It is normal for the mast to rotate - in fact it's designed to do so because it offers higher performance if you properly rotate the mast while underway. You should have some sort of rotation limiter that consists of a stainless steel "U" shaped bar coming off the mast just above the boom. A line should go from the boom, through this limiter, and back to a v-jam cleat on the boom. You can limit the amount of rotation while underway by controling the length of this line - but you can't eliminate rotation entirely. Beach cats are really meant to be left on the beach and I wouldn't recommend leaving it on a buoy for more than a day or so.