On my old flatty my prop nut came loose, but that was because the lock nut I was using was worn out, just replace your old nut with a new stainless steel lock nut and a new cotter pin. You really need to pull your v-drive apart to find out what gears you have, it's really easy to get apart, just make sure you have the proper amount of gaskets when you put it back together. As for the prop, it souds like the prop you have now is off a ski boat. A good steel three blade might be what you want, for a mild flatty they perform great. If all out speed is what you want I would go with a two blade 11x16 menkins. You can usually get a nice prop used for between 200-300 bucks, just make sure to have a prop shop check it out before you run it. When props come apart at wide open you would be lucky to stay dry, plus they usually ruin the bottom of the boat. Well now I am rambling so if you have any questions feel free to email me, I don't know much but I am more than happy to lend any knowledge I have