823 and an ordinary straightpile lighthouse which stood briefly there in 1828 but was destroyed by ice. Major Hartman Bache, a distinguished engineer of the Army Corps of Topographical Engineers, began work in 1848 and completed the task in 1850, at a construction cost of $53,317. Alexander Mitchell served as consultant. The screwpiles were turned by a 4-foot capstan worked by 30 men. To protect the structure from ice floes an ice-breaker consisting of a pier of 30 iron screwpiles 23 feet long and five inches in diameter was screwed down into the bottom and interconnected at their heads above the water reinforcing them together. Subsequently, though, the use of [[caisson lighthouse]]s proved more durable in locations subject to ice.
[[Image:Middlebay1940 300.jpg|left|thumb|200px|[[Middle Bay Lighthouse]] in [[Mobile Bay]]]]
Screwpile lighthouses were relatively inexpensive, easy to construct, and comparatively quick to build. They became especially popular after the [[American Civil War|Civil War]] when the Lighthouse Board adopted a policy to replace inside (bays, sounds, and rivers) light vessels with screwpile lighthouses. Most screwpile lighthouses were made with iron piles, though a few were made with wooden piles covered with metal screw sleeves (these sleeves were probably adopted because they were less expensive and easier to insert into the bottom, plus the sleeve protected the wood from marine-boring invertebrates). The typical screwpile lighthouse was hexagonal or octagonal in plan consisting of a central pile which was set first and then the six or eight perimeter piles were screwed in place around it.

[[File:Seven Foot Knoll Light.JPG|thumb|[[Seven Foot Knoll Lighthouse]], [[Inner Harbor]], [[Baltimore, Maryland]]]]
Metal screwpiles were used to form the foundation of many lighthouses built on sandy or muddy bottoms. The helicoidal or screw-like cast-iron flange at the end of the metal pile was augured into the bottom increasing the bearing capacity of the pile as well as its anchoring properties. Yet lighthouses built with these foundations were found to be vulnerable to ice floes. In areas such as the [[Florida Keys]], where the bottom is soft coral rock, diskpile foundation lighthouses were built. Wrought iron piles were driven through a cast-iron or [[semi-steel]] disk which rested on the sea floor until a shoulder on the pile prevented further penetration. The disk distributes the weight of the tower more evenly over the bottom. In coral reef areas where sand is also prevalent, a cast-steel screw was fitted to the end of the pile to give it more anchoring ability. Cofferdams were used generally in shallow waters where it was not necessary to deeply penetrate the natural bottom. The cofferdam enabled the water inside the dam to be pumped out and the foundation built "in the dry".

Perhaps as many as 100 spider-like, cottage-type (1½-storey wooden dwelling) screwpile lighthouses were built throughout the Carolina sounds, [[Chesapeake Bay]], [[Delaware Bay]], along the [[Gulf of Mexico]], at least two in [[Long Island Sound]] and one even at [[Maumee Bay]] (1855), [[Lake Erie]], [[Ohio]]. Few survive to this day; many were replaced with caisson-type lighthouses. The tall offshore skeletal tower type was built in exposed open water at major coastal sites where visibility over ten miles was required. Six offshore skeletal towers were built in Florida; three before and three after the American Civil War, as well as one in the Gulf of Mexico off [[Louisiana]] prior to the Civil War.