What type (material ) props are you using? All props will take a set after a while, and then the prop nut will need re-torquing. (especially when using wood props )

I have noticed, over the years, that many beginners don't get the prop nut quite tight enough. Don't be afaid to tighten that sucker down good 'n tight...the crank threads and prop nut are hardened Steel...they can take it.

I've never had the need for a lock washer...or loc-tite, or anything like that. I would think that the compressability of a lock washer just might make things worse?