An Open and Shut “Caisson”

CAISSON
/ˈkāˌsän,ˈkāsən/
Origin: late 17th century: from French, literally ‘large chest,’ from Italian cassone, the spelling having been altered in French by association with caisse ‘case.’

The caisson, a large floating gate, seals Dry Dock 1 from Boston Harbor. The original caisson built in 1833 was made of wood. In 1902, it was replaced with a steel caisson, which was the first steel hull constructed in the Charlestown Navy Yard. The second caisson remained in use until 2015, when a new caisson manufactured in Norfolk, VA by Steel America was installed in preparation for USS Constitution‘s current restoration.

Dry Dock 1 in the Charlestown Navy Yard and its companion dock in Gosport, VA were the first two public dry docks built in the United States. Both docks were built between 1827 and 1833 and were considered engineering marvels at the time. Gosport’s Dry Dock 1 opened first on June 17, 1833, making it the oldest dry dock in the country. Boston’s Dry Dock 1 opened at 5:30 AM on June 24, 1833 when Isaac Hull, a naval hero of the War of 1812, commanded Constitution across the dock’s sill. Vice President Martin Van Buren was there to witness the ship’s first ever dry docking.

Integral to the functioning of the dry dock is the caisson, or floating gate. The original wooden caisson (pictured below) was truly a vessel. When filled with water, the water-tight caisson would be sunk into place in the sill, sealing the seaward end of the dock. On its own, the caisson could float outside of the dock when not in use.

In the early years, Dry Dock 1 was flooded using the high tide cycle. “Filling gates” in the seaward walls of the dock were opened to admit the tide. When the height of the water in the dock equaled that of the harbor, the caisson would be emptied of its water and floated free from the dock’s sill. This opened the foot of the dock, allowing for the entrance of a vessel.

The 1833 caisson was replaced with a riveted steel caisson that was launched on October 31, 1901 and officially placed in service in 1902. The 1901 floating gate is 44.75 feet wide along the keel and flares to 63 feet at the top. It is 31.5 feet high from the keel to the top.

Tubes running through the 1901 caisson are used to flood the dry dock, as seen in the image below from Constitution‘s 1992-1996 restoration. Even though it is the Navy Yard’s oldest extant “vessel,” the 1901 caisson continues to function and has been used well into the 21st century. It was recently replaced by the new caisson for Constitution‘s current restoration.

The dock’s third caisson, manufactured by Steel America, arrived in Boston on February 27, 2015 and was installed in Dry Dock 1 on April 1, 2015. The 296-ton caisson was launched from its barge into Boston Harbor by “Chesapeake 1000,” the largest East Coast floating crane capable of lifting 1,000 tons.

The time-lapse video below shows the caisson being “picked” from the barge at 11:20 AM. The caisson’s keel was in the water by 12:45 PM.

The new caisson will remain in place for the duration of Constitution‘s dry docking, scheduled to conclude in the Autumn of 2017.

The Author(s)

Margherita M. Desy
Historian, Naval History & Heritage Command

Margherita M. Desy is the Historian for USS Constitution at Naval History and Heritage Command Detachment Boston.

Kate Monea
Manager of Curatorial Affairs, USS Constitution Museum

Kate Monea is the Manager of Curatorial Affairs at the USS Constitution Museum.