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Unfolded Offsets

A project log for A Plywood Canoe

I built this canoe 10 years ago. It was fun but various design flaws has limited it use. This project is about a salvaging the canoe.

agp.cooperagp.cooper 03/05/2017 at 07:070 Comments

Unfolded (Cut Out) Offsets

In order to cut out plywood pieces for your boat/canoe, the design needs to be "unfolded" from 3D to 2D. Now there are some very good programs on the Internet that can do this, my favourite is Carlson Design "Hull Designer" or "Hulls" (http://carlsondesign.com/projects/hull-designer). The only problem with this program is that the program needs to be run in compatibility mode (otherwise the behaviour is frustratingly strange).

The original canoe was unfolded using a spreadsheet (i.e. formula based). The problem is that building a design is a painfully slow process and there is not much hope of explaining its use. About three years ago I wrote a macro that reads a Table of Offsets (what you will often find in a boating books or magazines) and calculates the unfolded offsets directly. Like most home brew programs there are probably bugs that need to be fixed and improvements that can be made and no documentation.

The Offset Table

Here is a typical offset table for a canoe:

(source: https://s-media-cache-ak0.pinimg.com/originals/d5/70/16/d57016a2123565eb573042f4b155c4d4.jpg)

Although there is everything you need here (that is the above image) to physically build the canoe, it is still a tough job for a program to decode. Below is the offset table for my canoe:

Other than the offset table being rotated, there are two additional fields:

Two other comments are that although each "station" or "section" above have the same "X" co-ordinate, this is not a requirement, and although I have filled in each cell, again this is not a requirement.

It helps a lot if you run the "lines" or "chines" all the way though to a common point at each end of the table.

Running the Macro

After checking that you have filled out the yellow cells, click the blue arrow. A minute later (yes it is slow so don't panic!), it will update the "Unfolded Offset Table":

It will also update the 3D drawings:

The "Unfolded Offset Table" will need to be moved around to suit the plywood, so another page allows this. The "plot" button on this page will update the unfolded drawing:

AlanX

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