Now wait a minute!!!!!!!!!!!!!! You want to go from an aircraft .60 that spins around 16K to a buggy .21 that spins 30K+ THROUGH A GEARBOX? How big is this rigger? You may want to do more research on this before you go spend a lot of cash. You need to remember, a buggy engine doesn't make a lot of TORQUE. Torque is as important as RPM when getting a prop to push a boat. The buggy engine is designed to give max RPM, which is run through a gearbox to lower speed and increase torque. In your rigger, you won't have a gearbox so.............................