Carbon fiber is proving to be a gift to speedboaters. The material, bonded with epoxy resin, presents a significant alternative to fiberglass, allowing boatbuilders to create stronger, lighter vessels that are the nautical equivalents of racecars. One of these, the 51-foot Outerlimits GTX, debuted at the Miami Boat Show in February. The powerboat, from Outer-limits (www.outerlimitspowerboats.com) of Bristol, R.I., reaches a top speed of 138 mph when equipped with twin 1,200 hp Mercury Racing SCi engines. The 51-foot GTX, which begins at about $1.1 million, is the latest in a series that includes 39-, 42-, 44-, and 47-footers. Also at the Miami Boat Show, Outerlimits introduced the 50-foot Cat, its first power catamaran, which begins at $1.5 million.