Lofting

When I built Wee Lass, patterns for the stem, transom, and molds were printed on mylar.  The plans for Tammie Norrie had full size patterns included, but they were printed on paper.  Paper patterns have a reputation for being inaccurate; they distort with changes in humidity and can be distorted even during the printing process.  The plans for Tammie Norrie did have the lines plan and the table of offsets, so I could loft the patterns myself rather than trace from the templates printed on paper.

The table of offsets is essentially a table of coordinates, allowing you to determine the shape of the boat at a given location.  The lines plan consists of three views, taken from the side (profile), from above (plan view), and cross sections (body).  The lines are all inter-related, so if the lofting is done correctly, you have a series of smooth flowing lines on all three views.

Lofting is the practice of turning the table of offsets and the scaled lines on the plans into full size, from which patterns can be made.  Lofting has the advantage of not only verifying the paper patterns given on the plans, but also allows you to determine shapes that are not on the drawings.  For example, on of the things I wanted to determine was the shape of the bevel on the stem where the planks would lie.

A lot of people don’t like lofting, but I found it enjoyable, and an opportunity to learn a new skill.  Lofting is covered in many references; one of the most useful I found was in a chapter from “Building Small Craft” by Greg Rossel.  I confess I didn’t do an entire job of lofting the boat; I limited the effort to plotting the body sections, which would become the molds; the stem; and the transom.

2015 02 15 lofting table

First a lofting table was built, it is politely described as “mission adequate.”  A couple of 2x’s for framing, the table surface consisted of 1/2″ ply, then some 1/4″ ply on top of that which I would do the actual lofting.  The 1/4″ ply was given two coats of white paint so the lines would show up better.

2015 02 16 lines

Next a full size grid is laid out, and dimensions from the table of offsets are transferred to the lofting table.  Once the points are laid out, connect the dots with a long batten (a long flexible piece of wood) to give a smooth curve between the points.  In the photo above I’ve got the shear laid out on the plan view, and the batten is used to draw the line.

2015-03-08 Lofting the molds.
2015-03-08 Lofting the molds.

The same process is used to lay out the body sections; these are cross sections of the boat taken at specified intervals.  A true lofting exercise would have double-checked the dimensions in the table of offsets by plotting the lines in the plan and profile.  The designer enjoys a good reputation for being accurate, and the dimensions in the table of offsets are given to the nearest 1/16″, so I accepted them on faith rather than double-checking.  Any errors would show up when I lined off the hull.

2015-03-22 Expanding the transom.
2015-03-22 Expanding the transom.

The transom is raked (slanted aft); the table of offsets are all given perpendicular to the waterline.  Determining the true shape of the transom is called “expanding” the transom, and is not a difficult chore.  Instructions for how to do this are in “Building Small Boats.”

2015-03-08 Lofting the Stem
2015-03-08 Lofting the Stem

Plotting out the stem is no different than plotting the molds; just a matter of transferring points full size.

2015-04-19 Lofting provided the bevels on the stem.

Aside from not trusting the paper patterns, one of the things I wanted to determine was the bevel of the stem, which wasn’t shown on the plans.  Lofting helped me get this bevel, and you can see from the photo that I’m referring to the text to help me get there.  This is where lofting really shines; you can determine the shapes of items that are not included on the plans.  The photo shows that I’ve also increased the thickness of the inner stem; the thickness as shown on the plan likely assumes that the inner stem will be laminated.  I’m going with a built up inner stem, which will be wider than that shown on the plans.  The built-up stem is not only easier to make, but will also give me a larger gluing surface for attaching the fore end of the planks.

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