The Mahogany Sheerstrakes: Shaping and Hanging

I purchased four 5/4 Honduras mahogany planks, each 12′ long, to use for the sheerstrakes. Because the finished sheerstrakes are about 22′ long, I knew I would be making each of them up from two pieces of lumber. I planed the planks to slightly over the sheerstrake’s 1″ maximum thickness before cutting them to the spiled shape. This left a bit extra for rounding and sanding the thickest part of the sheerstrake.

I decided to do most of the shaping of the sheerstrakes by hand. But I did use my circular saw to make long parallel grooves of appropriate depth at about five locations spaced at fixed distances from what would be the sheerstrake’s top edge. This would give me reference lines that would indicate when I was getting close to planing off most of the unwanted material, after which I could start working toward a smooth and consistent final profile.

I did all of the final shape refinement using a cabinet scraper that I’d filed to an appropriate shape. Using the scraper was a slow process, but very effective. It’s important to keep the scraper very sharp and to scrape “with the grain.” (I found it necessary to shift directions as the grain in some of my planks changed.) Early on, I fitted my scraper with a small piece of wood that hung down over the top edge of the sheerstrake to act as a fence that would ensure that the molded profile maintained a fixed distance from the sheerstrake’s top edge.

As illustrated on the Flatfish construction drawing, the molded profile gets somewhat thinner approaching the transom and stem. That required carefully planing off some more material, and doing some additional hand scraping and  sanding. With all the hand work, I was glad I’d decided to make the sheerstrakes out of mahogany rather than oak (an option on the construction plan)!

Once the forward section of the sheerstrake was shaped and fitted against the adjoining cedar plank, I made the aft piece using the same method.

The two halves of the sheerstrake would need to be joined together with a scarfed joint. Anticipating this, I left extra material to remove near the scarfed ends when I created the planks’ molded shapes. I laid out the scarfs with an 8:1 slope, rough cut them on the bandsaw, then overlapped and planed them simultaneously with my 07 joiner plane. This method ensures a very close fit of the scarfs.

With the two pieces scarfed and ready to glue up, I needed a method to get them properly aligned so that the long continuous piece would fit properly against the adjacent cedar plank. I decided to use the spiling batten to draw the plank’s shape, from 6′ forward of the scarf to 6′ after it, on a board about 12′ long, then clamp the two pieces together and glue them directly on the marked board, aligned with the line I’d drawn on the board. I used epoxy thickened with silica to bond the pieces together.

After joining the two halves of what would be Justine’s port sheerstrake, I finished refining the molded shape around the scarfed joint. I also marked a second set of mahogany planks to make the starboard sheerstrake.

I did all of the work on the sheerstrakes working alone, including fastening them. I found that the best way to clamp them in position was to start amidships, and add clamps working forward and aft from there.

The sheerstrake’s forward end fastens into the top of the stem rabbet, so that end needed to be trimmed into the appropriate shape. Once that was done, with the sheerstrake clamped in position along its entire length, I put a piece of tape across the sheerstrake and its adjacent cedar plank near the amidships clamp, to serve as a reference mark for fore-and-aft positioning of the sheerstrake when I started fastening it in place. In the process of fitting and trimming the sheerstrake, I clamped it in place, marked it, and removed it numerous times. Eventually, it was ready to fasten.

All the sheerstrake’s fasteners are counterbored at least 1/4″ or so in order that they can be covered with mahogany “bungs” and concealed. And, as indicated on the construction plan, some of the fasteners are only temporary, as after the hull is turned over they will ultimately be replaced by copper rivets that also hold the sheer clamps in place.

I was reluctant to fasten the sheerstrake into the stem rabbet at this point because it took a lot of force to bring it into position with clamps. I decided it should be steamed before fastening, and I didn’t do that until after the hull was painted and Justine was right-side up and outside the barn. I also waited to caulk the entire sheerstrake joint until after the sheerstrakes were completely fastened.

I began planking by installing the garboards in August 2010, and finished installing the sheerstrakes in May 2014.