Just in case you think I am sitting around just drinking rum and Coke … well, I do a lot of that also but here is what has to happen to properly change a transmission coupling.
First the prop shaft has to be pulled out enough to get access to the transmission coupling - you have to be underwater to be able to pull on the prop shaft.
Second, unbolt the transmission coupling. No big deal
Third, find out that the new transmission coupling has a key slot that is a different size from the prop shaft.
Fourth, pull out the prop shaft. Wait, no, can't do that yet; have to take off that propeller. Underwater of course.
Fourth, a day later, get the propeller off. Ok, now pull out the prop shaft. Wait, no, not yet. Now the rudder is in the way. Ok, so lower the rudder. That’s after spending a day taking off the autopilot arm, the steering cables, the quadrant, the packing glands, and those four bolts that are rusted in place.
Now take out the prop shaft.
Fifth, take prop shaft to machine shop to get the key slot re-machined. Pick up the prop shaft the next day. Oops, never mind, the machinist didn't come to work that day. Wait another day. No, wait two more days.
Sixth, reinstall the prop shaft. Nope, not yet, now you can't get the prop shaft back in without completely disconnecting the rudder. Tie a couple lines around the rudder, run them to the genoa sheet winches, and lower the rudder slowly into water.
Oops, the tide is out and you must dig a hole in the ocean bottom to get enough space to lower rudder.
Seventh (?) put the prop shaft back into the boat. That's easy.
Eighth, reconnect transmission coupling to shaft. And find out that the engine is out of alignment.
Ninth, realign the engine to the shaft.
Tenth, put the rudder back into the boat. Find out that the rudder post is heavier than the rest of the rudder, so the rudder doesn't want to stand post up right but instead wants to lay on the ocean bottom.
Finally, get disgusted, take off your fins, put 5000 pounds of weight on, and walk on the ocean bottom. Pick up the rudder and shove it into the bottom of the boat. Oh, and make sure you lose the plastic ring that keeps the rudder from banging into the boat bottom in rough seas so you can find that later.
Eleventh, reconnect the packing gland, quadrant, autopilot, etc. etc. etc. When you find a few extra washers, just throw them overboard.
Then find out that the steering cables are now looser than before. Be sure to plan to fix them later.
Twelfth, reinstall the prop shaft, tighten down the locking nuts, and install a new cotter pin.
Thirteenth, put the new zinc fitting on the shaft. Find out your zinc is for a 1 inch shaft and your shaft is 1 1/4 inch. Go to the marine shop to find they don't have any in stock, but may be able to get one in two weeks. Plan a trip to the local shipyard for parts.
Look on the ocean floor around boat to find that plastic ring you lost. Recover it, and put it in the toolbox. You know you will be working on this boat again someday.
And that's it! In just a few simple steps you can replace the transmission coupling and assure yourself of days, possibly weeks, and maybe even months of carefree sailing.