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|Lighthouse Construction Types|
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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 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 the 1830s. The first screwpile lighthouse type built in the United States was at 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, construction cost $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 were screwed down into the bottom and interconnected at their heads above the water reinforcing them together.
Screwpile lighthouses were relatively inexpensive, easy to construct, and comparatively quick to build. They became especially popular after the 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.
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 power 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 diffuses 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-story wooden dwelling) screwpile lighthouses were built throughout the Carolina sounds, the 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.
Surviving examples of these construction types include:
SEVEN FOOT KNOLL LIGHT, MD (1855) (Relocated on shore)
HALF MOON REEF LIGHT, TX (1858) (Relocated on shore)
SOUTHWEST REEF LIGHT, LA (1858) (Relocated on shore)
THOMAS POINT SHOAL LIGHT, MD (1875)
HOOPER STRAIT LIGHT, MD (1879) (Relocated on shore)
DRUM POINT LIGHT, MD (1883) (Relocated on shore)
MOBILE MIDDLE BAY LIGHT, AL (1885)
ROANOKE RIVER LIGHT, NC (1903) (Relocated on shore)
DISKPILE WITH SKELETAL TOWER
CARYSFORT REEF LIGHT, FL (1852)
SAND KEY LIGHT, FL (1853)
SOMBRERO KEY LIGHT, FL (1858)
SHIP SHOAL LIGHT, LA (1859)
ALLIGATOR REEF LIGHT, FL (1873)
FOWEY ROCKS LIGHT, FL (1878)
AMERICAN SHOAL LIGHT, FL (1880)