As you see there are many way's to do what you want, ------ the laser
pen thing isn't that bad a choice but maby you need a bit experience
with other methods before you use that , but it make me wonder about a
method I would think --- havn't tried it though but often thought about
trying it --- emagine you draw chalk lines outside the hull
coresponding the ribs and take a foto from the end of the boat, do
likevise from the side . Now if the foto with chalk lines will bring
what you want, the laser pen is just the tool to make the chalk lines.
You can buy these water level laser pens everywhere at low price place
something in the picture you know the exact size of and I am sure the
foult will be minimal.