dlochner is correct, if you decide you want a new prop, you need to contact your local prop shop for the correct recommendation. Come prepared with the following information:
1) Engine max horsepower (see engine manual)
2) Engine max RPM (see engine manual)
3) Transmission ratio (see engine manual, or placard on side of transmission case)
4) Size of prop shaft (which you can measure at the aft end of the transmission)
5) Max clearance between hull and tip of prop shaft (the prop won't be this large, as there are "rules" regarding clearance)
6) Which way the shaft turns in forward gear (see engine manual)
7) Length of the boat at the water line (see owners manual, or sailboatdata.com)
The prop shop will also be able to intelligently discuss the advantages and disadvantages of two versus three blade props, and the advantages of specific blade shapes. You can have this discussion with a phone call, so aside from collecting the above info, your time investment is low.
If you decide a prop change is needed, used props are much less expensive than new ones, and can be found on eBay, local prop shops, and marine salvage yards.
Having said all that, I agree with the above posters that you should at least evaluate a few other things before spending any money on a prop. More info from you will help us help you more accurately.
Edit: And perhaps the most obvious question: is the hull fouled, or do you have it cleaned regularly?