You have a fitting called a grease cup. It is both a reservoir and a means of injecting grease into the shaft log. The cap which you turn is meant to protect the grease in the cup and to provide a means to compress it so that it will flow into the shaft log. It allows you to grease the packing without a grease gun.
Properly used, it is a very nice thing. I would keep it full of waterproof grease (boat trailer wheel bearing type) and give the cap a half turn or so every time you put the boat up. It will help to stop the water drip during periods at the mooring or slip and will also keep the shaft well lubricated.
A grease cup should be completely filled with new grease whenever you find that you have turned the cap fully down in the threads and it cannot be turned further. After filling, you should replace the cap, but do not turn it down more than a complete turn or so. You just want to have it contact the new grease so that each future turn will feed some into the log. Each time you turn the cap, the packing gets a little new grease.
These are also found on rudder post bushings where they provide similar "no grease gun" advantages.
Last edited by Goodnewsboy; 03-22-2007 at 06:26 PM.