Desiging the sails begins with their area. Beth has 90 square feet of sail, but our canoes are a bit smaller and probably we are less capable sailors than Mik intended for Beth, so the canoes will each have 80 square feet of sail. By guesstimation, I decide to have 60 square feet in the main and 20 in the mizzen. Both are simply scaled-down versions of OzRacer sails, which were designed for quick, cheap polytarp construction. One 8m x 4.5m heavyweight polytarp ran us AU$140 including tax and shipping. That was big enough to make all four sails with a lot of excess material for patches.
The main is a balanced lug sail and the mizzen is gaffer. The gaff-rigged mizzen requires no yard, and should be easier to handle and reef than a similarly sized lug. The balanced lug main ought to give us better performance than would a big gaffer, and also means the main mast is significantly shorter than would be necessary for a triangular main.
We took the tarp to a park and stretched it out with tent stakes in order to measure the sail panels. Like the hull panels, this involves making marks around the edges of the sail, then using a flexible batten to scribe a smooth curve through the points. After we cut out the pieces, the sails were taped together with double-sided tape, in which state they currently await the attention of our borrowed sewing machine.
The photo is Kate (with assistance from Annie the boxer) tracing out one of the big patches for one of the main sails.