When construction began on the Seven Foot Knoll lighthouse in 1855 it was among the first screwpile lighthouses to be build in the United States, and just the second to be build on the Chesapeake Bay. The screwpile design originated in England in the 1830s as a means of placing a lighthouse on the water where a traditional tower was not possible due to strong current or soft bottom. Unlike the typical lighthouse, which was build on a rock or masonry foundation, screwpile lighthouses made use of a series of iron piles which were screwed deep into the sea floor and then interconnected for stability. The lighthouse itself was then built on top of the assembled piles. The need for a lighthouse at the mouth of the Patapsco River was recognized in 1851 when Treasury Department’s Lighthouse board called for “a light-house on the Seven Foot Knoll off Bodkin Point…..on the same principle as that erected at Brandywine Shoal…” The Brandwine Shoal Lighthouse in Delaware Bay was the first screwpile lighthouse built in the United States and was completed in 1850.
The Seven Foot Knoll Lighthouse was fabricated and assembled by the Baltimore iron foundry of Murray and Hazelehurst at a cost of $30,340. In the early 1850s, this company had constructed a number of pre-fabricated iron lighthouse tower, and for the screwpile lighthouse at Seven Foot Knoll the company again opted for a pre-fabricated structure made from iron plates. The design offered the advantages of ease of construction on the water and great durability.
Seven Foot Knoll Lighthouse was built on a foundation of nine hollow, cast-iron screwpiles, each nine inches in diameter. The piles were driven into the floor of the Patapsco River and then filled with concrete. For stability, the piles were interconnected with an intricate series of iron tension rods. The lighthouse itself was built on top of the piles at a height of nine feet above the average high tide. The first floor was built on a deck made from ½” thick iron plates. The walls of the lighthouse were made from 5/16” thick iron boiler plate riveted together in the same way that a steam boiler or iron ship hull would have been fabricated in the mid 19th Century. Seven Foot Knoll was designed as both an aid to navigation and as a living space for the lighthouse keepers. The plan for the first floor features a parlor and sitting room, a kitchen and two bedrooms. The first floor also features two water cisterns which were used to collect and store rainwater from the roof as a supply of drinking water.
The second floor was the keeper’s primary workspace where all the tasks necessary for maintaining the beacon were carried out. The third story, or lantern, was built to house the Fresnel lens and beacon lamp some 42 feet above the water. The greatest challenge to the durability of the screwpile lighthouses was the potential for damage from ice in winter.
In January 1884, for example, one of Seven Foot Knoll’s screwpiles was broken by floating ice and as a protective measure some 150 oak piles, arranged in clusters, were driven into river bed to form a protective screen against floating ice. Later, between 1894 and 1903, over 700 tons of “riprap stone” were deposited around the base of the lighthouse for ice protection.