Chine fairness in boats

There's no escaping the math and the materials science if you want decent looking results. Stitch and glue requires a huge up-front effort for anything other than trivial shapes. Modeling is a little helpful, but if you are modeling for a plywood build, you must choose the model material very carefully. You can't use basswood, it is non-isotropic. Use plastic sheets like what circle templates are made of. Make sure you match the scale of the model to the bending modulus of the plastic so the forces on the panel seams are scaled to the real boat. This is a PINA to calculate and find the right sheets and get the scale you actually want.

Stitch and glue is like closing an overstuffed suitcase. As you close the zipper, you have to push along the top. For all but the smallest boats with the simplest shapes, you have to add temporary framing or sandbags as you stitch the hull so that the bending moment is distributed across the panel and not all concentrated along the edges. Panel development software doesn't apply edge forces. You want to bend the panel in the middle and not have to pull the panel in on the edge with the stitch.

Rabl's method is still the go-to system for backyard builders. I've commented on that before and will try to dig up the threads here in a minute. Predicting the curvature of the transverse sections and knowing the orientation of the panel's ruling lines is also very important. The ruling lines must not intersect on the panel or too near the edges. That is one of the biggest problems with eyeball designs. Rabl's method lets you control the focus points of the ruling lines. I designed a cruiser on A4 paper using Rabl's method and the focus points were 12 feet away - so clear an entire room. For a sketch on 8.5 x 11 paper, I usually work on an old painted door set up on sawhorses. I just draw all my extended lines on the door so I can keep them, at least until the next project when I repaint the door.

 

That is interesting. I never did apply steam but always wondered if it would be useful in achieving a more graceful curve. To be honest I thought it would have the opposite effect, creating sharper turns when stretching at two points in a chine along the length say. Will try it our.

 

That makes a lot of sense, the suitcase analogy. Yes the sam rabl material I could find online seems to be just what I'm looking for. Trying to fully wrap my head around it and will start drawing with it. Seems to me that it's a tricky balance as far as stretching stitch and glue boats. I feel like I'd want my design to require a little bit of force to stretch it into place, but too much and you end up with a lumpy edge. For example, stretching the beam just beyond its resting point can produce really nice rocker. Or would you design panels to the exact shape, with minimum force required?

 

This was designed on paper to get the most hull out of two four-foot wide sheets The three panels each side can be nested on a four foot sheet, so two sheets can be stacked and sawn out together. I then punched it in to Freeship in such a way that there was zero Gausian curvature on the Freeship model, which takes some math foolery with control points.

Development of intersection of two cones and two planes. https://www.boatdesign.net/threads/development-of-intersection-of-two-cones-and-two-planes.51910/#post-713716