An ultra light displacement boat (or ULDB) is a modern form of watercraft with limited displacement relative to the hull size (waterline length).
The possibility of further speed increase [...] will always attract ardent believers in their speed virtues, just as they have done in the past. The ULDB are, however, very capricious creatures in terms of performance. They may deliver the goods, provided there is just a right kind of wind and from the right direction to sail 'full and by'. And since weather is also capricious, the ULDB and weather seldom suit each other. 'Light displacement craft', Davidson remarked, 'are not new in principle'. For many centuries there have been canoes, proas and the like in the South Pacific and other places, with similar displacement in proportion to the sail area and hull length. Racing dinghies, or dinghy-like modern offshore racers, so common today, are typical examples of the same principle. In all instances the combination of the major design features: displacement, sail area, length and stability [i.e. —] power to carry sails effectively, is radically different from the combination found in the traditional seaworthy and wholesome yachts.— C. A. Marchaj, Sail Performance (1996)
ULDBs are competitive, even after 35 years with open ocean racing participation and podium finishes even today. The relative low cost to obtain, tough construction and readily easy modifications make an Olson or a Hobie an extremely competitive and fun boat. The boats do lack comfort, and are not designed for cruising; however, with multiple transpac races, and multiple Bermuda 1-2 entries, they are proving to be a stalwart competitor despite their older design.