Backbone

The backbone of a traditional wooden boat consists of the keel, stem, transom, frames (“ribs”), and floors. All these parts need to be in place on the construction molds before the hull’s planks are “gotten out” (cut to shape) and fastened. The backbone must be suitably strong and stiff so that the boat can withstand large forces imposed by wind and water. The butt of Justine’s mast connects to the backbone at the mast step, and a significant fraction of the forces produced by the mainsail and jib are transmitted to the hull via the backbone.

Components of the backbone should also be resistant to decay from moisture. Traditional wood species that are found in New England with the requisite properties are white oak (Quercus alba) and black locust (Robinia pseudoacacia), and I used both in Justine’s construction. I was extremely fortunate to obtain a supply of live oak (Quercus virginiana, not indigenous to New England) via my neighbor Scot Smith. The source was quite amazing: it had been excavated from an abandoned holding pond at Charlestown Naval Shipyard in Massachusetts! Some of this store of wood even ended up being used in the restoration of Mystic Seaport’s whaling ship the Charles W. Morgan. By the time I got my planks they had been re-sawn to 4/4 and 8/4 (1″ and 2″, respectively) planks and had about 2 years to air dry.

Building Justine’s backbone took me about 2 1/2 years, starting in January 2008 when I milled and glued up the mahogany planks for her transom and ending in July 2010 when the keel plank was bolted to the stem, floors, centerboard, and transom knee. Covering all this will take quite a few posts. In this one, I’ll simply show a few pictures that identify the major backbone components as some readers may not be familiar with the terminology.

The stem is the forward-most member of the hull. Justine’s stem consists of a single piece of black locust. Cleats on the barn floor hold the stem head in position, and construction molds 2–5 are beveled to support it.
Justine’s frames consist of 7/8″ square pieces of white oak. Heating in a steam box makes them pliable. After steaming, they are quickly clamped in place on the molds and allowed to cool. The clamps are subsequently replaced with metal “dogs” until the hull planking is in place.
Justine’s floors are made of live oak. They are triangular in shape and serve to connect pairs of frames at each station. Bolts through the floors tie the frames and keel plank (and/or stem forward, as seen here) together. Three copper rivets secure each frame to its corresponding floor. Here you see the frames with their floors at stations 2–5. Also seen are “dogs” and wooden wedges that hold the frames against their molds prior to planking the hull.
The transom is the after-most member of Justine’s backbone. Justine’s transom is positioned at about 45 degrees to horizontal. I carved her name in the transom while it was still horizontal and at a comfortable working height.
Justine’s keel plank is cut from a 16-foot piece of 8/4 (2″ thick) white oak. It is bolted to the stem forward, the transom and transom knee aft, and the floors and centerboard bedlogs in between.

In future posts, I’ll describe each element of the backbone construction in greater detail.