Sheer Clamps – Sam Allen's Boats

The first parts to go into the freshly painted hull are the sheer clamps. Justine‘s sheer clamps are Douglas fir pieces that are 1 3/8″ square in cross section and about 22′ long. I made them up from 12’ long pieces that were scarfed and joined with epoxy. The sheer clamps help keep the hull in its designed shape, they add stiffness, and they serve as a foundation for deck beams in the forward part of the boat and the mahogany covering boards and coaming aft.

In Herreshoff’s Fish class boats and many other designs of his, the ends of the deck beams rest on and fasten to the sheer clamps, while back in the cockpit area they support mahogany “covering boards” that lie atop the sheerstrake and sheer clamps. So the sheer clamps have to dip down in a “squiggle” trajectory amidships to perform these two different functions.

The sheer clamps extend from the stem to the transom along the top of the hull. They help hold the hull in the proper shape and stiffen it, and they also serve as a major component that helps to support the decking. Note the dip (“squiggle”) in the sheer clamps just forward of the centerboard trunk.

The sheer clamps’ squiggles occur over only about four frame spacings, or about 39″, so in this region there is an abrupt curve vertically, superimposed on the gentler horizontal curve from stem to stern. I figured this could only be achieved by steam bending, so I made a jig that would facilitate a compound bend.

I made this bending jig to impart both the “squiggle” between stations 11 and 15, as well as the long curved shape of the sheerclamp as it runs fore and aft.

I cut a 2″ square opening in the closed end of my steam box, and steamed the center section of the sheer clamp. I then clamped the hot piece in the jig and let it cool.

I steamed the midsection of the sheer clamp where the “squiggle” would be, then clamped it on the bending jig.

The sheer clamp also takes a rather sharp curve as it approaches the stem, so in a separate operation I steamed the forward section of the sheer clamp and put a suitable bend in it by re-clamping the midsection to the jig I’d made, and tying the end down to my anvil.

I also steamed the forward section of the sheer clamps, as they take quite a curve as the sheer clamp approaches the stem. I’ve put the sheer clamp in the bending jig, and have pulled the forward section down by tying it to my 120# anvil which is sitting on the ground.

Fortunately, the after end of the sheer clamp is only gently curved and it could be positioned properly without needing to be steamed. Aft of station 15, where the covering boards will go, the sheer clamp is positioned to lie fair along the bevel on the top of the sheerstrake, using the deckbeam mold as a guide.

I’m just starting to clamp the sheerclamp into position. I fitted the forward part first, and allowed the after end to run up over the sheerstrake and hang free for until the forward section is properly positioned and fastened.

The sheer clamp is fastened with rivets made from #8 copper nails. One rivets passes through the sheerstrake, frame, and sheer clamp at each station. Because it is fastened to the inside of the frames, the sheer clamp stands off the sheerstrake by the 7/8″ frame thickness. The holes in the sheeerstrakes are counterbored, the copper nails are inserted on that side, then the rivets are headed over burrs on the inside surface of the sheer clamps. A second rivet secures the sheerstrake to the frame at each station. (Prior to installing the sheer clamps, the sheerstrakes had been fastened temporarily to the frames with screws.) Once all the riveting is done, I bunged the counterbored holes in the sheerstrakes.

The sheer clamp is fastened with copper rivets that pass through the sheerstrake and frames. Rivets also attach through the sheerstrake and frames above the sheer clamp forward of the squiggle, and below it aft of the squiggle.

Back at the transom, the sheer clamps fit into oak blocks that provide the 7/8″ standoff from the sheerstrake and make a firm anchoring point for the clamps. The blocks are made from pieces of 8/4 white oak, rounded to fit against the curved inner surfaces of the sheerstrake and planking. They have a compound miter to fit against the transom, and a 1 3/8″ square rabbet to accommodate the sheer clamp.

Back at the transom, the sheerclamps fit into substantial oak blocks that have curved outer surfaces that fit against the curved inside surface of the sheerstrake and planking. I planed the curved outer face of the blocks first, then cut a compound miter for the fit against the transom. Finally I cut the large rabbet into which the sheerclamp will be bedded and fastened.

The block that holds the aft end of the starboard sheer clamp has been fastened in place, and I’ve cut the compound miter on the end of the clamp. It’s ready to bed and fasten.

Port sheerclamp fastened in place at the transom.

The bottom and outer surfaces of the sheer clamps got several coats of paint before fastening the sheer clamps in place, and the other surfaces painted afterwards.