These quotes are from the late Frank A. (RIP).....he gives a great explanation for proper prop nut torque.......
"The prop nut should be a nylock self locking nut and a little past snug should be tight enough. If the lower unit uses the Mercury style nut with tabbed locking washer, the nut should be snugged down to the next tab past snug and the locking tab bent over into the slot on the bronze hub"
"DO NOT USE AN IMPACT GUN!
The shaft splines keep the prop from rotating on the prop shaft and trying to back-off the nut.
The nut and washer simply keep the prop seated against the thrust washer behind the prop. It does not need to be "Killer" tight. If you snug the nut then give it a little extra, there is very little chance of losing the prop.
My 1988 factory manual does not give a torque value for that nut, however, I have heard values as high as 55 foot pounds given. In my opinion, this is way too much. Last time this question came up, I was so curious that I went down to the garage and checked the torque on all my engines. Average torque on prop nuts that I tightened by hand was 8 foot pounds. As stated above, being a nylock nut, it will hold itself on the threads quite nicely."
