As well as a double nut, I use a nylock nut (the ones with a nylon insert that locks onto the thread, but are still reversible). My engine is a four stroker. Being new to this hobby, and having seen a prop come off a long time ago, I reckoned it was best to be really sure. I understand that with two strokes one nut is OK, but fourstrokes are different; lot of torque to resist the starter and wind the prop off with the electric starter.