The logic of designing on software and shaping by hand is different. With software you start with the end product. By hand you work towards it. So by hand you shape your bottom then in putting in spiral vee you give four long rubs with a soft-backed sandpaper, and two short rubs between the fins and your concaves are in. Software you set where you want the concaves to be deepest and then let the algorithms fade the curves to match the rest of the shape. A hand shaper can use ply templates and use the stringer template on the rails to be sure of getting the same curve, but there is always some blending when the bottom is not flat and then you’re working with amount of light you can see under the template (and you’re bending the template to match the outline). Shapers who use profilers can put in concaves using a single profile by tilting/shifting the position of the board, so theoretically the rail and stringer curves are the same, just shifted relative to one another. But what do I know, I’ve only trained a shaper in sofware design, and picked up scraps in the process.
As to your question, use the stringer line and then lower or raise your rails relative to the stringer.Do this in the slices. If you want concave to flatten your rocker, then you have to start with a flatter stringer rocker and then lower the rails in the slice(s), in essence increasing rail rocker. If you tried this hand shaping, you would be “adding back” foam on the rails, an impossibility. You can use 3D layers in S3D to put in concave, but let’s keep it simple.
Redboards, a thought. Rather than the typical method of shaping from the perimeter and work your way in to skin the blank, plane right down the stringer before you skin the blank. Then work your way from the stringer outward, planing to the desired rail rocker. Your contour is the end result.
I read your first post and think I understand the question. The answer is the stringer. Yes it is true that the shaping software functions essentially backwards from how one would handshape. The basic program will give you stringer rocker which you manipulate in the side view and rail line rocker will be affected if you alter the slices with concave and vee. Unless you have an upgrade in the software you will lose the rail line rocker as soon as you go away from anything but flat on the slices. This is just as critical to the boards perfomance as stringer rocker and will alter how the board is going to ride. Depending on how you design the rocker and the slices in the program you need to keep in mind those differences. You can see this when you hand shape using measurements off an already finished board vs a raw blank. But the relationship isn’t as straightforward as it might sound. Concave especially can be tricky. You can tilt the rocker in relation to the rail line rocker by how you cut or not the concave at the center of the board.
I’ve transitioned from backyard to production handshaping and have developed my own way of dealing wth the essential mind f##k of rail and stringer rocker nuances. You have to start with a flat bottom, rail and stringer rocker being equal and know where you are going to end up rocker wise at the stringer when you are done. This is further complicated on you exit and entry rocker numbers. Is the concave consistent, does it transition to a double and then back to fat or is there vee a the tail tip. Example being I know that I have to start with an extra 1/8" of tail and nose rocker on the flat blank if I’m going do an 1/8" of concave at the center of the stringer if I’m going to accurately hit my rocker numbers on the finished board. Now this is a quick simple example but hopefully will illustrate at least my way of shaping. With that being said I know that a single concaved board at the center line will have less stringer rocker than rail rocker and it will ride in a certain way. Same as if the concave was put after the center line and essentially increases the stringer tail rocker as opposed to the rail rocker after center towards the tail. This alters again when you start adding vee.
Hopefully this isn’t too confusing but you need to always keep in mind how your actions or the machines actions are altering the relationships between rail and stringer rocker and more important on how it is affecting the ride characteristics and performance.
DMP and others, thanks for chiming in. DMP yes you understood the question and in fact what you described was what I have done on my latest design. I imported the blank into the software as a starting point for stringer rocker. I then flattened the stringer rocker in the area were I will have single concave. All of this was done in the profile view. Then go to slices and begin to add the concaves and or vee. In AKU there is an option to display rail rocker line which makes it crystal clear how much more or less rocker your rails will have than the stinger. ONce this is done, I take a look at the board using the various 3D views. Curves and contours that do not look right are pretty easy to spot from there.
Also what you described regarding adding the 1/8" in nose and tail when adding 1/8" concave in center hit home when I have hand shaped. I wanted 4" of stringer rocker in the nose of my last board. I also wanted 1/4" concave. So in trying to work with the rocker in the blank I moved the template until the nose hit the spot where I had 4.25 nose rocker prior to shaping.
Thanks for the insight. I believe Stumpy http://www.stumpsurf.com/ used to do it this way on the profiler. Cut the stringer then drop the blank, lift the tail, shift forward slightly and put in the rail rocker, ending up with a concave to flat that he blended rail to rail. The profiler now rusts in the yard, replaced by a wet-out table. Such is progress.