The center line in my postings was to make sure that the coaming was properly centered, not to eliminate twist.

Sight lines and winding sticks are the easiest, cheapest and most accurate ways to detect twist but you can use measurements, too. The problem with measurements is that they are more susceptible to errors than simply sighting, but if you're careful they will work.

The first thing you have to do is set up a reference for level. That's usually the hardest part. Once you have a reference level, you put the winding sticks on the boat and measure the distances from the tips to level and adjust the boat until they're the same. But you may not need to do that.

For your setup, you could try moving one end of your boat right up against the wall to combine your clearances into a 9 foot sight line from the bow and 21 feet from the stern. You can also increase the sensitivity of the test by making the sticks longer since the deflection of the tip is proportional to the length of the stick. A 1/16 inch deflect on a 3-foot stick would turn into a 1/4 inch deflection on a 12-foot stick.

You can also work on maximizing the visibility to make smaller differences easier to see. A gray wall, white back stick and black front stick with good lighting would show up very small differences.

Good luck,

Laszlo

At what point are you in your build?

Checking for twist is most useful - and correctable - when your panels are all in place, stitched together but not tacked / spot-glued. Easiest way is to take two long-ish, straight sticks that are stout enough to stay straight when placed across the gunwales / shearclamps near the bow and stern. It's quickly evident, once you set those in place then step back as far as you can to sight along the top edges of those stick, whether they're 'in the same plane' or not.

A centerline of some kind - stretched fish line or wire, even carpenters' string - is useful too strung between the bow & stern. Then you can take a level or plumb bob to read whether the centerline of the hull along the bottom deviates any from what the stretched line above defines.

If you have (or can borrow) a laser level that can be set for horizontal and/or vertical lines you can do both of these reads with ease. Only caveat being that you have to have set up your building platform with a level surface for your build.

I used one to check for twist when building my Waterlust kit. Set it to project a line from bow to stern with the horizontal positioned to illuminate those points. Then take a rule to where each frame meets the gunwales on either side and measure from the horizontal line to the top of the gunwale. Any difference in the numbers you get indicates something that needs correcting; when everything's square & plumb those numbers ought to be equal.

Laszlo, thank you for the clarification on that. I knew I read something but obviously remembered incorrectly. I will have to use that when I get to that point in my build. Sp, it seems that what you are describing with the fishing line might well be what i was remembering. Currently I am at that exact stage, everything is wired up and waiting to be tacked. With warmer weather approaching I'd like to get this done before summer is over. Thank you both for the opportunity to learn from your experience once again. It seems as though I will have to rearrange my basement a bit again but that's not an issue.