So how good are the bottom curves that come on a blank?
Do they work if kept in the finished shape, or a they just a starting point for the shaper to fine tune?
Can I be confident the supplied curve will be ok?
My first trip down to Greenlight to pickup a blank, I walk in asking all these complicated questions about rockers (what you're calling "bottom curve"). Guy behind the counter says, "Keep it simple and choose the blank based off the rocker you want." As I've read, blank design is the result of many many iterations of someone working on certain board shapes. Once they get a board that's tried and true, they work the rocker right into the measurements of the blank. As a matter of fact, the other day I was looking through a book on board design and came upon a Rusty Preisendorfer tri-fin board. I liked it so I started looking at blanks that would accomodate the same rocker target numbers and found one, the 65R, it matched it neary to the 1/16" of an inch all the way through. I looked at the notes on the blank, low and behold Rusty designed the blank.
The worst mistake I've made yet here was buying a blank and then trying to change the rocker to be quite different. Using hand tools, it was impossible to get it where I wanted without creating flat spots or other problems. Im sure there are people here who have the skill to pull it off. But for someone just starting out, you're better off just skinning the bottom of a blank and going with it. If it's your first build, there will be dozens of other things to learn.
Not only do you want to look at the blank's rocker, you also need to envision where on the blank you're going to put your template. The curve is the same but the end point measurements will vary by whether the shape is being taken from the front or center or rear of the blank.
Yes you can do more by moving that Masonite template forward or aft than you can do trying to carve something into a blank that isn't there to begin with. I always recommend the Ben Aipa shaping video that Fiberglass Hawaii did. Rocker change made easy. There's a man who knew how to keep it simple. Lowel
That which can be assorted without evidence was read in an illegal magazine.
I've mapped some of the stock blanks from the various manufacturers on the CAD, taken screen grabs to use as a background and drawn my design as an overlay to see what I could get out of the bottom rockers. I can move the design back-n-forth and actually see where the end points will be on the rocker at different positions. It's not a silver bullet for a couple reasons, but it makes for a more predictable starting point from which to proceed.
I've never used one, but for a long time I fantacized about building a router table to cut my own bottom rockers. Nowadays with the inexpensive CNC cuts there's little point to it.
Thanks all for the feedback. Unfortunately l can't go ahead with this project as no blanks are available! ( here in Oz) and probably not until end of Feb. :(