I have two things you need to look into. First, make certain that the hardened steel washer that is part of the engine is located at the front of the prop, away from the engine. The prop nut must bear against it. In this manner, the back of the prop is against an engine washer that has serrations like teeth, that lock the prop to the engine. I have seen cases where the hardened washer was placed behind the prop which negated the purpose of the serrated washer behind the prop. Second, you say you really tighten the prop nut. Well I have big, powerfull hands and have always used the hobby X type of prop wrench. After having more than a few props come off like you have, I stopped using the X prop wrench, and instead use a 10 inch adjustable wrench. True, this is overkill and I do not recommend this to you. You may damage the prop or even damage the engine. I am very carefull and now my prop never gets lose and I have never caused any damage. Try tightening your normal way, then use a larger wrench and tighten it a little more. Get some experience in that manner.