Keel Plank – Sam Allen's Boats

With the centerboard trunk, frames, stem, and transom in place, it’s time to work on Justine’s keel plank and thereby complete her backbone.

Scot and I found a suitable white oak plank for the keel at New England Naval Timbers, on the same trip that we got stock for the stem and frames. We picked over an extensive stickered pile of planks that had air dried for a couple of years to find one that was suitably long (over 16′) and straight, not overly wide, and not overly thick. It gave us quite a workout because there were many planks larger than I needed that had to be shuffled around during our search.

The keel plank is white oak, about 9″ wide, 2 1/2″ thick, and over 16′ long. It had been stickered and air dried at New England Naval Timbers. Before unloading it, I gave it a coat of linseed oil and painted the ends with red lead to slow the rate of drying. Scot helped me move it from the trailer to a shady spot up by my barn. It was so heavy I was close to my strength limit just carrying one end.

When in place on the hull, the keel plank is curved enough that it needs to be steamed and quickly clamped in place. Because it is over 16′ long, and my steam box is only about 10′, each end needed to be steamed in separate operations. As noted in earlier posts, the keel plank is bolted to the floors at stations 5–22, and amidships the bolts also pass through the centerboard bedlogs. A slot must be cut in the keel plank so the centerboard will pass through it as it’s lowered and raised. The ends of the centerboard trunk posts fit through the ends of the slot in the keel plank and serve to lock it into alignment both fore and aft and side-to-side.

All ready for the keel plank, with centerboard trunk in place.

Cutting the keel plank to shape

The easiest way to cut and fit a large part like the keel plank is to make a plywood template for it. I used pieces of 1/4″ plywood for the template and began by cutting the holes for the projecting ends of the centerboard trunk posts and clamping it into its curved shape fore and aft. Then I marked the positions of each station on the underside of the template, and from my lofting I took off the dimensions for the widths of the keel plank at each station. By clamping the template stock in place on the backbone, then marking the positions of the stations on the template, I knew that the curvature of the keel plank would be properly accounted for in my template layout.

I’ve made a template for the keel plank, using pieces of 1/4″ luan plywood. I had lofted the keel plank, so I used the lofting to mark the template for width at each station. The keel plank will extend from the transom to the stem, and it has a significant arc. Note how the centerboard trunk posts stick up through the ends of a slot in the template.

I had stored the keel plank stock outdoors, slightly off the ground and under a tarp. It remained there from August 2008 to October 2009 before I was ready to cut into it. So I figure it had at least three years of air drying before I started to use it.

Keel plank template on white oak plank. I’m going to make the first cuts oversize as I’ve still got a lot of material to remove and I don’t know if the plank will distort when the excess is trimmed away.

One can’t predict what internal stresses might be present in a piece of lumber. A plank that is straight might take a significant bend if parts of it are cut away. So it’s safest to make the first cuts so that there is some excess material because then you have a second chance to correct for any bend. I left an extra inch or so along the sides, and did not take any off the ends on these first cuts.

Here’s the oversize blank for my keel plank. It also needs to be reduced in thickness from about 2 1/2″ to 1 3/8″.

At 2 1/2″ thickness, the plank I purchased was nearly twice the 1 3/8″ thickness of the finished keel plank. So I removed the excess by taking many passes through my thickness planer. This is an opportunity to plane off parts of the plank that may have imperfections that are close to one surface or the other. And to a limited extent you can correct any tendency for the plank to bow or twist by using a power plane to bring one plank surface close to flat, then begin taking passes off the other side with the thickness planer.

Running the keel plank through my thickness planer. The planer shavings really piled up.

At this point the plank is the proper thickness, but overly wide and long. After placing the plywood template near the center of the plank I marked out the positions of the centerboard trunk posts and drew lines to mark the extent of the centerboard slot I needed to cut. The most expeditious way to cut the slot is with a circular saw, but I was leery of making the plunge cuts this would require. With a few tips from Scot (plunge very slowly, don’t risk turning the saw and jamming the blade in the kerf), I was confident I could make the cuts safely. Because the saw blade is circular, these blind cuts need to be finished with a hand (rip) saw at the ends. After these cuts were finished, I bored some large holes near the ends of the slot to release the long waste piece in the middle, and I squared up the ends using a large chisel. Finally, I lined up my template with the slot in the plank, and marked the plank for my final cuts of the keel plank’s profile.

I transferred the positions of the centerboard trunk posts from the template to the keel plank and used my circular saw to make careful plunge cuts so I could form the slot for the centerboard to pass through the keel plank. I bored holes to release the material from between the saw cuts, then cleaned everything up with chisels, a rasp, and sandpaper.

Prior to attaching the keel plank, its sides need to be beveled. This is detailed in the lines plan, lofting, and construction plan. In subsequent pictures you will see that the beveling has been done.

Adding the keel plank to the backbone

The keel plank was too stiff to fit to the rest of the backbone without steaming it to make it more compliant. I began by steaming the aft end and clamping it down to set its curved shape.

After steaming the aft end of the keel plank, we clamped it in place around the centerboard trunk, and used an overhead “shore” to get the required bend back toward the transom. I left it in place for a couple of days to cool and set its new shape.

The overhead “shore” is a 2 x 4 that has been cut to an appropriate length so it can be inserted between the barn ceiling (jammed against a joist) and the end of the keel plank. A shingle serves as a wedge to force the keel plank down in place on the transom and transom knee.

Once the after end of the keel plank had been permanently bent, I steamed the forward end. Here you see the curved aft end; the forward end is in the steam box.

After sufficient steaming, the forward half of the keel plank is removed from the steam box and clamped in position at the centerboard trunk while it’s still hot. I used an overhead shore (removed before this photo was taken) to force it quickly into position against the stem, then replaced the shore with the clamps you see here.

The keel plank is fastened to the backbone with 29 silicon bronze carriage bolts of various sizes and lengths. I had pre-drilled all of the floors for the bolts before riveting the floors to the frames. So I was able to drill upward through the pre-drilled holes in each floor, and drill a perfectly aligned hole in the keel plank. (Some extra-long drill bits were required!) Then all that remained was to drill counterbores on the outside surface of the keel plank to accommodate the bolt heads and the bungs that would cover them. (I’ll explain how I made the keel bolts in a future post.)