Go with sails
Ok...some arm-chair engineers and physicists out there are gonna pillory me for this, but screw the prop; go for the sails if the budget question is one or the other.Your boat's a SAIL boat, not a PROP boat and the drag produced by your prop is inconsequential when compared to a new suite of sails which, in the main, are the primary "engine" of your boat. True, props are important and folding props are extremely efficient compared to their fixed-pitch cousins and produce somewhat less drag when folded up, but your boat is driven by its sails more than it is your prop. Your lack of performance can probably be more closely linked to blown sails rather than a draggy prop. If you are not racing, a single knot or two isn't going to make a really big difference on the fixed-pitch/folding prop debate. Besides, a new suite of sails will probably make up that difference anyway.Good luck.