VII. Centerboard and Rudder
Building the Core Sound 20
April 5, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
Rainy weather in late May, early April has slowed our progress on the hull structure, so on rainy days like these, we spend our time in the garage working on the other pieces we'll need to sail, such as the centerboard and the rudder.
The centerboard is the retractable "fin" near the center of the boat that prevents the boat from slipping sideways (instead of moving forward) while sailing. It needs to be strong to bear the stresses that it will encounter under sail. It also needs to be abrasion-resistant to prevent the wood from getting damaged, which would lead to moisture penetration and rot.
Good, stiff, straight-grained, old-growth, dimensionally-stable planks of wood are hard to come by, so to build our centerboard, we began by making our own plank out of flat-sawn wood. We ripped several 2x6 boards of Douglas Fir into 1-1/8" thick strips (6' long), then turned each strip sideways, flipped every other strip end-for-end, and glued them back together with thickened epoxy. The result is a laminated plank with the grain running vertically through the faces like a piece of premium quarter-sawn wood. It is very stiff and very stable because (having flipped every other strip end-for-end) the stresses in the wood cancel themselves out).
Our laminated plank came out to be 6' long, 16" wide, and 1-1/8" thick. We laid out the profile of the centerboard on this plank and used a jig saw to cut it out. Then we capped it with another plank that will serve as the lever-arm to raise and lower the board. Finally, we used a block plane and random-orbital sander to ensure both faces of the plank were perfectly flat, producing a plank of consistent thickness. (A mechanical planer would've been ideal for this, but we don't have one.)
Next came the hard (labor-intensive) part: planing the leading and trailing edges of the plank down to produce a hydrodynamic "foil" shape that will glide more efficiently through the water. This part isn't an exact science, but generally speaking, the leading edge should taper fairly quickly, and the trailing edge should taper more gradually.
After consulting the plans, we decided to taper the leading 1-1/2 inches of the board down to a thickness of about 1/2", and to taper the trailing 3 inches of the board down to a thickness of about 1/4". In actual practice, we probably tapered slightly wider portions of the leading and trailing edges to ensure that our curves maintained a smooth, even "blend" into the thicker middle section. What matters most is that you clearly mark your target areas for fairing, and then (after you hit your target) work by sight and by touch to produce a fair shape that transitions smoothly from tip to tail.
To make the pivot hole for the board, we overdrilled the hole to 3/4" diameter, then filled it with thickened epoxy and re-drilled it to 5/8" diameter. This produces an epoxy "bushing" all the way around the hole to protect the wood from chafing against the pivot pin.
Next, we coated the entire centerboard in thickened epoxy to fill in any irregularities in the surface, including a few places where the block plane caused the grain to tear out. Then we sealed the entire centerboard in 6 oz. fiberglass cloth.
To add abrasion resistance to the leading edge (which is sure to strike underwater obstructions), we epoxied a 1-1/2" wide strip of kevlar felt to the leading edge, and blended it into the rest of the board using thickened epoxy. The kevlar should prevent underwater obstructions from gouging into the wood core and causing rot.
Finally, we covered the board in a second layer of 6 oz. fiberglass cloth and gave it several fill coats of epoxy. The second layer of glass is probably excessive, but given the amount of labor that goes into making the centerboard, we hope that with proper protection and maintenance, this board will never need replacement.
For the rudder, we laminated two pieces of 3/8" plywood together to produce a 3/4" thick slab of impressive stiffness and strength. Then we rough cut the rudder to the shape specified in the plans and used a block plane to taper the leading and trailing edges to a hydrodynamic foil shape.
Like the centerboard, the trailing edge gets tapered about twice as much as the leading edge. The glue lines in the plywood make it fairly easy to ensure that the plane is taking off the right amount of material for the taper. As long as the glue lines of each ply remain parallel, you can be fairly confident that the taper is consistent.
We cut the rudder cheeks out of 1/4" plywood, then laminated on 3/8" plywood "doublers" to provide strength and rigidity near the pivot point for the rudder. The inside faces of the rudder cheeks and the entire rudder are covered in 6 oz. fiberglass cloth to protect them from chafing and other kinds of damage.
Here, too, we cut the holes for the pivot pin to be oversized, then filled them with thickened epoxy and re-drilled the pin hole to create an epoxy "bushing" which will protect the wood from chafing against the pivot pin.
Next we glued the internal spacer block to one of the rudder cheeks. When the glue dried, we dry-fit the rudder cheeks together and drilled holes to attach the rudder pintles (the stainless pins that provide the "hinge" for the rudder to pivot).
Since the lower pintle is not wide enough to wrap around the doubled thickness of the lower half of the rudder cheeks, we used the router to cut a 5/16" deep mortise on each cheek.
Then we applied a few thin fill coats to seal the wood thoroughly. We also used Q-tips to coat the insides of the fastener holes with epoxy as well. (We overdrilled the fastener holes by 1/32" to allow for the thickness of the epoxy coating.) Once cured, a quick assembly confirmed that all the parts fit properly.
Weeks later, when we painted the hull, we sprayed the rudder assembly with four coats of VC Performance epoxy paint and then attached the stainless hardware. It turned out very professional looking.
To finish the rudder assembly and make it "steerable," we added one of the few fancy "showpieces" on our boat: A custom-designed mahogany tiller. Although the plans specify a straight tiller, we prefer the more luxurious looks of a curved tiller. We chose mahogany because the dark brown color will accent the lighter-colored Okoume deck nicely.
To make the curved tiller, we ripped a mahogany board into 1/4" thick strips and then epoxied them back together while clamped to a piece of plywood cut to the desired curve shape. When the epoxy cures, the pieces remain formed to the curve. The gluing process is a bit messy, but well worth the (minor) hassle.
After gluing up the pieces, we cut out the desired profile shape, rounded off the edges with a 3/8" roundover bit, and bored out the bottom with a 3/4" router bit until it fit over the rudder stock. (We also glued on a pair of "cheeks" to reinforce the neck where it fits over the rudder stock.) Then, to improve the fit, we coated the rudder stock in packing tape, filled the tiller neck with thickened epoxy, and pressed the tiller into place.
When the epoxy cured, the neck retained an exact, friction-fit mold of the rudder stock! Since the packing tape prevented the epoxy from adhering to the rudder stock, we can still remove our tiller handle, but when it is seated in place, it fits like a glove. We couldn't be more pleased.
All that was left to do was to drill the fastener holes and then sand down the tiller to a more refined shape. We progressed through 60-, 150-, and 220-grit to get a very fine finish, then we sealed the wood with three coats of epoxy. We're delighted with the results.
© 2008, Wesley Kisting