Piranha props are composite propellers designed with replaceable blades that fit into a universal hub assembly. When I installed one on the 90 hp Mercury outboard powering a 17-foot Boston Whaler, I noticed little difference in performance. The engine still wound up to maximum rpm, and the top speed, as reported by GPS, remained the same as with the stainless-steel prop normally run on this engine. During time-to-plane trials the stopwatch granted a half-second advantage to the stainless wheel. In all, this tells me that the Piranha can serve as a primary prop, not just a backup, if you so desire.