Approximating Cylinder Mold Boat Construction

[John R. Coil]

Cylinder mold construction is a way to trick plywood into accepting a small amount of compound curvature in narrow hulls. It is used in a stitch and glue process.

Essentially, for anyone unfamiliar with the technique, after vacuum bagging a panel — which has a simple profile curve along its width — long enough to cut either a port or start board hull panel on and these are then “tortured” to come together on the keel and compound curvature thereby induced.

The method tends to produce hulls to approximate dimensions rather than more exact. Still, within limits it makes it possible to rapidly achieve a fair surface.

My interest is actually in using these with a flat bottom to give the appearance of a rounded hull, where the 10:1 achievable LBR is instead run along the sides of a wider hull.

The problem is in approximating the form these panels will end up with. There will be radial bending that should flatten out the contours perpendicular to the length leading to, I’m supposing, the sheer bowing outward and a bit downward at the midpoint while the bow, midships, and bottom line remain fixed.

Does anyone have experience with the base construction technique to help qualify what sort of change to the sheer might be expected or suggestions on how to model it?

I’ve thought about starting with the profile curve and using rotate at each station (moving the model back and forth so that the station bring rotated is at 0) and then scaling the results, X and Y, to the right length before rotating that to match up to the bottom; however, that would be distortionless squash and stretch around the midpoint, and closer to modeling constant camber rather than cylinder mold.

Likewise, simply keeping the stations perpendicular to the centerline and moving them to match a curve, while it may allow that squash and stretch don’t really happen, still can’t yield the distortion that should occur.

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