Maybe I'm being too silly here but are you using the washer? You need to use the washer with a prop nut or else you won't be able to keep it tight. I suggest for fourstrokes 40 or below you can use a washer but anything larger, use a lock nut. If you are using a Y.S. PLEASE USE A LOCK NUT. I've had my Y.S. throw props and it's scary as hell. So for your 91, I suppose a lock nut is good. I will sometimes use one of the original nuts behind a prop nut to ensure the prop is secure.