The hole in the hull structure for accommodating the propeller shaft to the outside of the hull is called the stern tube. 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 tubecalled stern tube bearings. At the inner end of the stern tube there is a stuffing box containing the packing gland (figure 4-4), 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 compartmentsthe forward space which is the stuffing box proper, and the after space, provided with a flushing connection, 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.

The gland for the stuffing box is divided longitudinally into two parts. The gland bolts are long enough to support the gland when the latter is withdrawn at least 1 inch clear of the stuffing

Figure 4-4.Stern tube stuffing box and gland.

Figure 4-5.Details of underwater strut bearing. A. Longitudinal view. B. Cross-sectional view. C. Rubber stripping in the bearing.

box. This permits the addition of a ring of new packing, when needed, while the ship is water-borne.

Either braided flax packing or special semimetallic packing must be used (ships engineering drawings show the proper type of packing). This gland is usually tightened to eliminate leakage when the ship is in port, and is loosened (prior to warming up) just enough to permit a slight trickle of water for cooling purposes when the ship is underway.

More recent shaft seal designs utilize packing only for emergencies. These newer seals are of two types; rubber face seals and mechanical face seals. Both face seals are on a plane perpendicular to the shafting, against a gland ring for rubber face seal or against a seal ring for a mechanical face seal. Further, most face seals require seawater for both cooling and lubrication.

The rubber face consists of a rubber element that is clamped around the shaft just tightly enough to prevent rotational slippage and leakage underneath the seal, while at the same time, the seal is able to travel axially along the shaft. This axial motion is necessary so that the seal can maintain its position against the gland ring regardless of shaft position.

The mechanical face seal is a ring made of either a hard synthetic or a carbon compound. This ring is held tightly against the seal ring by springs mounted behind it.

NOTE: More information on face-type seals is available in manufactures technical manuals (Crane Co., Surface Ship, Seal Inc., Submarine).