A Stuffing Box is the oldest type of shaft seal from the first days of wooden boats and ships and the very first shafts that went thru the vessel hull.  They were used with both rudder shafts and propeller shafts as well as other shaft applications.

The Stuffing Box was a box that was built on the interior of the vessel around the shaft where it exited the vessel and it was stuffed with greased cloth and various other materials to keep the water out and also let the shaft turn at the same time.

Present day stuffing boxes work on the same principle as the first ones did. 

In addition, they are expected to leak just like the original ones did.

Marine Stuffing Box Catalog

Purchase ONLINE Marine Propeller Stuffing Box Packing

In the image above you can see that the greased cloth is replaced with "packing".  The seal mechanism that keeps the water out is the result of the packing tightly rubbing up against the turning shaft.  The packing can be tightened down with the "packing gland" that applies even pressure from the inboard end of the stuffing box and squeezes the packing up against the propeller shaft.

Over time this packing will wear and can be replaced.

The packing gland can be removed for complete replacement of the packing or you can just add another ring as necessary.

Some stuffing boxes are built with a grease zerk fitting so grease can be pushed into the stuffing box thus reducing friction and assisting with the seal of water.

Squeezing the packing up against the propeller shaft will create friction and heat.  While today's best packing can withstand more heat than older packing types there is still considerable heat that needs to be dealt with.  Read about packing that is made specially for stuffing boxes here:  Marine Stuffing Box Packing

A small amount of water present and leaking through the packing into the vessel will assist in heat removal.  Having your packing gland set to the right pressure on the packing is important.  Not enough will create excess leakage.  Too much will create inefficiency, shaft wear and too much heat.  The packing will wear and the gland will need to be tightened over time.

Because the packing is rubbing up against the propeller shaft it will wear grooves into the shaft over time.  With costly large diameter shafts a "shaft liner" is used.  Over time the shaft liner will wear instead of the more expensive propeller shaft itself.  With smaller shaft the added extra cost does not warrant the use of a shaft liner.

For larger shafts an Emergency Air Seal can be included outboard of the stuffing box.  If the shaft is damaged and bent and water is leaking into the vessel the shaft can be secured (stopped) and the air seal inflated to stop water from entering the vessel.  The shaft can not be turned with the emergency air seal inflated.   It will melt quickly from the friction.

Propeller Stuffing Boxes are flanged and bolted to the Stern Tube counter flange located on the inboard end of the stern tube.  The propeller shaft itself runs through the stern tube.