Page 223 - Introduction to Marine Engineering

204 Shafting and propellers

Sterntube bearing
The sterntube bearing serves two important purposes. It supports the
tailshaft and a considerable proportion of the propeller weight. It also
acts as a gland to prevent the entry of sea water to the machinery space.
Early arrangements used bearing materials such as lignum vitae (a
very dense form of timber) which were lubricated by sea water. Most
modern designs use an oil lubrication arrangement for a white metal
lined sterntube bearing. One arrangement is shown in Figure 11,4.

Figure 11.4 Oil lubricated sterntube bearing

Oil is pumped to the bush through external axial grooves and passes
through holes on each side into internal axial passages. The oil leaves
from the ends of the bush and circulates back to the pump and the
cooler. One of two header tanks will provide a back pressure in the
system and a period of oil supply in the event of pump failure. A
low-level alarm will be fitted to each header tank.
Oil pressure in the lubrication system is higher than the static sea
water head to ensure that sea water cannot enter the sterntube in the
event of seal failure.

Sterntube seals
Special seals are fitted at the outboard and inboard ends of the tailshaft.
They are arranged to prevent the entry of sea water and also the loss of
lubricating oil from the stern bearing.