Dogmoat is right you could go to a three blade... drop the dia. and increase the pitch and you would get about the same amount of thrust. Master Airscrew has a three blade 16x10 pusher for $19.95 at
http://www.masterairscrew.com/GoCart...66&RootID=1054 . I don't know if the thrust will be excactly the same but it should be close.