Here are some pictures of the boat shed a.k.a 'The Bubble'.  This is where it all happens.  A great, bright, cheerful place to work.

I saw a shed like this at a buddy's place and just came up with my own version that seems to work. 

I found that making the shed almost as high as it is wide works for the bending (mine is about 15' high and 16' wide).

You can figure out how long of material you need for the ribs by making a scale drawing on a piece of plywood, taking a couple of nails in it and bending a batten on it and then measuring the batten (and scaling accordingly).

I used 1x4 common-grade fir (3/4" x 3-1/2") for the bending stock and 2x4 (1-1/2" x 3-1/2") for the "spacers". 

I laid out the "ribs" on the floor of the shed using blocks to hold them in shape.  Each side of the rib (the
bent 1x4s) are glued and screwed to the spacers while blocked to shape on the floor. 

I built the two halves of each rib separately and then joined them together before raising them. 

After raising them you slip in the 2x2 (1-1/2"x1-1/2") longitudinals. 

The bottom of each rib slips over the edge 2x6 that the floor joists are nailed to.

The whole structure is covered first with "vapour-barrier" and then the top is covered with a tarp.

George Buehler has a similar shed described on his web site (see
http://www.georgebuehler.com/Shelter.html)

A similar design appears in Wooden Boat magazine issue No. 48 (Sept/Oct 1982).

I enthusiastically recommend this type of structure for a boat shed.
It's strong, light, inexpensive, quick to build, and easy on tarps (no sharp
corners).