On Saturday last [May 16, 1795], the keel of the United States frigate [Constitution]…was finished laying. It is 156 feet in length, and from the elegance of the workmanship, and the goodness of the timber of which it is composed…we anticipate that she will be one of the finest vessels that ever floated…” [“Federal Frigate,” Columbian Centinel, May 20, 1795.]



The keel is the backbone of a vessel. It runs from the forefoot in the bow to the deadwood of the sternpost. Joshua Humphreys, in his 1794 shipbuilding instructions, entitled Dimensions and sizes of materials for building a Frigate of forty-four guns, described exact materials and measurements for constructing the keel:

“Keel–Of good sound white oak, in three pieces; the middle piece to be not less than thirty feet, if to be had…”

In fact, Constitution‘s keel is made of four enormous pieces of white oak timber, equaling 150 feet in length, that were harvested in Trenton, New Jersey.


[Courtesy Naval History & Heritage Command Detachment Boston]
This plan of USS Constitution, drawn in 1996 and revised in 2011, shows the four sections of the keel and the approximately 12 foot scarfs linking the pieces of the keel together. [Courtesy Naval History & Heritage Command Detachment Boston]
Humphreys instructions continue:

“…scarfs not less than twelve feet, to be kept clear of the main and fore steps……the scarfs all to be tabled and bolted with five bolts, one an one-eighth inches diameter…”

A scarf joint is a long, angled overlap of two pieces of wood (see the illustration below). The overlapping nature of the joint, when fastened with bolts, is much stronger than if the two ends of the pieces were only butted and attached to each other. The use of scarf joints was and is common practice in wooden vessel construction. As can been seen in the 1996 plan above, Constitution‘s fore and mainmast steps sit directly above two of the keel’s several scarfs, contrary to Humphreys’s instructions.

[USS Constitution Museum Collection, 2331-68. © Stephen Biesty, 2015.]
A watercolor and ink illustration depicting a scarf joint in a detail of USS Constitution‘s keel. [USS Constitution Museum Collection, 2331.68. © Stephen Biesty, 2015.]


The entire keel structure is actually made up of several layers of wood. They are (from bottom to top, starting above the keel block in the illustration below): 1) The white oak keel, which projects from the bottom of the vessel; 2) The live oak V-shaped floor timbers, which sit directly on top of the keel and are the beginning timbers for the ship’s frames; and 3) The white oak keelson that sits on top of the floor timbers, which on Constitution is actually made of two timbers–the keelson and upper keelson. Protecting the keel from physical damage is the sacrificial “false keel.” This six-inch layer of white oak is attached to the bottom of the keel and acts like a bumper, taking the impact if the ship runs aground. When Constitution ran aground off the south coast of England in January 1879, the false keel performed exactly as intended. Eighty-five feet of the false keel ripped away as the ship was released from her grounding, with no damage to the actual keel. Together, these layers of white and live oak measure about six feet high for the entire keel structure.


[USS Constitution Museum Collection, 2331.72. © Stephen Biesty, 2015.]
A watercolor and ink illustration showing USS Constitution‘s keel resting on keel blocks as the ship’s frame is built up around it. [USS Constitution Museum Collection, 2331.72. © Stephen Biesty, 2015.]


When laid in 1795, Constitution‘s keel was straight and level. Overtime, a combination of the vessel’s weight, internal forces, and buoyancy caused the keel to hog, or curve downwards at the bow and the stern. Hog is a significant structural problem for wooden vessels. If left unchecked, the hog can cause a vessel’s keel to break, thereby destroying the ship. For most of the 20th century, Constitution‘s ship restorers at the Charlestown Navy Yard combated the recurring 14-inch hog in her keel. During the 1927-1931 restoration, Lieutenant John A. Lord noted, “The keelson, built of [layered] live oak…was in a decayed condition. The keelson was broken in two just abaft [the] foremast mast step.” Subsequently, both the keelson and upper keelson were replaced during the restoration. In a further attempt to strengthen the ship longitudinally and prevent future hogging, 18 x 18 inch assistant keelsons were installed.

When Constitution entered Dry Dock 1 in the Charlestown Navy Yard in September 1992, the keel had hogged again to 14 inches. The hog was visible inside the ship along the decks, as well as outside the ship in the dry dock. From 1992-1996, internal structural elements called for by Humphreys–including diagonal riders in the hold, standard knees at the bow and the stern, extra supporting stanchions on the berth deck, and other structural elements–which had long ago disappeared from the ship, were reinstalled. When the ship was launched from Dry Dock 1 in 1996 at the conclusion of the restoration, the hog had been reduced to zero.


[Courtesy USS Constitution Museum. Photo by Jack Pare.]
USS Constitution‘s keel had significant hogging before the 1992-1996 restoration, as visible in this 1992 photograph taken as water is drained from Dry Dock 1. [Courtesy USS Constitution Museum. Photo by Jack Pare.]

The reinstallation of these structural elements brought renewed strength to the ship’s keel and hull, making the project one of the two most significant restorations ever performed on Constitution. Within the first year after the ship’s relaunch in 1996, the then 200-year-old keel settled into a six-inch hog, which has not changed in the 20 years since. It is likely that during Constitution‘s active years in the early 1800s, she sailed with a several-inch hog in her keel.


In May 2015, keel blocks were set up once again in preparation for Constitution‘s dry docking. To accommodate the six-inch hog in her keel, the wooden tops of the keel blocks were custom designed to match the curve.


[Courtesy Naval History & Heritage Command Detachment Boston]
Keel and haul blocks are positioned in Dry Dock 1 in preparation for USS Constitution‘s dry docking in May 2015. [Courtesy Naval History & Heritage Command Detachment Boston]


The ship’s structure appears to have settled comfortably, so eliminating the hog is not part of this restoration. Nevertheless, ripples seen in the 1995 copper sheathing were a visible reminder that Constitution‘s keel had hogged slightly since her relaunch in 1996. The copper reacted to the movement, thus creating the ripples on its surface. When new copper is installed for the 2015-2017 restoration, it will be laid according to the keel shape that resulted from a stable six inch hog.


[Courtesy USS Constitution Museum]
A view of USS Constitution‘s keel from the floor of Dry Dock 1 in May 2015. Note the ripples in the copper sheathing, which indicate that the ship’s keel has settled since the last restoration in 1992-1996. [Courtesy USS Constitution Museum]


With the copper sheathing removed from Constitution‘s lower hull, the ship presents an extraordinary opportunity to learn about her remarkable construction. In March 2016, USS Constitution crew members and USS Constitution Museum staff, led by USS Constitution Museum Research Historian Matthew Brenckle and Naval History & Heritage Command Detachment Boston & USS Constitution Historian Margherita Desy, spent a morning examining the ship’s 221-year-old keel. This rare chance to view the ship’s keel won’t happen again for another twenty years.


[Courtesy USS Constitution Museum]
USS Constitution Museum Research Historian Matthew Brenckle and NHHC Detachment Boston USS Constitution Historian Margherita Desy discuss the building of Constitution and her keel with members of USS Constitution‘s crew and the USS Constitution Museum staff in March 2016. [Courtesy USS Constitution Museum]
[Courtesy USS Constitution Museum]
A USS Constitution Museum staff member has the opportunity to touch history, placing her hand on Constitution‘s 221-year-old keel. [Courtesy USS Constitution Museum]


Visitors to the Charlestown Navy Yard can learn more about the ship’s construction in a hands-on, minds-on exhibit, Forest to Frigate, at the USS Constitution Museum. The exhibit is a complement to USS Constitution‘s current restoration in dry dock. Visitors can follow the story of “Old Ironsides'” construction, from the establishment of the United State Navy, to the forests where the ship’s timbers grew, to her launch in 1797 as one of the original six frigates.


The activity that is the subject of this blog article has been financed in part with Federal funds from the National Maritime Heritage Grant program, administered by the National Park Service, U.S. Department of the Interior, through the Massachusetts Historical Commission, Secretary of the Commonwealth William Francis Galvin, Chairman. However, the contents and opinions do not necessarily reflect the views or policies of the Department of the Interior, or the Massachusetts Historical Commission, nor does the mention of trade names or commercial products constitute endorsement or recommendation by the Department of the Interior, or the Massachusetts Historical Commission.

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.

Phaedra Scott
Content Developer, USS Constitution Museum

Phaedra Scott was the Content Developer at the USS Constitution Museum from 2016 to 2017.