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The Rocker Curve The rocker curve is the rounded shape of the bottom as the hull is viewed from the side. It is created by assembling the forward web, centerboard well unit, and the after spine together. Referring to sheet 7/8 of the building plans, Offsets for Skin Panels & Tank Sides, the builder is given an exploded view of the three units and a set of reference numbers to plot out on the sheets of plywood that will become the spine of the boat. We will plot the units out on a separate sheet of paper then transfer the points onto the sheets of plywood. Begin by placing a long sheet of butcher type paper that is at least 28 inches wide by at least 16 foot 6 inches long. Place the paper so that the 16 foot edge lines up with the chalk line that was established earlier. After the paper is smoothed out and no wrinkles are present, tape the paper securely to the platform. You will be working on the paper several times before you are finished so make sure the paper is securely taped down. Begin at the left end of the paper, mark station A.a, and begin by measuring along the edge of the paper every 250 mm for 21 station to station T,t. At each station project a line 90 degrees to the edge of the paper across the width. It is recommended to repeat the measurements and mark the stations along the top edge of the paper to check to see if the lines projected across the width are indeed 90 degrees to the edges. After you are satisfied that the stations are located properly on the paper you may begin locating the points on the projected lines that will plot the rocker curve. The plans give the builder a set of numbers to locate at each station. We are only interested in the numbers that are assigned to the lower case letters. Locate these points on the line at each station. When you are satisfied that they are placed correctly strike a curve using a ¾ by ¾ inch batten made of western red cedar, preferably the batten should be as long as the curve you are striking. The more hands you can get to volunteer to hold the batten the better. Mark the curve by running a pencil along the batten. Once the curve is struck, the builder is ready to lay-out the exact rocker from this general curve. The curve created by these numbers will not line up with the measured curve outlined in the rulebook. This is just a general curve used to create the real rocker curve. For instructional purposes we will refer to this curve as the general curve. To create the actual rocker curve begin by locating at station A,a. a point 305 mm from the line that begins the general curve. Next, measure along the general curve for a distance of 4760 mm. Note that this distance is a curved measurement representing the contour of the bottom of the hull, and not a straight line measurement. From that station, draw a line towards the edge of the paper 90 degrees from the edge. Next, locate the actual measurement stations A through E. You will have to adjust for the thickness of the after transom when locating these stations. For example: our transom is 24 mm thick so when locating station A we will measure 765 mm along the general curve minus 24 mm to reflect the thickness of the transom. Locate the remaining stations as outlined in the rulebook under section 9.18 Rocker. Once all the stations are located project a line down towards the edge of the paper 90 degrees to the edge of the paper. At station A,a measure from the general curve down the projected line a distance of 305 mm and mark that point. At the station located at 4760 mm from station A,a, measure down that line towards the edge of the paper 407 mm and mark that point. Between these two points, strike a chalk line. This is the actual datum line from which all measurements for the rocker are taken. Measure along the general curve and locate the five measurement stations,A through E, used to measure the rocker curve. At each measurement station, project a line down from the general curve to the edge of the paper, intersecting the datum line. From the point where the projected line intersects the datum line, measure back along the projected line towards the general curve the distance the rocker is located from the datum and mark that point. Repeat this process for the remaining measurement stations. The measurement at station A should be 231 mm, at station B the measurement should be 164 mm, at station C the measurement should be 125 mm, at station D the measurement should be 137 mm, and at station E the measurement should be 229 mm from the datum line. If the builder is constructing the standard bow boat the rocker curve may be struck using the ¾ x ¾ batten used earlier and proceed with the building process. If the builder is constructing the wide bow boat then the measurements will have to be altered according to the previously stated plan. Once the builder is satisfied that the points are correct and the rocker will pass a measurers examination then the curve can be struck using the batten and the builder may proceed with the construction of the boat. The last step in lofting the pattern for the pieces requires the builder to plot out the top of the three different pieces that make up the spine of the boat. To do this, use the stations that were laid out every 250 mm for the original 21 stations. Next refer back to sheet 7/8 of the plans and now lay out the points that are assigned to the upper case letters to create the top of the forward web, centerboard well assembly, and the after spine. You can get creative at this point and alter the look of the centerboard well as we will do. We will not make our centerboard well straight across the top, as outlined in the plans. Just in front of the thwart, we will project the top of the well down to the mast step in a gentle convex curve. This will look more pleasing to the eye and excess weight will be removed. By doing this the crew will not have as hard a time crossing over from one side of the cockpit to the other. Make sure that the height of the forward web is correct and lines up with your forward bulkhead. Make sure that the over-all length of the boat is correct as the outer skin panels will be adhered to this spine unit. Before lifting the paper to transfer the patterns onto the plywood, use an awl to locate the seven points of measurement that will determine the rocker curve and pierce a tiny hole through the paper to locate those points onto the platform underneath. You can now lift the paper pattern and transfer the points onto the plywood.
© Fireball International, 2003
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