Oh man... you should really read up on here about "mashing" before you go for this. That's a huge batch of beer, way to much to waste with noobie mistakes. You will need about 12 gallons of water to mash, and 9 gallons to sparge. What are you mashing in, a bathtub? Also, how is it that you are making a 15 gallon batch of beer with 11 gallons of wort?