You have to hold either width or length constant while the other changes.
To create different sizes for any given model, a surfboard manufacturer/builder must re-size their template. There is no way to re-size a template without changing its curves. (To re-size without changing curve slopes, you would have to do a directly proportional re-size – which does not work well for re-sizing templates.)
When re-sizing, surfboard outline/curve slopes have to change.
The old standard for custom surfboard builders is to increase/decrease width by 1/8” for every 1”of length increase/decrease (up to 6”). But this method does not give the incremental changes needed for each width interval beyond the widepoint.
Stretching length (unlocking aspect ratio) changes width by the same percentage at each new interval beyond the widepoint. The curve’s slopes are different but the curve changes are still in the same positions relative to the original shape/template.
Lis Fish template: original length (65-inch), longer (74-inch) and shorter (56-inch). All three are the same width. (Dimensions displayed are for the 65-inch fish.)
