The propeller fits onto the tapered shaft tip and is tightened into place by two nuts that thread onto the end of the propeller shaft. One nut is small, the other nut is big. This brings up the question of which order should the nuts be placed?
You could either place the big one on first, tighten it all the way, and then place and tighten the smaller one. Or you could place the smaller one first and tighten it all the way, and then place and tighten the bigger one. So many options on such a tiny part of the boat that is instantly forgotten the moment the boat goes into the water.
The proper order is the small nut first and the big nut second.
When you tighten the small nut against the propeller, the force of the propeller is then pushed onto the threads of the nut and shaft. When you tighten the big nut, the lateral pressure from the small nut is transferred to the big nut which has more threads and transmits the force more evenly to the threads of the shaft tip.
The reason to have two nuts is simple, when they are tightened against each other, they will apply lateral pressure away from each other and seize to the threads that they are pushing against. When the two nuts are tightened into each other, they will not come loose as the entire assembly spins furiously without any inspection. As a safety precaution, the end of the shaft will have a cotter pin just in case the nuts come loose. A loose propeller is a problem that can quickly be remedied. A lost propeller is an expensive problem that is not easy to fix when out at sea!
Next time you are on the hard, take a look at the nuts at the end of the propeller shaft and make sure that the small one is next to the propeller, followed by the big one.