Lofting, Part 2

Lofting in the living room.  My three
sons help fit the batten to the curve
so I can trace it on the plywood.

I spent last night laying out the station lines and marking the points of the various curves.  This morning, I drafted my sons to help hold the batten in place while I drew the curves in for the side and bottom panels.

There are four major lines to draw in - each side panel has one curved and one straight edge, and the bottom panel is curved on both edges.  There are also a number of smaller curves to trace in, along the cutout in the stern end of the bottom, and the bow and stern decks.  On the longer curves, I tacked brads in at each station.  It was then a relatively easy thing to have the boys press the batten against the brads while I drew the curve in.  Brads weren't necessary on the smaller curves.

Everything went really well; the markings came out clear and it really looks like these shapes would make a boat if you joined them.  There was one point on the bow that I had marked at the wrong spot, but that was an easy fix.  More perplexing was a station point near the bow that just didn't match the rest of the curve.  It happened on both side panels, at the same (shared) station line.  I quadruple-checked the depth of the curve at that point, and at the adjacent points, and never found an explanation, so we ignored that line and drew the curve in using all of the rest of the points.  The only explanations that make any sense are that there's an error in the plans (i.e. that they give the wrong depth for that station), or that I mis-measured the location of that entire station line (i.e., that the depth is correct and would fit the curve fine if I moved it along the line).

Before I cut those panels out, I'll recheck the station line and contact the Wharram folks for verification of measurements given in the plans.

Stern post and stem post, traced onto office
paper to avoid cutting the plans up.  The stern
is on the left and the bow is on the right - the
difference in length gives some indication of how
much deeper the boat is at the bow than the stern.

The plywood now has the shapes of both decks, both side panels, and the bottom panel drawn in.   There are many smaller pieces that fit in the gaps between; most of those are either butt straps, lashing reinforcement pads, or stem & stern posts.  The butt straps are just patches to join the long pieces together, so they and the lashing pads don't need to be precisely shaped.  But the stem and stern posts are critical, and oddly shaped, so the plans include full-sized (1:1 scale) patterns of these pieces.

I was not willing to cut my plans up to trace the stem and stern posts, so I taped several sheets of paper together and traced them onto that paper, then cut the shapes out.  They came out really nicely.  Their shape somehow evokes the hull's form better (for me) than the side panels do.

Next I'll transfer the stem and stern post shapes to the ply, and then (assuming I get an answer on that point that didn't match the curve) I'll begin cutting various hull panels out.  That's an exciting prospect!

Lofting, Part 1

After two months of fiddling around with paddles and outrigger timber, I finally had a good opportunity to begin the work on the hull.  The first step is "lofting," which is the process of transferring the shape of the various hull panels from the plans to the plywood itself.

Because the plans are smaller than boat they depict, the panels can't just be traced from them.  Instead, you draw a number of reference lines ("stations") at measured intervals along the plywood, and then mark precise points at each station according to the plans.  These points are then joined in a continuous, curved line and traced using a flexible straight edge from your scrap lumber pile - a "batten". 

Lofting in progress. The longitudinal
chalk line is the center line for the
bottom panel; dimly visible are the
transverse station lines.

The Wharram Melanesia is a narrow, low-cut, 16-foot boat, made from two sheets of plywood laid end-to-end.  To loft the patterns onto the plywood, you lay the two sheets out together, mark the stations and the centerline of the bottom panel, and then get your battens and scribe the lines.  These lines become your guides for cutting the patterns out.  Both side panels and the bottom panel fit in the 4-foot width of the plywood, along with several miscellaneous parts like the stem and stern posts, the bow and stern decking, and various butt plates and lashing pads.

Lofting has to be done right or you'll be buying new plywood and starting over.  One reason is that it's a tight fit, squeezing all of these shapes into two sheets of plywood.  If a cut wanders much, it will probably end up in the next panel.  But the most important reason the lofting and cutting have to be precise is that these shapes will be "tortured" into a hull shape by joining the curved panels along their edges.  If you get the curves wrong, the hull formed by joining the faulty curves together will be skewed.  So lofting is where it all starts.  Get it right, and you've laid a solid foundation for the rest of the project.

This is the aft end of the bottom
panel. It has a "swallowtail" cutout
shape; the interior edges of this
cutout are stitched together
to form a flattish, curved
bottom in the stern.  The
last 4-5" of the tips are
traced from a full-sized
pattern included in the plans.

For these reasons, I'm taking this step nice and slowly.  My eldest son (12) helped me chalk the stations and the bottom panel's centerline in.  The only error I found so far came when I mismarked one end of the centerline - I had it off by an inch!  The error became obvious as I drew the center panel in, and we redid that part.

We stopped short of actually drawing the curves in, because it was getting to be past everyone else's bedtime.  It's nearly impossible to hold a batten in a complex curve and trace it faithfully when you only have two hands for the job.  So I contented myself with making all the marks necessary to draw the curves when people wake up tomorrow.  We'll take care of that in the morning, then I'll be ready to start cutting pieces out whenever time permits.

More news as it happens.