You are actually better off cubing, steaming, mashing and then add it to the boil in a muslin bag to avoid it going everywhere. Steaming helps break out the starch. Then use Amylase on it, as this will then break the starch down to fermentable sugars. Other wise you will just be stuck with starch that does nothing. Or you can do as above, but instead of adding to the boil, add to the fermentation bucket. I did a sweet potato pie mead that way in the summer. It actually tasted a little like a beer, too. Check out the mead section or the GF section for more sweet potato recipes and how to use them.