Changing A Boat Prop NEED HELP...why Does My Boat Take Off In Start, Can't Set It In Neutral?

NEED HELP...why does my boat take off in start, can't set it in neutral? - changing a boat prop

the first time with the boat propeller rotates in principle. seems that theres not neutral. What could be wrong. Oil changes and the reduced speed. has grinding.1973 Johnson 85 hp outboard motor sounds waived. Thanks