>RebelCat 5 Update 10-27-09

Two New Centerboards

I'll get right to it. The original centerboard (CB) was made from plywood and began coming apart when water got inside - that's the nature of plywood, unless you get the expensive marine grade.

I made two new CBs, one from Oak, the other from an ordinary 2 x 8, both with aluminum sheet added. First the 2 x 8...

2 x 8 board from which to make a centerboard This is actually a old board, and I chose it because it had dried out completely, and it was straight. I'm using the best part, not the ends which have small splits.
The same 2 x 8 with cuts made to remove bad parts Using a jigsaw, I cut the rough shape of the CB.
Using electric planer to shape board Using an electric planer, I begin to create the foil shape, but only on one side. The other side will be covered with aluminum sheets, so it need not be shaped. Leave it flat.
Aluminum sign scored to make two halves This is an aluminum sign from a salvage yard, 1/8 inch thick. I scored it with an angle grinder cutting disc until it could be bent on the score.
Aluminum sign split into two equal parts Bending back and forth splits the sign into two equal halves.
Aluminum placed on epoxied board and drilled Here, the shaped board has been coated with boat epoxy resin, and the two aluminum pieces have been drilled through while on the board. The holes are countersunk and screws attach the aluminum to the board. The aluminum still has to be cut to length. Note that you will have to measure the underside of your deck for the width, to know how long your alum. CB blade can be. Mine is a 30" deck, minus the two 2 x 6s (actually 1.5" thick), so I have about 27" to tuck my CB under the deck. I make the blade 26 3/4" and test it before any final touches.
Applying putty to smooth edge of aluminum on board The aluminum has been screwed to each side of the board, and a putty, made from wood flour and epoxy is spread to blend the edge of the aluminum with the board.
Aluminum now fixed to board with screws, putty added

Spread the putty smooth to even out the edge of the aluminum. The screws have already been put in to hold the aluminum to the board - notice the putty in the countersunk holes over the screw heads.

If you look at the top part of the CB (at the bottom in this photo), you see that I got carried away while shaping the edge of the wood. The top should be flat at the top where it will fit inside the CB trunk. Fortunately, I left the other side flat, so the CB doesn't wobble. Shaping the trailing edge of the CB above the aluminum is not needed.

In the background is the CB made from an oak plank. It has already been painted.

Sanding cured putty on side of centerboard

When the putty is hard, sand well to blend the edge. You will sand off some epoxy from the board, but you will coat it again later.

Since both CBs are nearly identical, I'll show next the stages with an oak plank 3/4" thick, half as thick as the other. I've already shaped it with planer and sander, and now I coat it with epoxy.

Coating the oak CB with epoxy resin The oak plank has already been drilled for the aluminum. Here, it is coated with epoxy resin.
Attach aluminum to aok with screws Countersink holes, drip in some epoxy and then screw down tight. The epoxy will prevent water from entering through screw holes.
Drill and rivet aluminum on oak CB Both CBs now are completed the same way. Clamp the trainling edges of the aluminum together, drill both and rivet. I used both 1/8 and 3/16. Smaller is better and easier to peen and smooth later.
Drilling pivot hole in oak CB The oak CB needed another piece of wood laminated to it at the top to make it the right thickness for the CB slot on the cat. Here I'm drilling out a 1/4" hole to a 1/2" hole where the pivot bolt goes.
Making the pivot hole ragged with a saw With a keyhole saw, I enlarge the hole to maybe 3/4", making it quite ragged.
Closeup of ragged pivot hole in oak CB I know, it looks unprofessional now, but wait... Actually, the more rough and ragged you make this hole, the better it will be for the next step. On the rudders, I actually cut slots around the hole, making it look like a sun icon.
Drilling final hole in epoxy plug What you missed here you can see on the rudder update. I filled that ragged hole with epoxy (masking tape on the back first) and fibers, let it set hard, then drill the final 1/2" hole in the epoxy. This way, no wood is exposed to water and the hole is very clean.
Completed pivot hole in oak CB

See, nice hole. All of the CBs and rudders were done this way. If a 1/2" pivot hole is drilled in the wood directly, water will cause the wood to swell, making it difficult to remove the pivot bolt later.

Wet wood also tends to rot sooner or later. By making a larger hole in the wood and filling it with epoxy and fibers, then drilling the 1/2" hole, water never reaches the wood, and the hole is precise.

I'm not sure, but I might have invented this technique.

Oak CB ready to paint The oak CB ready to paint. The aluminum has been trimmed to match the wood and riveted on the trailing edge.
Inserting foam into the space inside the CB

The oak CB I filled with camping-mat foam. The other I just made foam plugs for top and bottom (shown here) to keep the water from swirling inside. A hole in the foam is needed to let water in and out, otherwise it has to drain slowly later.

Both of these new CBs perform well. The best is the thicker one, and I have heard from a boatbuilder that it creates a more efficient underwater foil and aids in pointing and speed.

RebelCat 5 now points even better than with the original CB, and it was already quite good for a cat. The 2 x 8 was cheaper too. I made both so you could see the results. I now use only the cheaper one, because it performs so well.

See also the Update on the new Rudders here.


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