The stem tube is located where the shaft passes through the hull of the ship. The shaft is supported within the stern tube by two stern tube bearings; one on the inner side, and one on the outer side of the hull. The construction of the stern tube bearing is basically the same as that of the strut bearing, which is described later in this chapter.

The point where the shaft passes through the hull must be sealed to prevent seawater from entering the ship. This is accomplished primarily by use of either packing or mechanical seals. Stem tube packing (fig. 12-20) is used only on older ships as a primary sealer.

This packing method uses a stuffing box that is flanged and bolted to the stern tube. The afterspace contains a flushing connection to provide a constant flow of water through the stem tube (from inside the ship to outside the ship) to lubricate, cool, and flush the bearings. This flushing connection is supplied by the firemain. A drain connection is provided both to test the presence of cooling water in the bearing and to allow seawater to flow through the stern tube to lubricate the packing when the ship is underway. This is done where natural seawater circulation is used.

The gland for the stuffing box is divided longitudinally into two parts. The packing

Figure 12-20.-Stern tube stuffing box and gland.

material used, Teflon-impregnated asbestos (PTE), is according to MIL-P-24377 and has replaced all previously used packing material. Gland leakage is required to prevent packing from heating up, crystallizing, and scoring the shaft sealing surface within the gland. Usually, the gland is tightened and the flushing connection is closed to eliminate leakage when the ship is in port. It is loosened just enough to permit a slight trickle of water for cooling purposes when the ship is underway. Whenever packing is added to a stern tube, be sure that the gland is drawn up evenly by using a rule to measure the distance between the gland and the stuffing box.

In some installations, mechanical seals are used to seal the stern tube. Two of the major advances of these seals are (1) they will operate maintenance free for extended periods, and (2) there is no leakage into the shaft alley.