Octagonation

octagon.GIF

Here is a fun fact for those out there who need to shape an object so as to have a smooth elliptical cross section throughout, and lack detailed plans or templates.  To make the Ulus’s ama, the shape of the elevation view  is given by a table of offsets that one  uses to create the plywood sheerweb, and the planview shape is laid out by hand using mainly one’s eye aided by some long battens.   I did some rough scaling from a rendering in the plans, but no dimensions were available.    At each station along the length of the ama there is thus defined a rectangular cross section into which fits the the curved cross section of the final shape.

But how to actually form the shape so that it will not be un-fair?  I decided that an elliptical cross-section throughout would be simplest.   The best approach is to first shape the rectangular lofted solid into a octagonal lofted solid, and then take it from there.  This is how round spars and oar looms are made.   With a little geometry I figured out that for an ellipse incribed in a rectange, the sides of the enclosing octagon lying on the original rectangle sides are 1/(1 + √2) ≈ 0.414 times the length of the corresponding rectangle side.  So for many stations along the length of the ama we measured these distances and then used battens and string lines to mark the foam ama core preparatory for final shaping with handsaws and sanding boards.  It’s turning out pretty well, as you will seen in the next update.

 img_5500.jpg

img_5502.jpg

1 Response to “Octagonation”


  1. 1 bastcilkdoptb

    I’m still learning from you, but I’m improving myself. I definitely enjoy reading all that is posted on your website.Keep the aarticles coming. I liked it!

Leave a Reply