From a beginner to a beginner: I had the same problem. I believe that the problem is with the size of the nut that comes with most engines (it is tiny). Go to Tower's website and look at hubs from Harry Higley http://www2.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=++&FVSEARCH=harry+hub&FVPROFIL=+ +
These are large nuts (not trying to be funny) that you can really crank down onto the prop with a socket wrench (just grab that blade with a glove!)
This will solve your problems, unless you need/desire to use a spinner. ps you will need the proper rubber insert for your electric starter if you use these types of hubs