Screwpile lighthouse
A screw-pile lighthouse is a lighthouse which stands on piles that are screwed into sandy or muddy sea or river bottoms. The first screw-pile lighthouse to begin construction was built by blind Irish engineer Alexander Mitchell. Construction began in 1838 at the mouth of the Thames and was known as the Maplin Sands lighthouse, and first lit in 1841. However, though its construction began later, the Wyre Light in Fleetwood, Lancashire, was the first to be lit (in 1840).
In the United States, several screw-pile lighthouses were constructed in the Chesapeake Bay due to its estuarial soft bottom. North Carolina's sounds and river entrances were also once home to many screw-pile lights. The characteristic design is a 1 1⁄2-storey hexagonal wooden building with dormers and a cupola light room.
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 1833. 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.
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, 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 lighthouses proved more durable in locations subject to ice.
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