Whereas a catamaran has two hulls of equal size and the same proportions, a proa has one hull and a small outrigger. The latter serves partly as a float (to hold up) and partly as a weight (to hold down). But a proa is normally sailed with the wind on the outrigger side, and instead of tacking through the wind she swings her sail round to sheet at the other end, and what was the stem becomes the stern and vice versa. That may involve shifting the rudder (or steering oar) from one end of the craft to the other.