Birdwatcher-2 Building the Off-Centerboard
Birdwatcher-2 Sailboat, designed by Phil Bolger
The Off-Centerboard of the Birdwatcher-2 is roughly 3 x 5 feet, and made from three pieces of plywood. The center is weighted by 109 lbs of steel.
A 3+ inch hole in the 5/8-inch thick maple board will make both bearing surfaces for the off centerboard. These go in the forward bottom corner of the centerboard trunk, one on each side.
A 3 inch diameter, 5/8-inch thick disk is placed on each side of the off-centerboard to serve as a bearing. No holes are cut in the centerboard case to support the board. These disks were made from eastern hard maple that was scrap from an earlier project.
The Off-Centerboard is assembled from three sheets of plywood. The center is 1/2-inch and cut out to contain the steel weight. The sides are 1/4-inch plywood. I used Douglas Fir marine for all layers. After assembly the board is sheathed in fiberglass. Here the first two layers are being epoxied together. Filler in the epoxy ensures that there is contact throughout. Clamps are fine around the edges, but temporary dry-wall screws hold the interior areas.
The plans call for a piece of 1/2-inch steel sheet to fill the center. In a moment of re-cycling enthusiasm, I decided to use some 1/4-inch sheet left over from an old centerboard project. Of course nothing was the right shape, so I cut pieces and welded/screwed them together to be approximate. I then filled the remaining space with lead (shot-gun reloading shot) filled epoxy. This all turned out to take days and I would not do it this way again! But, it did use up most of my scrap steel. The lead-loaded epoxy was an interesting experiment, and I thought that part went well. I bought a 25-pound bag of tiny lead balls for about $20 to do this with. I used about 10 pounds and now have scrap lead! Just don't let those lead balls get away from you until they are epoxy coated!
As of 9 July the centerboard is finished, the outside is sheathed and the bearing disks are attached. I find I need a picture of this, but it is a bit buried in the garage, so this will need to wait.
Here is the Off-Centerboard Trunk Case. It is still in two halves, and needs epoxy and some sheathing on the inside. The bearing arrangement prevents having holes in the side of the case. To take up the extra space for the bearings, battens are added. These battens also reduce the friction between the case and the board. After the case is sitting in the bottom of the boat, the amount of rocker will be measured and the bottom (long) edge trimmed to fit. This will remove about 2-1/2 inches on the bearing end.
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This was last revised 10 July 2005. Bob Larkin, Corvallis, Oregon