You can now buy a laser level for under $20. I bought one at Costco. With the sweep line (instead of pinpoint) I mounted it on a camera tripod about 3 feet above the centerline of my strong back (and five feet behind the aft end) and it covered the whole length.
It was a cute trick but I found a chalkline to be more helpful. For a tight clean chalk line get some fisihing monofiliment run it through a piece of 220,320 or 400 sandpaper-that'll give the slick monofiliment some tooth to hold chalk. (Infact you don't even need the chalk box-you could run a piece of chalk along the stretched monofiliment-you don't need much-before you lower it to the two end points on your strongback.
Your strongback could be bowed, but so long as its stable and you follow the chalkline you can make your forms true. And it never hurts to trace over a faint chalkline with a straightedge and sharp pencil.

Will

: I have a saber saw with a laser beam sighting line. I was playing with that
: last night. It doesn't have sufficient power to project a line down the
: entire length of the boat, and the beam gets quite wide after a few feet.

: I bought a bunch of string at lunchtime today....