Centerboard and Rudder
The centerboard and rudder is made from sitka spruce. Start by glueing up strips of sitka spruce 2 inches wide x 53 inches long to create a blank of wood 1 ¼ x 18 x 53. After the epoxy has cured, scrape the excess epoxy from the joints and run the blank through a surface planner to a thickness of 28 mm. The shape of the centerboard can vary slightly, depending on the builder’s design philosophy. The tip of the board can reflect several design thoughts. Most centerboards have a straight or cut off end while some boards have rounded ends that resemble the tip of a spitfire airplane wing. The plans suggest a profile with a tip that angles back towards the trailing edge. Regardless of the shape of the tip, the main body of the centerboard must conform to the dimensions outlined in the rule book as diagramed by the Centerboard Profile Sheet. We chose to make a cut off tip with the end if the board being 90 degrees to the leading edge projecting through point “R” on the trailing edge. We also choose to create a foil shape based on a fat NACA curve of .0090, with the maximum draft located 30 percent from the leading edge. Lay the shape of the board out so that the glue joints run parallel to the trailing edge. This practice maximizes the rigidity, minimizing the twisting that the board may undergo wile traveling through water.
After the shape is laid out, cut the centerboard from the blank using a bandsaw or a sabre saw with a heavy duty blade. Using the above information on cross sectional shapes, locate the “A” and “B” dimension on the centerboard and mark them on the board. Next, locate from the leading edge of the board the point 30 percent back form the leading edge on both “A” and “B” dimensions. Do this in both sides of the centerboard and strike a line between both points the length of the centerboard. Now locate a line that runs the entire distance around the perimeter of the centerboard at half the thickness of the board. Also, draw a line near the top of the board that represents the bottom of the boat when the centerboard is completely down. These lines are placed on the board as guides and should not be removed during the shaping process. Divide the distance between the leading edge and the 30 percent point into four equal parts at both the “A” and “B” dimension and strike a line between the two dimensions through these joints. Do the same for the distance from the 30 percent mark and the trailing edge. You will now have a board with 8 lines that run along the leading and trailing edge but are not parallel to one another.
On the bottom end of the board lay out the NACA curve you have choose to use. Transfer each line around the end of the board to determine the thickness from the surface of the board to the edge of the curve you wish to create. Joint the edge of a straight piece of scrap wood to create a fence to run a router along. Set the fence up and clamp it on the surface of the board so a 3/8 inch straight router bit can run along the leading side of the 4 lines that runs along the length of the centerboard along the leading edge. You will do likewise for the 4 lines running along the trailing edge. The router bit will have to run along the trailing side of those lines. Begin at the line closet to the edge of the centerboard that runs along the leading edge. Set the router bit to a depth that corresponds to the thickness from the surface of the board to the edge of the curve at that line. Run the router along that line from the bottom end of the board to the line that represents the bottom of the boat when the board is down. Before resetting the router to another depth, do the other side of the centerboard to create a symmetrical shape.
After you have run the router along all the lines, you will have a board with the general foil shape defined by a series of steps created by the router. To finish the shaping process, use a jack plane or sure form tool to smooth the surface of the board down to the level of the steps into a smooth curve. When you have finished using the plane, you will have the 30 percent lines and the line running around the perimeter remaining. The last step in the shaping process is to use 80 grit abrasive paper and sand the final shape. When completed, your centerboard should have a maximum thickness of 28mm. If you are going to wrap the board in fiberglass the finish thickness should be less then 28mm to allow you to build it back up to 28 mm with the glass cloth and resin.
The rudder design is totally open with no set of dimensions to determine it’s shape. The rules only state that the rudder not be more then 25 mm in thickness below the water line and weigh no less then 3.0 kg as a completed assembly with the tiller, extensions, and fittings. Except for the tiller extension the entire unit may not be greater then 100 mm in transverse width. We choose a shape that has a leading and a trailing edge that are parallel to one another with about 2 feet of rudder below the waterline. The tiller will attach directly to the rudder to eliminate the need of a rudder head. Once a shape is chosen, follow the same procedure to shape the rudder as you did for the centerboard.
Here is some photo's of the students using the plunge router to create a series of steps to make their centerboard. This is suppose to represent a 30 maximum draft 0900 NACA curve. We got our information from two article off the Net the first was an article published back in 1931 by a Robert Pinkerton, Effects of Nose Shapes on the Characteristics of symmmetrical Airfoils. The other one was published in 1997 by Craig O'Donnell entitled Much, Too Much Boat Technology for My Own Good. One really needs to be on the Physics side of life to totally understand what if being translated here but we got a great shaped board out of it. I will also forward on to you the technical writings my students put together to explain what they did.
