Shear Clamp, Breast Hook, Quarter Knees, Seat Rails

Just a relook at the lines that define the shape of a flat bottom boat’s side, showing the difference between the sheer and chine lines.

Sheer and Chine Lines
The Sheer Clamp serves the purpose of reinforcement. The sheer clamp it self is a long piece of white oak that runs along the inside of the boat (along the sheer) . It is composed of several different components:

  • Sheer clamp: long run of 3/4″ x 1-1/4″ white oak
  • Sheer clamp spacer: 1-1/4″ x 1-1/2″ x 1/2″ thick white oak. Beveled bottom (for drip)
  • Quarter Knee: attaches the aft part of the sheer clamp at the transom
  • Breasthook: attaches the sheer clamp at the stem

The relationship of the sheer clamp with it’s spacers is shown below

Top View of Small Section of Sheer Clamp Components
This particular design keeps the sheer clamp spaced 1/2″ away from the plywood side so the future boat operator can secure 3/8″ to 1/2″ line through the spaces (dock lines, fenders, etc).

Sheer Clamp Rabbet

The first step in the build processs is “notching” the top end of the ribs to provide the rabbet for the sheer clamp to sit in. This was done by clamping a “fence” on the circular saw for a distance of 1″ (1/2″ plywood + 1/2″ space) with the depth set at a 1-1/4″ . The saw was the on both sides, producing a vertical cut. The excess was cut off with a handsaw (note: this may not be a 90-deg cut as it follows the sheer line).

Circular Saw Fence (1″ spacing)

1-1/4″ Vertical Cut for 1/2″ Spacing
Completed Rabbet

The spacers are used to help beef up the sheer clamp structure plus the smaller space (between ribs) will make future line securing easier. They are made from white oak and are 1-1/4″ wide, 1-1/2″ tall, and 1/2″ thick. The 1-1/2″ height has a bevel on the bottom so rain water can drip off. There are a bunch of these to be made, so we used appropriate sawing stops, etc.

The spacers were glued on (Titebond II) at equal distances between the boat ribs

Spacer for Sheer Clamp
Attached Spacers
Quarter Knee

The Quarter Knee attaches the aft end of the sheer clamp. It is made in a similar fashion as the knees used to support the bottom but the sides/ends are compound angled to meet the angles of the boat’s side and transom angles. Additionally a 1/2″ notch has to be created for keeping the sheer clamp 1/2″ off the sides.

Installed Quarter Knee
Starting the Breasthook

The purpose of the breasthook it to provide a little stiffening up forward and it also is atteches the forward end of the sheer clamp. The breasthook is built and installed bit by bit (not as a whole assembly and plopped in). The first installed piece (starter piece) is attached to the boat’s side and it is 1″ wide by 1-1/4″tall by 10″ long. The forward end is compound angled to meet the angles of side and stem. The aft end was provided a rabbet notch to accept the sheer clamp.

As note: We took a little bit of the top angle of the starter piece because the breasthook’s top is should be an arc (it’s not flat). And the arc is created by angling several pieces of wood across the breasthook. And the starter piece removed a little angle. It sounds confusing but if you put up a “square cross section ” piece of wood up there you will see what I’m talking about.

Breasthook Starter Piece

Installation of the Sheer Clamp

Before the sheer clamp was installed, we epoxy coated the inside sides of the boat as access was easy. We also rounded over the inside edges of the sheer clamp (inside meaning inside the boat and not meaning the inside of sheer clamp spaces).

We are now a part of the build where the quarter knee is installed, as is the first pieces of the breasthook. The proper fitting of the sheer clamp is more than taking out a tape measure, it is cut to fitted length while dry fitted (no adhehesive yet).

Dry Fit: This is done by butting up the sheer clamp in quarter knee, and working stern to forward, setting the sheer clamp in the rib notches, then clamping into place. This process is continued until the sheer clamp sits on top of the first breasthook piece and the cut point can be identified and marked. Note: During this process if the stem gets in the way the sheer clamp can be handsawn back a little. Once the breasthook cut mark has been obtained, the sheer clamp can be removed and cut per the mark.

Wet Fit: The final installation was pretty straight forward as the dry fit was pretty much identical. The sheer clamp (sitting on saw horses) was wet out by epoxy coating the entire faying side (faying be a surface that touches another surface). The quarter knee, spacers, rib notches, and breasthook’s faying surfaces were wet out as well.  A mixture of West Systems epoxy with 406 Silica was mixed to a ketchup constistancy and applied to the previously wet out components. Then the sheer clamp was then installed exactly like the dry fit.

Sheer Clamp Installed in the Quarter Knee
Sheer Clamp Installed in the Breasthook Starter Piece

Completing the Breasthook 

So we looked at 3 different styles/methods of completing the breasthook and some took us down a complicated trail, which is a clear sign of being on the wrong path. We settled on the easiest solution.

The sides were each angled at 8-10 degrees up in the breasthook area, and the goal is to level it out at the top of the breasthook (top of the arc). This was done with 3 different parts: the starter piece (already installed), the pie piece (for lack of a better word), and the keystone piece. We already took about 3 degrees of the Starter Piece.

The angle pieces were cut to about 12″ long. The “pie angle” is half of the boat’s entry angle, which was 40-degs. The outboard angle of the pies edges were reduced about 5 degrees. The aft of the pie piece (fat end) was then provided a smooth curve on the bandsaw. Then it was epoxied into place (the clamps needed 20-deg wedges and needed a clamp aft to prevent slipping).

After drying, the clamps were removed and only the single keystone piece remained. The angle was easily obtained and cut on the table saw. It was then epoxied in place, no clamping needed.

The breasthook is complete at this point except for final sanding, which will round over the assemblage Of all the flat pieces.

Pie Piece
Pie Pieces Clamped in Place
Keystone Piece
Keystone Piece Installed
Seat Rails

The seat rails are used to support the seats and their nice because it gives flexibility on seat location (fore and aft). They were constructed of 3/4″ x 1-1/4″ white oak roughed in at a 16′ length. They were installed onto the ribs with stainless screws, located 8″ below the shear clamp. They run stem to stern.

Installation was done with several “dry fits”, first clamping the stem end on the forward-most  rib then running it back, making initial cuts purposefully long, then remove adjust, etc. the compound angle at the stem was a little trick, so the proper angles were obtained with a scrap piece of wood before attempting with the 16-ft’r. Both ends were epoxied and screwed.


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