I thought of this technique to create straight lines might be of interest to other model builders. Maybe it's been posted before, but I've never seen it.
I had pulled a casting of one of my models and when I cut along the midline, which was captured in the molding, the halves did not line up. Either through the casting sitting in a position that caused it to bend before completely curing, or through the rubber mold not being completely seated in its hardback, this midline had turned out curved, so the two halves did not meet in the middle.
I didn't want to have to go back to scratch and redo my molds, but I didn't want to cut up more castings that didn't line up either, so I figured a way to get perfectly straight lines, at least marked on these hull castings, from now on. Whether or not they can be cut straight is for another story.
Because of the length of these hulls, 70 inches, and the curves at the bow and stern, there is no way to use a straight edge to get a straight line from end to end. So I pulled out a laser leveling tool my wife had gotten for me years ago. You know, the kind you hang on a nail on the wall and then use the laser light line to hang your pictures straight. No, I wasn't going to hang my hull on the wall and use it that way, but the laser line that comes out of these things is as perfectly straight as anything else we can use. And it doesn't bend like even metal rulers do.
Shining this laser light down the length of the hull made a perfectly straight line for the midline. Just a few problems to figure out. At the end I started at, the light would hit a good portion of the curved part of the hull, but at the other end it disappeared, and the hull is so long I couldn't really locate the correct spot to line it up on the other end because I couldn't see where it ended as once the hull started to curve away again, the light stopped touching it.
So I simply stood a piece of cardboard with a straight vertical line drawn on it up at the far end, with the drawn line lined up with the midline at that end.
Now when I shined the laser from the far end, all I had to do is make it line up on that drawn vertical line and it would be straight down the center. (Make sure your hull is set up so that this midline, if cut straight down to the other side, would hit the midline on that other side, in other words—the top center, or bottom center of the hull half, is lined up at a straight 90 degree angle to the laser light. Otherwise, your new line will also be curved compared to the other half once they are cut. You can see in the long photo below how the laser line cuts across the stern of the hull. This line goes straight down to meet the correct midline on the other side.)
At this point, I just made marks with a sharpie along this laser line every three or four inches. When I got to the end where the light stopped touching the hull because of the curve to the bow, it was only about 8-10 inches left. From there I could use a ruler to continue the line all the way to the bow.
From here I used painter's tape to run the length of the midline, from dot to dot. Then I cut along the edge of the tape. This helped too, because I could see immediately if my cut was lined up with the tape edge or not, keeping the cut lined up over the whole distance. Víola. The two halves once again line up perfectly.
Hope this will be of some help to others of you. It should work for determining those difficult waterlines some boats require painting along for color demarkation too. Good luck.