Spanker?
A spanker is a large sail fitted to the after-most mast (mizzen)on ships with several masts, generaly hermaphrodite rigs. I think you'd be better off with a spinnaker. My guess is that the maker would be more than happy to provide all the information you could ever need to enable you to fly the sail properly. Failing that, most of the sailmakers have informative articles available at their web sites.