My friend & I built one for a school competition. The objective was to design & build a boat that could sail the fastest, with the only rule being that it could have a maximum sail area of 1 A4 size sheet of paper.
As we weren’t told what the competition rules & conditions (especially wind) were, we could only assume a straight line path and little or no wind. Thus, we couldn't optimise the boat much other than minimise weight.
We used a catamaran design because catamarans has a much lesser risk of capsizing than monohulls and thus doesn’t need to be ballasted or widened, both which reduce speed. Wikipedia explains why.
The catamaran hulls were built from flexible Tamiya PVC sheet. We cut the shapes out, and joined 2 such shapes together to form each hull. Joining the hull together was a framework of ice cream sticks. We forgot a keel, though.
The sail was cut out from the school magazine. Holes were cut out and light paper “doors” were pasted over them, similar in concept to a check valve. This is to reduce the effect of a headwind on the boat, which would make the boat go backwards.
Our competitors had rather different ideas. One also built a crude catamaran – using 2 bottles for the hulls and a piece of Styrofoam linking them together. Another simply used a huge piece of Styrofoam as the hull and a piece of corrugated plastic for the keel. Yet another used plastic wrap and a wire framework, which subsequently sank.
We won, though we could have been a lot faster if we had put a keel. The wind kept on changing direction, and with it our boat.