I don't consider myself an expert, and I'm sure there are many other ways to do it, but here’s how I shape rocker for a 9-6 noserider:
It starts with the blank. I order a US Blanks 10’2”B. (the “New Green” density is excellent for noseriders) This blank’s “Natural” rocker is 4-11/16” NR and 3-11/16” TR…just the opposite of what I’m looking for. I’m shooting for 4-1/4”NR and 4-1/2” TR, with most of the NR in last 12” or so (just as JP points out), and most of the TR in the last 24”-18” or so. I order the blank with the rocker adjusted “-1N” (decrease nose rocker at tip by 1” starting from center) and “+1/2T18” (add ½” to the tail rocker starting at a point 18” from the tail). Remember that when adjusting rocker you need to increase stringer thickness accordingly to prevent “snap-back” or the tendency of the foam to return to its natural, molded-in, rocker. (This is also a good opportunity to specify glue color, which is a nice “detail” to add with basswood stringers) The blank I start shaping is then about 10’2” tip-to-tip, measured on deck, with 3-11/16”NR and 4-5/16”TR (Approx of course. This glue-up work is done by hand at the blank factory, so there will be some variation, which you take into account as you shape.)
Next, I cut about 1”-2” off the tail length to clean up the end. I flip the blank over, deck-side up and measure 9’6-1/4” from the tail to the nose and place a cut mark here on the nose. I draw a line at this mark across the deck and perpendicular to the stringer. Then I cut on the line, holding the saw perpendicular to the floor (some cut at an angle). I cut most of length from the nose because it will help to keep the rocker curve in the front half of the board flat until I flip it in the last 12” or so. After cutting, the nose tip will be 1-1/2” thick, which gives me plenty of adjustment room.
Now my blank has been reduced from 10’1” in length to just a little over 9’6” and has the approximate rocker curve I desire. I flip the blank over, bottom-side up, and re-measure the NR and TR on the bottom. By cutting off the ends of the blank, I have reduced the original rocker at both ends. I record this somewhere for later reference. This is an important step because these measurements tell me how far off my existing end-point rockers are from my target. I then skin the blank and rough-shape the bottom. As I plane foam from the bottom I must keep in mind the targeted thickness and the final rocker numbers, all the while trying to maintain the existing bottom curve and create a nice foil. (I personally check and re-check rocker as I go, while more experienced shapers can just eye-ball it.) As I plane the bottom, I try to follow the existing curve, keeping the curve gentle (not flat) from the center to about 12” from nose and about 18” from tail. At these points, give or take a few inches depending upon the how the bottom curve looks and where it breaks from what I want (thanks JP), I accelerate the curve until it meets my targeted rocker numbers. As JP stated above, the nose concave will counter the accelerated curve in the nose by flattening rocker along the stringer, minimizing water “push”. I personally leave ¼” less than the desired rocker in the tail to allow for adding V, which I do next. (I invariably gain ¼” tail rocker while smoothing out the V and bringing it to flat in the last 3” or so of the tail). Final rocker numbers aren’t as important as the overall arc of the curve, with a more “relaxed” curve in the front 2/3 and a more accelerated curve in the last 1/3. “Remember no abrupt changes in direction or flat spots. You want a smooth continuous curve throughout” (Can you tell I’ve memorized JPs video?)
What you should end up with is enough curve in the tail to (1) act as a lever to lift the rail line of this long surfboard out of the water so it can turn, and (2) to create downward suction on the tail to counteract your weight when on the nose. And, just enough curve in the nose to keep from pearling on takeoff, but flat enough to maximize glide.
