The propeller, shaft, stern tube and stuffing box 

Look for wear in the stern tube bush by trying to move the shaft sideways. If you need to replace this you might need to remove the shaft depending on what type of bush you have. For a cutless bearing usually after removing the propeller with a proper prop puller, you can loosen the grub screw or clamp bolt on the side of the bronze housing and grip the cutless bearing with a pair of vice grips, and twist it out.

Most others require that the shaft be removed.

Don’t forget to replace your zinc anode.

When pulling the shaft out from the coupling at the back of the gearbox, if it is tight, place a slightly smaller diameter spacer between the end of the shaft and the gearbox coupling, insert some longer coupling bolts through the two halves of the coupling and evenly tighten them up to press the shaft out. Do not use a hammer as this could burr the end of the shaft and when putting the shaft back into the coupling Do not be tempted to hit the propeller end of the shaft to drive it home, while using the gearbox and engine as a dolly. This will result in the rear gearbox bearing being damaged. While the shaft is out check for wear and straightness. To check the latter lay it across two pieces of 100x 50mm (4 x 2") on edge on a flat surface. Roll it back and forth and watch to see if the ends wobble. If it does, or it is badly worn at the gland end or stern bearing end, get a new shaft made.

Check the stern tube. This connects the outer bearing housing with the inner stuffing box or shaft seal. This is usually a threaded length of copper or bronze tube. Several boats have had a hard to find leak from this area and have found that the stern tube had developed a hole or broken. To remove this assembly you need to unscrew the two outer lag screws, which hold the outer housing to the hull and then unscrew the outer housing and stern tube. Get your local marine engineering shop to turn you up a new one. Reinstall bedding it down in plenty of high quality marine sealant. ie: Sika flex or M5200.

Don’t forget to check your shaft to motor alignment. This should be done each year as the motor mounts do slowly compress. The easiest way is to make sure that the shaft and gearbox couplings are in line and that an equal gap exists between the coupling faces when they are in close proximity. Use a feeler gauge or a set space for this.