Sunday, July 21, 2019

Garboard sheathing

Using a router with guides, a notched "shelf" has been cut along the edges of the keel to accept the edge of the anticipated garboard.  This will provide backing and bonding surface during assembly.

Creating a rosin paper pattern for the garboard stem ends.  The garboard will be sheathed in three sections (ends and a center piece) of 4.8mm plywood connected by butt blocks.

A simple basin, plastic over 2x4s, has been created to soak the plywood for several hours prior to clamping it in place on the hull frame. A few small objects keep the wood off the bottom of the basin and bricks keep it from floating.

When wet, the plywood is flexible enough to clamp in place without using excessive force.  It will be allowed to dry for a few days (while I work at the clinic) before the clamps are removed, and it is checked and trimmed further.

What I am discovering is that the plywood is not bending according to the designed developable parallel projection.  Instead, the plywood is bending according to an alternate projection which induces lower stresses.  By finding some of the ruling lines (where a straight rule will lie flush with the surface), it appears that the alternate projection is a conic projection with the focus below the keel and about two feet back from the stem. I have not encountered this before.  Perhaps this is part of the concept of "tortured plywood" as described in the Gougeon text.  Long, slender plywood forms will seek to relieve stress when possible by adopting an alternate shape.

This is occurring only near the bow ends where maximum curvature is expected.  When a sheet of thin, flat, flexible, sheet of clear plastic is wrapped around the bow section, it follows the designed developable shape perfectly and lays snugly against the frames as designed.  The more rigid (but slightly distorted) plywood relieves internal stresses and takes on a flatter profile; resisting conformation to the designed shape.  So I am forced into iterations of fitting the ply and reshaping the frames and bow profile taper to accept the altered plywood shape, a time-consuming and exacting process with an inferior result.  Lesson learned: when a lower stress conformation is available, the plywood will adopt it.  But how to anticipate such an occurrence?

The best way to examine this occurrence will be to bend ply around a surface where only the edges are defined and then see what type of curvatures are present at the frame intersections; a future project.

 Future project completed:  I constructed a replica of the forward frames and used stiff cardboard to simulate the sheathing.  The cardboard (like the flexible plastic) exactly conformed to the original frame shapes.  This convinces me that the plywood, by being reduced to only four laminations, may not have a balanced strain pattern when stressed and, thus, not react/bend in a predictable pattern.  If I build a similar hull in the future, I think solid planking, 1.5" to 2" wide and 5.5mm to 6mm thick will work well.