The important thing is whether the forms are properly placed relative to each other - not the strongback. Any warp in the external strongback really doesn't matter.

Run a builder's string down the keel line to confirm straight keel line and proper rocker at each station. Confirm that each form is plumb to the floor with a plumb bob. Pay special attention that the bow and stern points are alligned with the keel line (do it again after laying the first few strips). After laying the first pair of strips, eye down the strips for fairness and adjust any form that doe snot yield a fair curve.

John