: Just how precise do they have to be?
: Allison
I'd shoot for 1/32" or about the thinkness of your string. An 1/8" is a large number here. It's said most wood work can be easily done to 1/64" tolerances. A difference of an eighth inch in the rise of stair treads will cause the average person to trip.

If you run a longitudinal batten (say 3/4" by 3/4") over your forms you'll clearly see any humps and hollows in the strip. You don't want these speed killers reflected in your hull. Just relax and trust your eyes, you have the ability to see very small unfair spots. All the effort preparing the forms will be rewarded in the actual building process and final product. Tack a few battens on your forms and use them to guide your adjustments. As the battens will want to bend in fair curves, they should help you push your forms to where you want them.
Hope this helps.
Geo. C.