Thursday, February 15, 2018

Sportfisherman 26 Sheer Clamps

Since my last post about Kevin Agee's project he has been working on the second 9mm layer of  the bottom skin. With this layer there is no chance of using clamps along the keel to assist by holding that edge tight against the structure while you work elsewhere on the panel. For that reason this second layer is most easily done in narrower pieces than what worked for the first layer.

Kevin is dry-fitting this layer from the bow, toward the stern. In the bow the pieces are approx. 300mm (12") wide, which has worked well. Further aft the twist reduces rapidly so wider pieces can be fitted without problem.
Second layer of 9mm bottom skin being dry-fitted.
The sheer clamp is the next major piece of longitudinal framing that must be added to the skeleton. This is a fairly complicated component and must be laminated in place. The sheer clamp defines and strengthens the top edge of the hull and joins the hull to the deck. The complication comes from the fact that it curves on plan and in profile, at the same time twisting to conform to the changing angles of the upper edge of the hull side. At the transom the hull has tumblehome, so the side leans inward at the top. At the bow it has a lot of flare, so the side leans outward at a large angle.

To further complicate it, this boat has a broken sheer, meaning that there is a hard break in the side profile instead of having smooth curves that flow from one end of the boat to the other. That means that the sheer must be done in two sections. The aft flat part of the sheer clamp is done first, then the forward curved part is built off the aft section.

Kevin laminated the aft section of the sheer clamp during the week, ahead of me arriving to help with the more complicated forward section.
This photo shows the twist that is formed in the aft part of the sheer clamp as it changes from tumblehome at the transom to the beginnings of flare at the break in the sheer.
The aft part of the sheer clamp is run through to the next permanent frame forward of the break and will be trimmed off flush with the front face. The first layer of the forward part of the sheer clamp is also in position, dry-fitted so that we can check fairness of the curve and also mark the bevels on the frames and the stem.
Preparing the framing to receive the sheer clamp needs to be done carefully, so should not be rushed. The frames are not difficult, the angles can be established with a shallow cut with a back saw to show angle and depth, followed up with a rasp to take the frame edge down to the saw cut.

The Stem is more complicated. The angle of the bevel for the skin must be marked and cut first in the area where the sheer clamp will fit. This angle can be established by running a batten across the forward 3 or 4 frames to check that it contacts all of them correctly and lies flat against the bevel on the stem. Once that stem bevel angle is correct the rebate to receive the sheer clamp can be formed.
In this photo the first layer of the sheer clamp has been glued in place. The narrow piece of stem bevel that shows above the sheer clamp is the bevel for the inside face of the hull side and is flush with the edge of the sheer clamp.  The broad piece of bevel that shows below the sheer clamp extends from centreline on the front face to the aft edge of the stem to form the bonding surface for the sheer clamp.

A slightly different perspective of the intersection.
The sheer clamp must also be prepared with its ends trimmed to properly meet the aft sheer clamp and the stem. The aft end is relatively simple, with an angle in only one direction that is easy to mark, rough-cut with a saw, then fine-tune with a sharp plane.
The junction of the two sheer clamps at the sheer break. The cut angle for a good fit is quite easy to work out.
The forward end is more complicated so it is best to first work out the cut angles (which run in two directions so that the bevel is not parallel to any of the surfaces of the timber) on a length of off-cut of identical size to the sheer clamp. Cut this off-cut and fine-tune to a perfect fit, then transfer the angles to the sheer clamp on the workbench, where they can be accurately drawn, cut and planed. Test fit on the boat.

Now comes the part that demands ingenuity, to deal with the enormous amount of twist in the bow sections. We were working with very nice clear Douglas fir, which is stiff and not keen to twist as we needed. It didn't work to rely on clamps alone to do this work, the timber came in most of the way but still needed about 20 degrees of twist. We set up a Spanish windlass to do all of the work of pulling the timber in against the stem so that the clamps would only have to work on the twist. 
This was our setup for the dry-fitting phase of the first layer of the sheer clamp.
We were not confident of being able to pull it in totally with slippery glue complicating the work, so we decided to leave it clamped for two days to set in some of the bend. When we returned to it the pre-set of the timber made it easier to pull it back in. Again, we used a Spanish windlass to apply the bending forces. We used a different clamp setup to do the twisting, with the handles of the clamps lashed together to apply the twisting force.
Our setup for the gluing phase. The Spanish windlass is visible just above the sheer clamp, pulling the two sides against the stem. The two black clamps with their handles tied together are twisting the wood to align the outer surface with the front face of the stem.
We were working with timber thickness to laminate the sheer clamps in three pieces. Doing it in four thinner strips will make it easier to pull to shape.

The next step will be to laminate the other two layers onto the sheer clamp, which should be completed in the next few days.