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Sunday, November 16, 2014

Kingfisher Drift Boat Build - Aft Knee Brace Construction

In a previous post I discussed the design process of building the aft knee brace for the Kingfisher. It is an involved process to say the least. In essence the aft knee brace is constructed with 3 pieces of 3mm Okoume plywood that are approximately 24" x 48" each. They are cut to size and then must be bent into a sort of "S" curve in a form constructed to the desired shape. The form ended up looking more like a big question mark. There is an inner and outer form that will be clamped together once the 3 pieces of Okoume plywood are steamed.

The first decision that had to be made is which direction to bend the plywood. In 3mm thick plywood it bends way easier one way than the other. The way I wanted the grain in the plywood to be oriented was unfortunately the hard way. Just my luck. Prior to steaming the plywood I could bend the dry plywood easily in one direction and hardly at all in the other. I'm steaming it right? I figured what the heck I might as well give it a try. So onward I went.

I had to construct a plywood box in which to steam the 3 pieces of plywood. I made a box that was about 30"wide x 54" long and 6" deep. It was like a plywood coffin. I devised a method to separate the 3 sheets as they laid in the box. I attached a radiator hose to one end of the box, fired up my propane hot plate with a 5 gallon bucket of water attached to the other end of the hose and steamed the pieces for 2 hours. I did some research on the internet where I found information stating that the marine adhesives would hold up to the steaming process.

Here's a photo of the steam box with the radiator hose coming into the bottom. There are 4" screws installed at the sides to sepatate the 3 sheets when they are placed in the box.

The box is closed up and the 3 sheets are inside and being steamed. I placed the box on a slight incline so the water vapors that condense can drain out of the low side. I placed an oven thermometer to see the temperature. The 3 pieces inside were steamed for 2 hours.

I took the 3 pieces out of the steam box and place them in the form and clamped the assembly together. This was not easy and it took four attempts to get in done.  The two opposing forms where hard to keep aligned. After much frustration They finally came together. You can see the "Question mark shape which are the 3 pieces of 3mm Okoume plywood clamped tightly together.

The three pieces of 3mm plywood were saturated and wet when they were clamped together. I left them clamped in the form for 10 days to enable them to dry back out before gluing them together.

Here's another view of the forms clamped together. Thank goodness for bar clamps!

After 10 days it was time to glue the 3 pieces of 3mm Okoume plywood together. I round sanded the surfaces that were to be glued with 80 grit paper. I lined the form with 3 mil plastic so the pieces would not end up glued to the form. I then flow coated each surface with clear epoxy.  This took about 3 ounces for each side that was coated (4 sides). I used one side of the form and used pieces of 1x2x24 oak across the sheets with a clamp on each end. I stared at one end and worked lengthwise along the sheets. This took lots of patience and clamps.

Here's another view of the clamps with the 1x2 pieces of oak stretched across.

I let the epoxy set up for 48 hours to make sure it set up properly. I removed all the clamps hoping that I would not have much spring back and that the plywood would stay in the same relative shape as it was in the form. Epoxy is strong stuff and it came out perfect.

The next step was too layout the final shape of the knee brace and cut it out. This was a very important step and one that I took my time doing in order to get it right the first shot. If it was done wrong I'd have to start over from the beginning. I mainly worked of photos of the knee braces designed by Jason Cajune and just winged it. 

Here's what it looks like. There are "horns" made out of 4/4 mahogany that will attach at the top which will form the actual "Knee Brace"  You can see a vertical piece made from 5/4 mahogany that reinforces the brace from flexing downwards and two braces on the back side to keep the brace from leaning forward. This unit will get epoxied to the bottom with biaxial tape and then additional 4" fiberglass tape and be installed in a permanent fixed position.

Here's another view showing the two braces on the other side.

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Have any Questions or Comments? Let me know, Clay.