In the 1960s and early 1970s, boats were made of heavy fiberglass and nylon. The boats were high volume and weighed over 65 pounds (30 kilos). In the early 1970s Kevlar was used and the boats became lighter as well as the volume of the boats was being reduced almost every year as new designs were made. A minimum boat weight was introduced to equalize competition when super light materials began to effect race results. The I.C.F also reduced the width of the boats in the early 1970s. The gates were hung about 10 cm above the water. When racers began making lower volume boats to sneak underneath gates, the gates were raised in response to fears that new boats would be of such low volume as to create a hazard to the paddler. Their low volume sterns allow the boat to slice through the water in a quick turn, or 'pivot'. Usually boats are made with carbon fiber, Kevlar, and fiberglass cloth, using epoxy or polyester resin to hold the layers together. Foam sandwich construction in between layers of carbon, Kevlar, or Aramid is another technique in use to increase the stiffness of slalom boats. Recently, the minimum length of these boats were reduced from 4 meters down to 3.5 meters, causing a flurry of new, faster boat designs which are able to navigate courses with more speed and precision. The shorter length also allows for easier navigation and less boat damage in the smaller man made river beds that are prevalent in current elite competitions.