Looking up "draught" in the OED for pronounciation notes, which can't be copied into here because it's full of symbols that ordinary HTML can't render. Apparently the pronounciation "drawt" showed up in late Middle English but went out around the 18th century; the Scots currently pronounce it "dracht" (guttural ch, as in 'loch') and everyone else has gone over to "draft," a pronounciation which the OED editors apparently don't like because it makes the connection with "draw" less obvious. (A draught is something which is drawn, as a breath, a drink, a pen across a page, or an animal's load)
The more logical spelling "draft" is therefore coming into use, but is apparently the accepted spelling only for certain senses of the word, e.g. a drafting table, or a gust of wind in a room, and not e.g. a draught of beer or a draught animal.
