Building a Wood Duck 12 - Graphite bottom, invisible hatch hold-downs

Today's theme is bottoms and tops. First, the finishing of the bottom and the application of a graphite/epoxy mix. Then some work on the hatch and the final installation of the coaming.

The theory behind a graphite/epoxy bottom is to take the standard epoxy and to make it so slippery that rocks, oyster shells, glass, gravel, etc. do not get a purchase in the surface and so are unable to start a gouge. Graphite/epoxy is not a "bulletproof" hard surface, as I've seen it described so often by folks who confuse it with aerospace graphite composites. It is, in fact, somewhat softer than plain epoxy, but much slipperier.

Click on any image below for a larger view

Filling the weave

Since the bottom will be covered by the graphite/epoxy mix, I filled the weave with a thick slurry of phenolic microballooons and epoxy. This let me economize on the epoxy, while yielding an easy-to-sand surface. The horrible purple color will be covered by the graphite.

Hull masked

Once the bottom was filled and sanded, I masked the hull with fine line tape. I used the tops of the stitch holes on the side panel as my boundary line. Not only does it actually make sense to protect the seam, but it also covers all the holes and joints.

Black bottom I

I used a 50/50 mix, by volume, of graphite and mixed epoxy/hardener. The epoxy and hardener need to be thoroughly mixed first, then the mixture is slowly poured into the graphite. It's like mixing cocoa powder and milk - adding the powder to the milk gives a lumpy mess while the reverse order gives a smooth paste.

Graphite powder starts flying with the slightest puff of air. It also stains anything it touches dark black. Definitely wear gloves and eye protection and at least a dust mask, as well as old clothes.

Black bottom II

Once the goop is mixed, just pour it onto the bottom and spread it with a short nap roller. Tip it out with a dry foam brush to get rid of the air bubbles. Two coats should be plenty. Unsanded it will be a striking glossy black with a slight orange peel finish. Sanded it will be a smooth pencil lead grey. I didn't sand mine. The graphite is effective UV protection, so there's no problem with leaving the bottom exposed to the sun.

Hatch straightening

My hatch cover ended up with a lengthwise curve which made it bow up when in place on the boat. So I cut a kerf laterally across the doubler, clamped the cover to a flat workbench, filled the kerf with epoxy/woodflour mix and glued a longitudinal stiffener to the doubler. The stiffener and hatch form a t-beam, which strongly resists bending. The operation was successful, reulting in a perfectly flat hatch cover.

Hold-down hardware

These are the bits needed to make an internal bungee hold-down system. There are 4 bungie loops (the D-shaped bits drilled for the bungie cords) and two hooks. The bungie loops are cut from 1 1/4 inch hardwood dowel that's been drilled and had a flat side planed on it. They go into the hatch compartment. The hooks are cut from plywood and attach to the bottom of the hatch cover.

For those who don't want to take the trouble to make their own, CLC sells a kit with lovely trapezoidal drilled bungie blocks and tacky metal hooks for $35.

Bungie loop in place

To install the bungie loops, rough up the surface in the hatch compartment and simply bed them in epoxy/woodflour putty.

Last bits

This image is quite a milestone - all the wooden bits are used up. Everything in the kit box has been attached. There are only 2 pieces now - the boat and the hatch cover.

In the foreground, the hatch cover has received its bungee hold-down hooks, which are held in place with masking tape while the epoxy is curing. (The stiffener bar has also been smoothed.) In the background, the coaming is held in place for its final cure with every spring clamp in the shop.

Page 1 - In the beginning
Page 2 - Hatching a Duck
Page 3 - Stem & stern pieces
Page 4 - Interior fillets & glass
Page 5 - Glassing under the deck
Page 6 - Installing footbrace mounting studs
Page 7 - Building the coaming, attaching the deck
Page 8 - Exterior glass
Page 10 - Final outfitting

Copyright © 2009 László I. Mórocz. All Rights Reserved.