OB props are lightly snugged up just enough to take the fore & aft play out. The prop shaft has a hole which lines up with a hole in the nut for a cotter pin. Some nuts (not many like this anymore) have slots in the top instead of a hole. You just line the holes (or slots) up enough to get the cotter pin in. The only time you need a wrench if if the fit is so tight you have to use one to get it all the way on.
I won't go into breaking shear pins or ruining a slipping rubber hub by tightening too much...
