This is where the shaft meets the water, well actually it passes through the hull of the ship. Eventually the shaft must exit the inside of the ship and pass thought the hull. This device helps keep the water on the proper side of the hull.

The propeller shaft is supported in the stern tube by two bearings, one at the inner end and one at the outer end of the stern tube. These are called stern tube bearings. At the inner end of the stern tube there is a stuffing box containing the packing gland, which is generally referred to as the stern tube gland. The stern tube gland seals the area between the shaft and stern tube but allows the shaft to rotate. The stuffing box is flanged and bolted to the stern tube.

Its casing is divided into two compartments, the forward space which is the stuffing box itself, and the after space, provided with a flushing connection. This is designed to maintain a positive flow of water through the stern tube for lubricating, cooling, and flushing. This flushing connection is supplied by the firemain. A drain connection is provided both for testing for the presence of cooling water in the bearing and for permitting sea water to flow through the stern tube and cool the bearing when underway, where natural seawater circulation is employed.

Some stern tubes employ an inflatable collar with is pressurizes according to the outside water pressure. This helps keep the water out of the ship. This system has benefits over the packing gland type of seal, however neither method is 100% perfect. This is one area of the ship that has extra special inspections and maintenance. This is a relatively small piece of equipment compared to other pieces of machinery, but its importance to the ship cannot be measured. It is pretty much sink or swim.