Week 5 – Drafting and Dovetails

Drafting

In my previous training and work I have always considered technical drawing or drafting to be a communication tool for a design. The interesting thing, and my main takeaway, about drafting in the boat building world is that the act of drafting is also a design tool.  I have written a lot of words about this topic for those like detail. For those that prefer a big picture assessment think of drafting as a way to communicate a boat’s design and a tool that works to make hull shape pleasing to the eye in all directions.

Now for the detail.

In the most basic sense, drafting in the context of boatbuilding is communicating a 3d object in 2d format.  Many boats have complex hull shapes that should be fair in all directions.  A fair line or surface is one that is free of any extraneous bumps or hollows.  The practice of drafting, then subsequently lofting, helps to generate lines of a boat that meet that criteria.  Taking the time to do this work up front will reduce the amount of fairing (the act of making something fair) once the hull is constructed.  

Drafting starts with the table of offsets, a collection of measurements, in a non typical format the describes the shape of the hull.  From a builders perspective, the table of offsets comes from the plans.  One possible way to develop these offsets is from a half hull model (which would be developed by modifying a current design or by eye).   Check out the video from of Halsey Herreshoff using cool old measurement tool to generate an offset table.  In modern times computer software can help develop the shape.  But here at boat school we are old school and are going to draft with pencil and paper.  Mainly because we will use the same process to loft our boats.  The process of lofting generates the hull shape in a 1:1 scale.  These lines are used to fabricate the molds and patterns needed to build a boat.

A lines drawing of a boat typically has 4 views, sheer (profile or side view), half breadth (plan or top view), body plan (cross sections from bow to stern also called mold stations), and diagonals (cross sections from the center line out approximately perpendicular to the planking).  In our drafting the first 3 views are displayed on top of one another.  They can also be separated, which is more pleasing to the eye, but more challenging to draft.  All of these views have the same references in different planes.  This creates an interrelation of lines from from one view to another.  The methodology we follow to generate these lines is to start from outermost part of the boat.  Once the points are plotted a small spline, is used to connect the dots forcing the line to be fair.  Then the lines that divide the boat in half are plotted then quarters and so on.  As one moves through the layout points that are taken from the table of offsets can only be used once.  If one needs to plot a point that has already been used they must measure it from another view.  A number of lines will be generated purely from measuring other views of the drawing.  This rule helps force everything to be fair in all directions. If a line can’t be drawn fair (can’t get the plastic spline to hit all the points) there may be an issue with a line in another view.  Once all the lines are drawn then a measurement prove out is done at each mold station and any corrections are made.  If you have a good set of plans drafting probably isn’t necessary but lofting might be to get all the molds and patterns.

Dovetails and Shoulder Boxes

When I started getting more serious about woodworking my friend Tom told me (he may deny this) that he had no desire to learn how to cut dovetails by hand.  So I decided, since I wanted dovetails in my woodworking that the Leigh dovetail jig was the thing for me.  Thats right, one of my first pricey woodworking purchases was a dovetail jig … what was I thinking??  So here I am 7 years later hand cutting dovetails for a grade.  After we cut a few sample joints we were let loose making a shoulder box.  I am not sure of the history of this particular shipwright box but I think this same design has been built by students for years.  I believe all of the boat building instructors are former students and I think they all still have their boxes.  The point of the box was to show that we could follow a plan and build to a design.  We also made “trays” or tiny boxes to live in the larger box.  The only design criteria for these was that they had to fit in the larger box.  I am featuring some of my classmates work, to show the variety.  Its fun to see the different designs and uses of scraps of boat building material we could scrounge from around the shop!

Someone had a question about spars

I had a question about spar making, specifically the process of going from 8 sides to 16 then subsequently 32 on a long spar such as a boom.  The process we were taught was to layout the end of the 8 sided piece by drawing, a circle then tangent lines create a series of small triangles.  Those lines are then connected along the length of the spar with a batten and material is removed as described previously.  This method works for a spar with the same diameter from one end to other or even one that has a taper.  The issue comes in what the diameter is larger in the center compared to the ends.  When this spar shape is desired you have to focus on maintaining equal sized facets on all sides.  The key is being methodical as material is removed.

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