Construction of the Boat BW: I Have My Kneeds

Design of the Knees

Knees are braces. They are located on the inside of the side panels, at the top of the side panels. They provide a horizontal member for support of the sheer deck. They also connect the side panel to the sheer deck to make the panel and deck an integrated structure. This is how I built the Lady C (V-20) over twenty years ago. This system accommodates a bright finish and leaves few plywood edges exposed. I used AutoCad and designed the knees and deck.

Sheer deck design

Knee design

Cutting the Knees

I designed the knees so that they could be cut with a pattern cutter in a nested arrangement.

Knees to be cut with a pattern cutter

I ripped a sheet of meranti plywood to 1'-2½" x 96". Then coated both sides with 10 oz. fiberglass set in epoxy.

Squeegee applying epoxy to fiberglass

The excess F/G will be cut and sanded away after the epoxy cures

I designed a cutting jig to cut each blank exactly the same.

Cutting jig used to cut blanks out of epoxied meranti.
Each blank will be pattern-cut into a pair of knees.

Layout of the pattern knee on a blank

Cutting of the pattern knee.
The cuts were made outside of the lines.
The pattern knee was then sanded to the lines.

Blanks ready for pattern cutting

I used a flush trim bit in a router to pattern-cut the knees.

Pattern cutting of knees. Pattern knee is on the bottom.
Due to the shape of the blanks, one pass with the router creates two knees.

Twenty knees. One, the pattern, was cut with saber saw.
Nineteen were cut with flush trim bit following the pattern

The final cut of the knees is the vertical edge for the inwhale.
That edge was cut with the chop saw.

Knees are great places to route cables, wires, fuel lines etc. beneath the shear deck. The largest item to pass through the knees will be the steering cable. The helm end of the cable requires a 2-1/8 inch diameter hole. The engine end of the cable requires a 1½ inch diameter hole. However, I recall that the engine end of the cable has a long stiff rod that does not pass around corners so I am prepared to route the helm end through the knees.

Drilling 2-1/8 inch diameter hole in each knee.

After some sanding to clean them up, the edges of the knees, that will be exposed, were coated with unthickened epoxy.

Completed knees.

This is my first time working with meranti. It is beautiful, and without any stain, the epoxy darkens the meranti to a rich red-brown color. This will contrast wonderfully with the honey-colored rubrails, inwhales and spray rail.

After 5 months of hanging from the ceiling, it is time to install the knees. But, first, the backer must be installed.

When I made the side panels, I scarphed together 9mm plywood sheets to make 4-foot x 24-foot panels. Then I glassed the portion of the 4-foot width that would be the side panels. The remainder of the 4-foot width will become the backer and sheer deck.

Remainder to be used for backer and shear deck is on the left,
with no fiberglass on one side. The side panel portion is on the
right, glassed and weave filled.

I cut the 24 feet x 3½ inches x 9mm backers out of the remainder.

Then I glued the backers along the top of the side panels. The backers get installed using the same method as the inwhales and rubrails when I built the Sea Scull. That is, butter a few feet of the side panel with thickened epoxy, clamp the backer to part of the buttered section, use the thickened epoxy that oozes out from clamping to butter some more of the side panel, clamp the backer to more of the buttered section, use the thickened epoxy that oozes out from clamping to butter some more of the side panel,so on and so forth.

Gluing of backer to side panel.
Temporary screws from the outside
provide clamping. Scrap blocks
on the inside provide "meat" for
the screws to grip. The temporary
clamping screws will be removed
and the holes will be covered
by the rubrail.

Completed gluing and clamping
of the starboard backer

After the backer epoxy cures, the clamping screws and blocks are removed.

Completed backer

Now the knees can be installed.

Knees dry-fit before gluing

Close-up of knee and side panel with backer

Tracy O'Brien's V-20 design has the top edge of the forward knees cut at increasingly flatter angles to provide a flat sheer deck for the length of the boat. I decided to leave all knees with a 20° angle, which will cause the sheer deck in the fore of the boat to slope outward. Let's wait and see how this looks. If I don't like it, I won't do it on the next boat.

At this point I realized that I was short by two knees. Originally, the BW was designed to be 22 feet long and would require twenty knees, ten per side. So I fabricated twenty. But, when stitching the hull, I realized that I could not bend the 9mm meranti plywood side panels as radically as needed to achieve the steep stem that I desired. So, I had to make the stem less steep, which added nearly a foot to the length of the BW. With the BW a foot longer than planned, I need two more knees, one for each side.

I used one of the twenty knees as a pattern to cut two more knees. They were cut on the router table with a flush trim bit.

22 knees filleted in-place

Originally, the sheer deck was going to be two layers of 9mm (3/8) meranti. But I came across some very nice 5-ply 15/32 ACX fir plywood at the local big box store. I decided the first layer, which is the lower unseen layer of the sheer deck, will be the ACX fir plywood instead of meranti.

15/32 ACX for first layer of sheer deck

Coating the A side (underside) of the ACX with 4oz. fiberglass

Then I began installing the ACX. The ACX was epoxied to the top of the side panel which is about an inch wide thanks to the backer. Epoxy fillets were place on the underside. Temporary clamping screws secure the ACX until the epoxy cures.

Starboard aft segment of sheer deck ACX layer.
The knees were positioned so that the butt joints
in the ACX are at knees.

Scribing of the starboard middle segment of the ACX layer

Completed starboard sheer deck ACX layer.The underside of theACX was pre-coated with 4oz fiberglass.

ACX layer pre-coated before laminating
9mm meranti top layer

Eight-foot segments of meranti were scarphed into 24-foot segments previously. Now the 24-foot segments of layer of 9mm meranti can be laminated to the ACX to form the top and final layer of the sheer deck.