Sunday, April 26, 2020

Solving the hull sheathing conformation

Spring is here but Pikes Peak still is snow covered.

Wow! Has it been so long since posting previously?  Over six months?  That has been time for three vacation trips and a major bathroom renovation.  It is nice to stay busy.  Now with the pandemic, things have slowed down some, and it's back to the guideboat project.

In the back of my mind has been that failure of the garboard sheathing to conform to my design dimensions.  The plywood when bending aligned itself with a different curvature, lower stress pattern.  Today, I did a simple investigation.  By laying a straight edge (I used a four-foot level) on the hull sheathing and rotating around a tangent point it until the entire edge contacted the surface, I was able to discern some of the actual (approximate) ruling lines inherent in any developable surface.  I had designed the hull bottom to be a parallel projection; the X:Y:Z ratio was 6/2.4/-0.8.  It appears that, instead, the plywood adopted a conic projection toward the hull end.

Ruling lines marked on the sheathing plywood.

I marked some of the ruling lines on the hull surface.  As you sequentially view the ruling lines from midships toward the ends, The first line is consistent with the design slope.  The second line converges with the first line toward the hull midline.  Converging lines indicates a conic focal point in that area.  Subsequent ruling lines at the hull ends only slightly converge but with steepening deadrise (Y/Z slope).  Looking at these lines and viewing the hull surface from various angles suggests to me a conic projection, or several conic projections, with focal points located forward and across the midline (keel).  On my previous guide boat design, I had used a conic projection in that area.  I used a parallel projection on this hull, expecting that it would be superior, and it is for general purposes.

Initial convergence toward the keel of the ruling lines from midships proceeding toward the ends.

Slight convergence with increasing deadrise for marked ruling lines.

I think I have solved this puzzle.  The plywood was thin enough, and with only a four-ply unbalanced stress resistance, to relieve stress by distorting slightly at each of the frames creating an unpredictable conformation with a lower induced-bending-strain pattern; at least in this situation of a long, comparatively slender hull panel.  I will make sure that I never face this situation again.  I want predictability; not repetitive cut and fit construction.