loose prop

I had occasion to pull props for re-pitching on a Bertram 31, and was puzzled to see dis-coloration at the forward end of the shaft taper, and when I cleaned up the keys, there was a groove worn on both sides exactly at the height of the shaft key way depth. I decided to lap the props to the shaft and found that while the prop slid forward with a definite stop( no key in place), the forward end of the taper would never make contact with the bluing. Took the props to the local machinist who also makes shafts and fits couplings to shaft, etc. he immediately set his taper reamer up on a vise and turned the prop by hand, which revealed that the prop taper bore was not straight, but convex and not making contact the entire length at all. We got such a pile of Nibral shavings that I had to make sure the prop hub was not now sliding so far forward that the nuts couldn't tighten. Thankfully, there were still threads for the nuts to turn onto. I also made new keys that fit properly in width and height. Result was the boat has never run so smooth and quiet.
I was shocked that such a poorly machined taper could be sent out by the Manufacturer, these props had always been tuned to Class 1, but still the boat was not happy and completely smooth before this last work.
My SOP now will to always check the prop tapers on a reamer, not the mandrel the prop shops typically use.

Isn't it time to consider a new/different prop tech?

Insanity. Doing the same thing over and over again and expecting a different result.

Something is not right. I'm almost willing to put money that the keyways are too long.

Keys

You should take the time to see what the keyway actually does, if properly matched and lapped the keyway is not needed - look at many engine gears these days that have no keyways just a lapped surface and tightened by bolt.

I'm aware of the fact a keyway is not needed. I have seen on a handful of boats were the key was just a touch too long, as you tightened the nut, it pushed the key up the taper and didn't allow the prop to seat all of the way against the shaft, yet the nut would tighten, etc. Then you'd haul the boat and the nuts were loose because the use allowed the props to slide up a little further. Aside from improperly lapped props, the key being a touch too long is the only other reason I've seen the prop nuts being loose.

Loose prop nuts can cause the shaft to crack, usually at the keyway area, I've seen that a few times also.

Also if you're props are Michigan/Federals they had some back in that era where the hubs weren't cut 100% properly, but close.

You need to make sure the key slides all the way through the hub. When they come back from the prop shop they usually have a slight burr on the large side of the bore. All it takes is a couple of passes with a file. The torque is not causing the problem. If that wheel is properly on the taper it will stay tight. Do you know who did the machine work on the taper? I would check the key first, then the hub for cracks, then blue the shaft.